natural selection definition essay

ENCYCLOPEDIC ENTRY

Natural selection.

Natural selection is the process through which species adapt to their environments. It is the engine that drives evolution.

On the Origin of Species

English naturalist Charles Darwin wrote the definitive book outlining his idea of natural selection, On the Origin of Species. The book chronicled his studies in South America and Pacific islands. Published in 1859, the book became a best seller.

Photograph by Ian Forsyth via Getty Images

English naturalist Charles Darwin developed the idea of natural selection after a five-year voyage to study plants, animals, and fossils in South America and on islands in the Pacific. In 1859, he brought the idea of natural selection to the attention of the world in his best-selling book, On the Origin of Species .

Natural selection is the process through which populations of living organisms adapt and change. Individuals in a population are naturally variable, meaning that they are all different in some ways. This variation means that some individuals have traits better suited to the environment than others. Individuals with adaptive traits — traits that give them some advantage—are more likely to survive and reproduce. These individuals then pass the adaptive traits on to their offspring. Over time, these advantageous traits become more common in the population. Through this process of natural selection , favorable traits are transmitted through generations .

Natural selection can lead to speciation , where one species gives rise to a new and distinctly different species . It is one of the processes that drives evolution and helps to explain the diversity of life on Earth.

Darwin chose the name natural selection to contrast with “artificial selection,” or selective breeding that is controlled by humans. He pointed to the pastime of pigeon breeding, a popular hobby in his day, as an example of artificial selection. By choosing which pigeons mated with others, hobbyists created distinct pigeon breeds, with fancy feathers or acrobatic flight, that were different from wild pigeons.

Darwin and other scientists of his day argued that a process much like artificial selection happened in nature, without any human intervention. He argued that natural selection explained how a wide variety of life forms developed over time from a single common ancestor.

Darwin did not know that genes existed, but he could see that many traits are heritable—passed from parents to offspring.

Mutations are changes in the structure of the molecules that make up genes , called DNA . The mutation of genes is an important source of genetic variation within a population. Mutations can be random (for example, when replicating cells make an error while copying DNA ), or happen as a result of exposure to something in the environment, like harmful chemicals or radiation.

Mutations can be harmful, neutral, or sometimes helpful, resulting in a new, advantageous trait. When mutations occur in germ cells (eggs and sperm), they can be passed on to offspring.

If the environment changes rapidly, some species may not be able to adapt fast enough through natural selection . Through studying the fossil record, we know that many of the organisms that once lived on Earth are now extinct. Dinosaurs are one example. An invasive species , a disease organism, a catastrophic environmental change, or a highly successful predator can all contribute to the extinction of species .

Today, human actions such as overhunting and the destruction of habitats are the main cause of extinctions. Extinctions seem to be occurring at a much faster rate today than they did in the past, as shown in the fossil record.

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natural selection , process that results in the adaptation of an organism to its environment by means of selectively reproducing changes in its genotype , or genetic constitution.

A brief treatment of natural selection follows. For full treatment, see evolution: The concept of natural selection .

In natural selection, those variations in the genotype (the entire complex of genes inherited from both parents) that increase an organism’s chances of survival and procreation are preserved and multiplied from generation to generation at the expense of less advantageous variations. Evolution often occurs as a consequence of this process. Natural selection may arise from differences in survival, in fertility , in rate of development, in mating success, or in any other aspect of the life cycle . All such differences result in natural selection to the extent that they affect the number of progeny an organism leaves.

Gene frequencies tend to remain constant from generation to generation when disturbing factors are not present. Factors that disturb the natural equilibrium of gene frequencies include mutation , migration (or gene flow ), random genetic drift , and natural selection. A mutation is a spontaneous change in the gene frequency that takes place in a population and occurs at a low rate. Migration is a local change in gene frequency when an individual moves from one population to another and then interbreeds. Random genetic drift is a change that takes place from one generation to another by a process of pure chance. Mutation, migration, and genetic drift alter gene frequencies without regard to whether such changes increase or decrease the likelihood of an organism surviving and reproducing in its environment . They are all random processes.

Natural selection moderates the disorganizing effects of these processes because it multiplies the incidence of beneficial mutations over the generations and eliminates harmful ones, since their carriers leave few or no descendants. Natural selection enhances the preservation of a group of organisms that are best adjusted to the physical and biological conditions of their environment and may also result in their improvement in some cases. Some characteristics, such as the male peacock ’s tail, actually decrease the individual organism’s chance of survival. To explain such anomalies , Darwin posed a theory of “ sexual selection .” In contrast to features that result from natural selection, a structure produced by sexual selection results in an advantage in the competition for mates.

Volume 2 Supplement 2

Special Issue: Transitional Fossils

• Evolutionary Concepts
• Open access
• Published: 09 April 2009

Understanding Natural Selection: Essential Concepts and Common Misconceptions

• T. Ryan Gregory 1  

Evolution: Education and Outreach volume  2 ,  pages 156–175 ( 2009 ) Cite this article

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Natural selection is one of the central mechanisms of evolutionary change and is the process responsible for the evolution of adaptive features. Without a working knowledge of natural selection, it is impossible to understand how or why living things have come to exhibit their diversity and complexity. An understanding of natural selection also is becoming increasingly relevant in practical contexts, including medicine, agriculture, and resource management. Unfortunately, studies indicate that natural selection is generally very poorly understood, even among many individuals with postsecondary biological education. This paper provides an overview of the basic process of natural selection, discusses the extent and possible causes of misunderstandings of the process, and presents a review of the most common misconceptions that must be corrected before a functional understanding of natural selection and adaptive evolution can be achieved.

“There is probably no more original, more complex, and bolder concept in the history of ideas than Darwin's mechanistic explanation of adaptation.” Ernst Mayr ( 1982 , p.481)

Introduction

Natural selection is a non-random difference in reproductive output among replicating entities, often due indirectly to differences in survival in a particular environment, leading to an increase in the proportion of beneficial, heritable characteristics within a population from one generation to the next. That this process can be encapsulated within a single (admittedly lengthy) sentence should not diminish the appreciation of its profundity and power. It is one of the core mechanisms of evolutionary change and is the main process responsible for the complexity and adaptive intricacy of the living world. According to philosopher Daniel Dennett ( 1995 ), this qualifies evolution by natural selection as “the single best idea anyone has ever had.”

Natural selection results from the confluence of a small number of basic conditions of ecology and heredity. Often, the circumstances in which those conditions apply are of direct significance to human health and well-being, as in the evolution of antibiotic and pesticide resistance or in the impacts of intense predation by humans (e.g., Palumbi 2001 ; Jørgensen et al. 2007 ; Darimont et al. 2009 ). Understanding this process is therefore of considerable importance in both academic and pragmatic terms. Unfortunately, a growing list of studies indicates that natural selection is, in general, very poorly understood—not only by young students and members of the public but even among those who have had postsecondary instruction in biology.

As is true with many other issues, a lack of understanding of natural selection does not necessarily correlate with a lack of confidence about one's level of comprehension. This could be due in part to the perception, unfortunately reinforced by many biologists, that natural selection is so logically compelling that its implications become self-evident once the basic principles have been conveyed. Thus, many professional biologists may agree that “[evolution] shows how everything from frogs to fleas got here via a few easily grasped biological processes ” (Coyne 2006 ; emphasis added). The unfortunate reality, as noted nearly 20 years ago by Bishop and Anderson ( 1990 ), is that “the concepts of evolution by natural selection are far more difficult for students to grasp than most biologists imagine.” Despite common assumptions to the contrary by both students and instructors, it is evident that misconceptions about natural selection are the rule, whereas a working understanding is the rare exception.

The goal of this paper is to enhance (or, as the case may be, confirm) readers' basic understanding of natural selection. This first involves providing an overview of the basis and (one of the) general outcomes of natural selection as they are understood by evolutionary biologists Footnote 1 . This is followed by a brief discussion of the extent and possible causes of difficulties in fully grasping the concept and consequences of natural selection. Finally, a review of the most widespread misconceptions about natural selection is provided. It must be noted that specific instructional tools capable of creating deeper understanding among students generally have remained elusive, and no new suggestions along these lines are presented here. Rather, this article is aimed at readers who wish to confront and correct any misconceptions that they may harbor and/or to better recognize those held by most students and other non-specialists.

The Basis and Basics of Natural Selection

Though rudimentary forms of the idea had been presented earlier (e.g., Darwin and Wallace 1858 and several others before them), it was in On the Origin of Species by Means of Natural Selection that Darwin ( 1859 ) provided the first detailed exposition of the process and implications of natural selection Footnote 2 . According to Mayr ( 1982 , 2001 ), Darwin's extensive discussion of natural selection can be distilled to five “facts” (i.e., direct observations) and three associated inferences. These are depicted in Fig.  1 .

The basis of natural selection as presented by Darwin ( 1859 ), based on the summary by Mayr ( 1982 )

Some components of the process, most notably the sources of variation and the mechanisms of inheritance, were, due to the limited available information in Darwin's time, either vague or incorrect in his original formulation. Since then, each of the core aspects of the mechanism has been elucidated and well documented, making the modern theory Footnote 3 of natural selection far more detailed and vigorously supported than when first proposed 150 years ago. This updated understanding of natural selection consists of the elements outlined in the following sections.

Overproduction, Limited Population Growth, and the “Struggle for Existence”

A key observation underlying natural selection is that, in principle, populations have the capacity to increase in numbers exponentially (or “geometrically”). This is a simple function of mathematics: If one organism produces two offspring, and each of them produces two offspring, and so on, then the total number grows at an increasingly rapid rate (1 → 2 → 4 → 8 → 16 → 32 → 64... to 2 n after n rounds of reproduction).

The enormity of this potential for exponential growth is difficult to fathom. For example, consider that beginning with a single Escherichia coli bacterium, and assuming that cell division occurs every 30 minutes, it would take less than a week for the descendants of this one cell to exceed the mass of the Earth. Of course, exponential population expansion is not limited to bacteria. As Nobel laureate Jacques Monod once quipped, “What is true for E. coli is also true for the elephant,” and indeed, Darwin ( 1859 ) himself used elephants as an illustration of the principle of rapid population growth, calculating that the number of descendants of a single pair would swell to more than 19,000,000 in only 750 years Footnote 4 . Keown ( 1988 ) cites the example of oysters, which may produce as many as 114,000,000 eggs in a single spawn. If all these eggs grew into oysters and produced this many eggs of their own that, in turn, survived to reproduce, then within five generations there would be more oysters than the number of electrons in the known universe.

Clearly, the world is not overrun with bacteria, elephants, or oysters. Though these and all other species engage in massive overproduction (or “superfecundity”) and therefore could in principle expand exponentially, in practice they do not Footnote 5 . The reason is simple: Most offspring that are produced do not survive to produce offspring of their own. In fact, most population sizes tend to remain relatively stable over the long term. This necessarily means that, on average, each pair of oysters produces only two offspring that go on to reproduce successfully—and that 113,999,998 eggs per female per spawn do not survive (see also Ridley 2004 ). Many young oysters will be eaten by predators, others will starve, and still others will succumb to infection. As Darwin ( 1859 ) realized, this massive discrepancy between the number of offspring produced and the number that can be sustained by available resources creates a “struggle for existence” in which often only a tiny fraction of individuals will succeed. As he noted, this can be conceived as a struggle not only against other organisms (especially members of the same species, whose ecological requirements are very similar) but also in a more abstract sense between organisms and their physical environments.

Variation and Inheritance

Variation among individuals is a fundamental requirement for evolutionary change. Given that it was both critical to his theory of natural selection and directly counter to much contemporary thinking, it should not be surprising that Darwin ( 1859 ) expended considerable effort in attempting to establish that variation is, in fact, ubiquitous. He also emphasized the fact that some organisms—namely relatives, especially parents and their offspring—are more similar to each other than to unrelated members of the population. This, too, he realized is critical for natural selection to operate. As Darwin ( 1859 ) put it, “Any variation which is not inherited is unimportant for us.” However, he could not explain either why variation existed or how specific characteristics were passed from parent to offspring, and therefore was forced to treat both the source of variation and the mechanism of inheritance as a “black box.”

The workings of genetics are no longer opaque. Today, it is well understood that inheritance operates through the replication of DNA sequences and that errors in this process (mutations) and the reshuffling of existing variants (recombination) represent the sources of new variation. In particular, mutations are known to be random (or less confusingly, “undirected”) with respect to any effects that they may have. Any given mutation is merely a chance error in the genetic system, and as such, its likelihood of occurrence is not influenced by whether it will turn out to be detrimental, beneficial, or (most commonly) neutral.

As Darwin anticipated, extensive variation among individuals has now been well established to exist at the physical, physiological, and behavioral levels. Thanks to the rise of molecular biology and, more recently, of genomics, it also has been possible to document variation at the level of proteins, genes, and even individual DNA nucleotides in humans and many other species.

Non-random Differences in Survival and Reproduction

Darwin saw that overproduction and limited resources create a struggle for existence in which some organisms will succeed and most will not. He also recognized that organisms in populations differ from one another in terms of many traits that tend to be passed on from parent to offspring. Darwin's brilliant insight was to combine these two factors and to realize that success in the struggle for existence would not be determined by chance, but instead would be biased by some of the heritable differences that exist among organisms. Specifically, he noted that some individuals happen to possess traits that make them slightly better suited to a particular environment, meaning that they are more likely to survive than individuals with less well suited traits. As a result, organisms with these traits will, on average, leave more offspring than their competitors.

Whereas the origin of a new genetic variant occurs at random in terms of its effects on the organism, the probability of it being passed on to the next generation is absolutely non-random if it impacts the survival and reproductive capabilities of that organism. The important point is that this is a two-step process: first, the origin of variation by random mutation, and second, the non-random sorting of variation due to its effects on survival and reproduction (Mayr 2001 ). Though definitions of natural selection have been phrased in many ways (Table  1 ), it is this non-random difference in survival and reproduction that forms the basis of the process.

Darwinian Fitness

The meaning of fitness in evolutionary biology.

In order to study the operation and effects of natural selection, it is important to have a means of describing and quantifying the relationships between genotype (gene complement), phenotype (physical and behavioral features), survival, and reproduction in particular environments. The concept used by evolutionary biologists in this regard is known as “Darwinian fitness,” which is defined most simply as a measure of the total (or relative) reproductive output of an organism with a particular genotype (Table  1 ). In the most basic terms, one can state that the more offspring an individual produces, the higher is its fitness. It must be emphasized that the term “fitness,” as used in evolutionary biology, does not refer to physical condition, strength, or stamina and therefore differs markedly from its usage in common language.

“Survival of the Fittest” is Misleading

In the fifth edition of the Origin (published in 1869), Darwin began using the phrase “survival of the fittest”, which had been coined a few years earlier by British economist Herbert Spencer, as shorthand for natural selection. This was an unfortunate decision as there are several reasons why “survival of the fittest” is a poor descriptor of natural selection. First, in Darwin's context, “fittest” implied “best suited to a particular environment” rather than “most physically fit,” but this crucial distinction is often overlooked in non-technical usage (especially when further distorted to “only the strong survive”). Second, it places undue emphasis on survival: While it is true that dead organisms do not reproduce, survival is only important evolutionarily insofar as it affects the number of offspring produced. Traits that make life longer or less difficult are evolutionarily irrelevant unless they also influence reproductive output. Indeed, traits that enhance net reproduction may increase in frequency over many generations even if they compromise individual longevity. Conversely, differences in fecundity alone can create differences in fitness, even if survival rates are identical among individuals. Third, this implies an excessive focus on organisms, when in fact traits or their underlying genes equally can be identified as more or less fit than alternatives. Lastly, this phrase is often misconstrued as being circular or tautological (Who survives? The fittest. Who are the fittest? Those who survive). However, again, this misinterprets the modern meaning of fitness, which can be both predicted in terms of which traits are expected to be successful in a specific environment and measured in terms of actual reproductive success in that environment.

Which Traits Are the Most Fit?

Directional natural selection can be understood as a process by which fitter traits (or genes) increase in proportion within populations over the course of many generations. It must be understood that the relative fitness of different traits depends on the current environment. Thus, traits that are fit now may become unfit later if the environment changes. Conversely, traits that have now become fit may have been present long before the current environment arose, without having conferred any advantage under previous conditions. Finally, it must be noted that fitness refers to reproductive success relative to alternatives here and now —natural selection cannot increase the proportion of traits solely because they may someday become advantageous. Careful reflection on how natural selection actually works should make it clear why this is so.

Natural Selection and Adaptive Evolution

Natural selection and the evolution of populations.

Though each has been tested and shown to be accurate, none of the observations and inferences that underlies natural selection is sufficient individually to provide a mechanism for evolutionary change Footnote 6 . Overproduction alone will have no evolutionary consequences if all individuals are identical. Differences among organisms are not relevant unless they can be inherited. Genetic variation by itself will not result in natural selection unless it exerts some impact on organism survival and reproduction. However, any time all of Darwin's postulates hold simultaneously—as they do in most populations—natural selection will occur. The net result in this case is that certain traits (or, more precisely, genetic variants that specify those traits) will, on average , be passed on from one generation to the next at a higher rate than existing alternatives in the population. Put another way, when one considers who the parents of the current generation were, it will be seen that a disproportionate number of them possessed traits beneficial for survival and reproduction in the particular environment in which they lived.

The important points are that this uneven reproductive success among individuals represents a process that occurs in each generation and that its effects are cumulative over the span of many generations. Over time, beneficial traits will become increasingly prevalent in descendant populations by virtue of the fact that parents with those traits consistently leave more offspring than individuals lacking those traits. If this process happens to occur in a consistent direction—say, the largest individuals in each generation tend to leave more offspring than smaller individuals—then there can be a gradual, generation-by-generation change in the proportion of traits in the population. This change in proportion and not the modification of organisms themselves is what leads to changes in the average value of a particular trait in the population. Organisms do not evolve; populations evolve.

The term “adaptation” derives from ad + aptus , literally meaning “toward + fit”. As the name implies, this is the process by which populations of organisms evolve in such a way as to become better suited to their environments as advantageous traits become predominant. On a broader scale, it is also how physical, physiological, and behavioral features that contribute to survival and reproduction (“adaptations”) arise over evolutionary time. This latter topic is particularly difficult for many to grasp, though of course a crucial first step is to understand the operation of natural selection on smaller scales of time and consequence. (For a detailed discussion of the evolution of complex organs such as eyes, see Gregory 2008b .)

On first pass, it may be difficult to see how natural selection can ever lead to the evolution of new characteristics if its primary effect is merely to eliminate unfit traits. Indeed, natural selection by itself is incapable of producing new traits, and in fact (as many readers will have surmised), most forms of natural selection deplete genetic variation within populations. How, then, can an eliminative process like natural selection ever lead to creative outcomes?

To answer this question, one must recall that evolution by natural selection is a two-step process. The first step involves the generation of new variation by mutation and recombination, whereas the second step determines which randomly generated variants will persist into the next generation. Most new mutations are neutral with respect to survival and reproduction and therefore are irrelevant in terms of natural selection (but not, it must be pointed out, to evolution more broadly). The majority of mutations that have an impact on survival and reproductive output will do so negatively and, as such, will be less likely than existing alternatives to be passed on to subsequent generations. However, a small percentage of new mutations will turn out to have beneficial effects in a particular environment and will contribute to an elevated rate of reproduction by organisms possessing them. Even a very slight advantage is sufficient to cause new beneficial mutations to increase in proportion over the span of many generations.

Biologists sometimes describe beneficial mutations as “spreading” or “sweeping” through a population, but this shorthand is misleading. Rather, beneficial mutations simply increase in proportion from one generation to the next because, by definition, they happen to contribute to the survival and reproductive success of the organisms carrying them. Eventually, a beneficial mutation may be the only alternative left as all others have ultimately failed to be passed on. At this point, that beneficial genetic variant is said to have become “fixed” in the population.

Again, mutation does not occur in order to improve fitness—it merely represents errors in genetic replication. This means that most mutations do not improve fitness: There are many more ways of making things worse than of making them better. It also means that mutations will continue to occur even after previous beneficial mutations have become fixed. As such, there can be something of a ratcheting effect in which beneficial mutations arise and become fixed by selection, only to be supplemented later by more beneficial mutations which, in turn, become fixed. All the while, neutral and deleterious mutations also occur in the population, the latter being passed on at a lower rate than alternatives and often being lost before reaching any appreciable frequency.

Of course, this is an oversimplification—in species with sexual reproduction, multiple beneficial mutations may be brought together by recombination such that the fixation of beneficial genes need not occur sequentially. Likewise, recombination can juxtapose deleterious mutations, thereby hastening their loss from the population. Nonetheless, it is useful to imagine the process of adaptation as one in which beneficial mutations arise continually (though perhaps very infrequently and with only minor positive impacts) and then accumulate in the population over many generations.

The process of adaptation in a population is depicted in very basic form in Fig.  2 . Several important points can be drawn from even such an oversimplified rendition:

Mutations are the source of new variation. Natural selection itself does not create new traits; it only changes the proportion of variation that is already present in the population. The repeated two-step interaction of these processes is what leads to the evolution of novel adaptive features.

Mutation is random with respect to fitness. Natural selection is, by definition, non-random with respect to fitness. This means that, overall, it is a serious misconception to consider adaptation as happening “by chance”.

Mutations occur with all three possible outcomes: neutral, deleterious, and beneficial. Beneficial mutations may be rare and deliver only a minor advantage, but these can nonetheless increase in proportion in the population over many generations by natural selection. The occurrence of any particular beneficial mutation may be very improbable, but natural selection is very effective at causing these individually unlikely improvements to accumulate. Natural selection is an improbability concentrator.

No organisms change as the population adapts. Rather, this involves changes in the proportion of beneficial traits across multiple generations.

The direction in which adaptive change occurs is dependent on the environment. A change in environment can make previously beneficial traits neutral or detrimental and vice versa.

Adaptation does not result in optimal characteristics. It is constrained by historical, genetic, and developmental limitations and by trade-offs among features (see Gregory 2008b ).

It does not matter what an “ideal” adaptive feature might be—the only relevant factor is that variants that happen to result in greater survival and reproduction relative to alternative variants are passed on more frequently. As Darwin wrote in a letter to Joseph Hooker (11 Sept. 1857), “I have just been writing an audacious little discussion, to show that organic beings are not perfect, only perfect enough to struggle with their competitors.”

The process of adaptation by natural selection is not forward-looking, and it cannot produce features on the grounds that they might become beneficial sometime in the future. In fact, adaptations are always to the conditions experienced by generations in the past.

A highly simplified depiction of natural selection ( Correct ) and a generalized illustration of various common misconceptions about the mechanism ( Incorrect ). Properly understood, natural selection occurs as follows: ( A ) A population of organisms exhibits variation in a particular trait that is relevant to survival in a given environment. In this diagram, darker coloration happens to be beneficial, but in another environment, the opposite could be true. As a result of their traits, not all individuals in Generation 1 survive equally well, meaning that only a non-random subsample ultimately will succeed in reproducing and passing on their traits ( B ). Note that no individual organisms in Generation 1 change, rather the proportion of individuals with different traits changes in the population. The individuals who survive from Generation 1 reproduce to produce Generation 2. ( C ) Because the trait in question is heritable, this second generation will (mostly) resemble the parent generation. However, mutations have also occurred, which are undirected (i.e., they occur at random in terms of the consequences of changing traits), leading to both lighter and darker offspring in Generation 2 as compared to their parents in Generation 1. In this environment, lighter mutants are less successful and darker mutants are more successful than the parental average. Once again, there is non-random survival among individuals in the population, with darker traits becoming disproportionately common due to the death of lighter individuals ( D ). This subset of Generation 2 proceeds to reproduce. Again, the traits of the survivors are passed on, but there is also undirected mutation leading to both deleterious and beneficial differences among the offspring ( E ). ( F ) This process of undirected mutation and natural selection (non-random differences in survival and reproductive success) occurs over many generations, each time leading to a concentration of the most beneficial traits in the next generation. By Generation N , the population is composed almost entirely of very dark individuals. The population can now be said to have become adapted to the environment in which darker traits are the most successful. This contrasts with the intuitive notion of adaptation held by most students and non-biologists. In the most common version, populations are seen as uniform, with variation being at most an anomalous deviation from the norm ( X ). It is assumed that all members within a single generation change in response to pressures imposed by the environment ( Y ). When these individuals reproduce, they are thought to pass on their acquired traits. Moreover, any changes that do occur due to mutation are imagined to be exclusively in the direction of improvement ( Z ). Studies have revealed that it can be very difficult for non-experts to abandon this intuitive interpretation in favor of a scientifically valid understanding of the mechanism. Diagrams based in part on Bishop and Anderson ( 1990 )

Natural Selection Is Elegant, Logical, and Notoriously Difficult to Grasp

The extent of the problem.

In its most basic form, natural selection is an elegant theory that effectively explains the obviously good fit of living things to their environments. As a mechanism, it is remarkably simple in principle yet incredibly powerful in application. However, the fact that it eluded description until 150 years ago suggests that grasping its workings and implications is far more challenging than is usually assumed.

Three decades of research have produced unambiguous data revealing a strikingly high prevalence of misconceptions about natural selection among members of the public and in students at all levels, from elementary school pupils to university science majors (Alters 2005 ; Bardapurkar 2008 ; Table  2 ) Footnote 7 . A finding that less than 10% of those surveyed possess a functional understanding of natural selection is not atypical. It is particularly disconcerting and undoubtedly exacerbating that confusions about natural selection are common even among those responsible for teaching it Footnote 8 . As Nehm and Schonfeld ( 2007 ) recently concluded, “one cannot assume that biology teachers with extensive backgrounds in biology have an accurate working knowledge of evolution, natural selection, or the nature of science.”

Why is Natural Selection so Difficult to Understand?

Two obvious hypotheses present themselves for why misunderstandings of natural selection are so widespread. The first is that understanding the mechanism of natural selection requires an acceptance of the historical fact of evolution, the latter being rejected by a large fraction of the population. While an improved understanding of the process probably would help to increase overall acceptance of evolution, surveys indicate that rates of acceptance already are much higher than levels of understanding. And, whereas levels of understanding and acceptance may be positively correlated among teachers (Vlaardingerbroek and Roederer 1997 ; Rutledge and Mitchell 2002 ; Deniz et al. 2008 ), the two parameters seem to be at most only very weakly related in students Footnote 9 (Bishop and Anderson 1990 ; Demastes et al. 1995 ; Brem et al. 2003 ; Sinatra et al. 2003 ; Ingram and Nelson 2006 ; Shtulman 2006 ). Teachers notwithstanding, “it appears that a majority on both sides of the evolution-creation debate do not understand the process of natural selection or its role in evolution” (Bishop and Anderson 1990 ).

The second intuitive hypothesis is that most people simply lack formal education in biology and have learned incorrect versions of evolutionary mechanisms from non-authoritative sources (e.g., television, movies, parents). Inaccurate portrayals of evolutionary processes in the media, by teachers, and by scientists themselves surely exacerbate the situation (e.g., Jungwirth 1975a , b , 1977 ; Moore et al. 2002 ). However, this alone cannot provide a full explanation, because even direct instruction on natural selection tends to produce only modest improvements in students' understanding (e.g., Jensen and Finley 1995 ; Ferrari and Chi 1998 ; Nehm and Reilly 2007 ; Spindler and Doherty 2009 ). There also is evidence that levels of understanding do not differ greatly between science majors and non-science majors (Sundberg and Dini 1993 ). In the disquieting words of Ferrari and Chi ( 1998 ), “misconceptions about even the basic principles of Darwin's theory of evolution are extremely robust, even after years of education in biology.”

Misconceptions are well known to be common with many (perhaps most) aspects of science, including much simpler and more commonly encountered phenomena such as the physics of motion (e.g., McCloskey et al. 1980 ; Halloun and Hestenes 1985 ; Bloom and Weisberg 2007 ). The source of this larger problem seems to be a significant disconnect between the nature of the world as reflected in everyday experience and the one revealed by systematic scientific investigation (e.g., Shtulman 2006 ; Sinatra et al. 2008 ). Intuitive interpretations of the world, though sufficient for navigating daily life, are usually fundamentally at odds with scientific principles. If common sense were more than superficially accurate, scientific explanations would be less counterintuitive, but they also would be largely unnecessary.

Conceptual Frameworks Versus Spontaneous Constructions

It has been suggested by some authors that young students simply are incapable of understanding natural selection because they have not yet developed the formal reasoning abilities necessary to grasp it (Lawson and Thompson 1988 ). This could be taken to imply that natural selection should not be taught until later grades; however, those who have studied student understanding directly tend to disagree with any such suggestion (e.g., Clough and Wood-Robinson 1985 ; Settlage 1994 ). Overall, the issue does not seem to be a lack of logic (Greene 1990 ; Settlage 1994 ), but a combination of incorrect underlying premises about mechanisms and deep-seated cognitive biases that influence interpretations.

Many of the misconceptions that block an understanding of natural selection develop early in childhood as part of “naïve” but practical understandings of how the world is structured. These tend to persist unless replaced with more accurate and equally functional information. In this regard, some experts have argued that the goal of education should be to supplant existing conceptual frameworks with more accurate ones (see Sinatra et al. 2008 ). Under this view, “Helping people to understand evolution...is not a matter of adding on to their existing knowledge, but helping them to revise their previous models of the world to create an entirely new way of seeing” (Sinatra et al. 2008 ). Other authors suggest that students do not actually maintain coherent conceptual frameworks relating to complex phenomena, but instead construct explanations spontaneously using intuitions derived from everyday experience (see Southerland et al. 2001 ). Though less widely accepted, this latter view gains support from the observation that naïve evolutionary explanations given by non-experts may be tentative and inconsistent (Southerland et al. 2001 ) and may differ depending on the type of organisms being considered (Spiegel et al. 2006 ). In some cases, students may attempt a more complex explanation but resort to intuitive ideas when they encounter difficulty (Deadman and Kelly 1978 ). In either case, it is abundantly clear that simply describing the process of natural selection to students is ineffective and that it is imperative that misconceptions be confronted if they are to be corrected (e.g., Greene 1990 ; Scharmann 1990 ; Settlage 1994 ; Ferrari and Chi 1998 ; Alters and Nelson 2002 ; Passmore and Stewart 2002 ; Alters 2005 ; Nelson 2007 ).

A Catalog of Common Misconceptions

Whereas the causes of cognitive barriers to understanding remain to be determined, their consequences are well documented. It is clear from many studies that complex but accurate explanations of biological adaptation typically yield to naïve intuitions based on common experience (Fig.  2 ; Tables  2 and 3 ). As a result, each of the fundamental components of natural selection may be overlooked or misunderstood when it comes time to consider them in combination, even if individually they appear relatively straightforward. The following sections provide an overview of the various, non-mutually exclusive, and often correlated misconceptions that have been found to be most common. All readers are encouraged to consider these conceptual pitfalls carefully in order that they may be avoided. Teachers, in particular, are urged to familiarize themselves with these errors so that they may identify and address them among their students.

Teleology and the “Function Compunction”

Much of the human experience involves overcoming obstacles, achieving goals, and fulfilling needs. Not surprisingly, human psychology includes a powerful bias toward thoughts about the “purpose” or “function” of objects and behaviors—what Kelemen and Rosset ( 2009 ) dub the “human function compunction.” This bias is particularly strong in children, who are apt to see most of the world in terms of purpose; for example, even suggesting that “rocks are pointy to keep animals from sitting on them” (Kelemen 1999a , b ; Kelemen and Rosset 2009 ). This tendency toward explanations based on purpose (“teleology”) runs very deep and persists throughout high school (Southerland et al. 2001 ) and even into postsecondary education (Kelemen and Rosset 2009 ). In fact, it has been argued that the default mode of teleological thinking is, at best, suppressed rather than supplanted by introductory scientific education. It therefore reappears easily even in those with some basic scientific training; for example, in descriptions of ecological balance (“fungi grow in forests to help decomposition”) or species survival (“finches diversified in order to survive”; Kelemen and Rosset 2009 ).

Teleological explanations for biological features date back to Aristotle and remain very common in naïve interpretations of adaptation (e.g., Tamir and Zohar 1991 ; Pedersen and Halldén 1992 ; Southerland et al. 2001 ; Sinatra et al. 2008 ; Table  2 ). On the one hand, teleological reasoning may preclude any consideration of mechanisms altogether if simply identifying a current function for an organ or behavior is taken as sufficient to explain its existence (e.g., Bishop and Anderson 1990 ). On the other hand, when mechanisms are considered by teleologically oriented thinkers, they are often framed in terms of change occurring in response to a particular need (Table  2 ). Obviously, this contrasts starkly with a two-step process involving undirected mutations followed by natural selection (see Fig.  2 and Table  3 ).

Anthropomorphism and Intentionality

A related conceptual bias to teleology is anthropomorphism, in which human-like conscious intent is ascribed either to the objects of natural selection or to the process itself (see below). In this sense, anthropomorphic misconceptions can be characterized as either internal (attributing adaptive change to the intentional actions of organisms) or external (conceiving of natural selection or “Nature” as a conscious agent; e.g., Kampourakis and Zogza 2008 ; Sinatra et al. 2008 ).

Internal anthropomorphism or “intentionality” is intimately tied to the misconception that individual organisms evolve in response to challenges imposed by the environment (rather than recognizing evolution as a population-level process). Gould ( 1980 ) described the obvious appeal of such intuitive notions as follows:

Since the living world is a product of evolution, why not suppose that it arose in the simplest and most direct way? Why not argue that organisms improve themselves by their own efforts and pass these advantages to their offspring in the form of altered genes—a process that has long been called, in technical parlance, the “inheritance of acquired characters.” This idea appeals to common sense not only for its simplicity but perhaps even more for its happy implication that evolution travels an inherently progressive path, propelled by the hard work of organisms themselves.

The penchant for seeing conscious intent is often sufficiently strong that it is applied not only to non-human vertebrates (in which consciousness, though certainly not knowledge of genetics and Darwinian fitness, may actually occur), but also to plants and even to single-celled organisms. Thus, adaptations in any taxon may be described as “innovations,” “inventions,” or “solutions” (sometimes “ingenious” ones, no less). Even the evolution of antibiotic resistance is characterized as a process whereby bacteria “learn” to “outsmart” antibiotics with frustrating regularity. Anthropomorphism with an emphasis on forethought is also behind the common misconception that organisms behave as they do in order to enhance the long-term well-being of their species. Once again, a consideration of the actual mechanics of natural selection should reveal why this is fallacious.

All too often, an anthropomorphic view of evolution is reinforced with sloppy descriptions by trusted authorities (Jungwirth 1975a , b , 1977 ; Moore et al. 2002 ). Consider this particularly egregious example from a website maintained by the National Institutes of Health Footnote 10 :

As microbes evolve, they adapt to their environment. If something stops them from growing and spreading—such as an antimicrobial—they evolve new mechanisms to resist the antimicrobials by changing their genetic structure. Changing the genetic structure ensures that the offspring of the resistant microbes are also resistant.

Fundamentally inaccurate descriptions such as this are alarmingly common. As a corrective, it is a useful exercise to translate such faulty characterizations into accurate language Footnote 11 . For example, this could read:

Bacteria that cause disease exist in large populations, and not all individuals are alike. If some individuals happen to possess genetic features that make them resistant to antibiotics, these individuals will survive the treatment while the rest gradually are killed off. As a result of their greater survival, the resistant individuals will leave more offspring than susceptible individuals, such that the proportion of resistant individuals will increase each time a new generation is produced. When only the descendants of the resistant individuals are left, the population of bacteria can be said to have evolved resistance to the antibiotics.

Use and Disuse

Many students who manage to avoid teleological and anthropomorphic pitfalls nonetheless conceive of evolution as involving change due to use or disuse of organs. This view, which was developed explicitly by Jean-Baptiste Lamarck but was also invoked to an extent by Darwin ( 1859 ), emphasizes changes to individual organisms that occur as they use particular features more or less. For example, Darwin ( 1859 ) invoked natural selection to explain the loss of sight in some subterranean rodents, but instead favored disuse alone as the explanation for loss of eyes in blind, cave-dwelling animals: “As it is difficult to imagine that eyes, though useless, could be in any way injurious to animals living in darkness, I attribute their loss wholly to disuse.” This sort of intuition remains common in naïve explanations for why unnecessary organs become vestigial or eventually disappear. Modern evolutionary theory recognizes several reasons that may account for the loss of complex features (e.g., Jeffery 2005 ; Espinasa and Espinasa 2008 ), some of which involve direct natural selection, but none of which is based simply on disuse.

Soft Inheritance

Evolution involving changes in individual organisms, whether based on conscious choice or use and disuse, would require that characteristics acquired during the lifetime of an individual be passed on to offspring Footnote 12 , a process often termed “soft inheritance.” The notion that acquired traits can be transmitted to offspring remained a common assumption among thinkers for more than 2,000 years, including into Darwin's time (Zirkle 1946 ). As is now understood, inheritance is actually “hard,” meaning that physical changes that occur during an organism's lifetime are not passed to offspring. This is because the cells that are involved in reproduction (the germline) are distinct from those that make up the rest of the body (the somatic line); only changes that affect the germline can be passed on. New genetic variants arise through mutation and recombination during replication and will often only exert their effects in offspring and not in the parents in whose reproductive cells they occur (though they could also arise very early in development and appear later in the adult offspring). Correct and incorrect interpretations of inheritance are contrasted in Fig.  3 .

A summary of correct ( left ) and incorrect ( right ) conceptions of heredity as it pertains to adaptive evolutionary change. The panels on the left display the operation of “hard inheritance”, whereas those on the right illustrate naïve mechanisms of “soft inheritance”. In all diagrams, a set of nine squares represents an individual multicellular organism and each square represents a type of cell of which the organisms are constructed. In the left panels, the organisms include two kinds of cells: those that produce gametes (the germline, black ) and those that make up the rest of the body (the somatic line, white ). In the top left panel , all cells in a parent organism initially contain a gene that specifies white coloration marked W ( A ). A random mutation occurs in the germline, changing the gene from one that specifies white to one that specifies gray marked G ( B ). This mutant gene is passed to the egg ( C ), which then develops into an offspring exhibiting gray coloration ( D ). The mutation in this case occurred in the parent (specifically, in the germline) but its effects did not become apparent until the next generation. In the bottom left panel , a parent once again begins with white coloration and the white gene in all of its cells ( H ). During its lifetime, the parent comes to acquire a gray coloration due to exposure to particular environmental conditions ( I ). However, because this does not involve any change to the genes in the germline, the original white gene is passed into the egg ( J ), and the offspring exhibits none of the gray coloration that was acquired by its parent ( K ). In the top right panel , the distinction between germline and somatic line is not understood. In this case, a parent that initially exhibits white coloration ( P ) changes during its lifetime to become gray ( Q ). Under incorrect views of soft inheritance, this altered coloration is passed on to the egg ( R ), and the offspring is born with the gray color acquired by its parent ( S ). In the bottom right panel , a more sophisticated but still incorrect view of inheritance is shown. Here, traits are understood to be specified by genes, but no distinction is recognized between the germline and somatic line. In this situation, a parent begins with white coloration and white-specifying genes in all its cells ( W ). A mutation occurs in one type of body cells to change those cells to gray ( X ). A mixture of white and gray genes is passed on to the egg ( Y ), and the offspring develops white coloration in most cells but gray coloration in the cells where gray-inducing mutations arose in the parent ( Z ). Intuitive ideas regarding soft inheritance underlie many misconceptions of how adaptive evolution takes place (see Fig.  2 )

Studies have indicated that belief in soft inheritance arises early in youth as part of a naïve model of heredity (e.g., Deadman and Kelly 1978 ; Kargbo et al. 1980 ; Lawson and Thompson 1988 ; Wood-Robinson 1994 ). That it seems intuitive probably explains why the idea of soft inheritance persisted so long among prominent thinkers and why it is so resistant to correction among modern students. Unfortunately, a failure to abandon this belief is fundamentally incompatible with an appreciation of evolution by natural selection as a two-step process in which the origin of new variation and its relevance to survival in a particular environment are independent considerations.

Nature as a Selecting Agent

Thirty years ago, widely respected broadcaster Sir David Attenborough ( 1979 ) aptly described the challenge of avoiding anthropomorphic shorthand in descriptions of adaptation:

Darwin demonstrated that the driving force of [adaptive] evolution comes from the accumulation, over countless generations, of chance genetical changes sifted by the rigors of natural selection. In describing the consequences of this process it is only too easy to use a form of words that suggests that the animals themselves were striving to bring about change in a purposeful way–that fish wanted to climb onto dry land, and to modify their fins into legs, that reptiles wished to fly, strove to change their scales into feathers and so ultimately became birds.

Unlike many authors, Attenborough ( 1979 ) admirably endeavored to not use such misleading terminology. However, this quote inadvertently highlights an additional challenge in describing natural selection without loaded language. In it, natural selection is described as a “driving force” that rigorously “sifts” genetic variation, which could be misunderstood to imply that it takes an active role in prompting evolutionary change. Much more seriously, one often encounters descriptions of natural selection as a processes that “chooses” among “preferred” variants or “experiments with” or “explores” different options. Some expressions, such as “favored” and “selected for” are used commonly as shorthand in evolutionary biology and are not meant to impart consciousness to natural selection; however, these too may be misinterpreted in the vernacular sense by non-experts and must be clarified.

Darwin ( 1859 ) himself could not resist slipping into the language of agency at times:

It may be said that natural selection is daily and hourly scrutinizing, throughout the world, every variation, even the slightest; rejecting that which is bad, preserving and adding up all that is good; silently and insensibly working, whenever and wherever opportunity offers, at the improvement of each organic being in relation to its organic and inorganic conditions of life. We see nothing of these slow changes in progress, until the hand of time has marked the long lapse of ages, and then so imperfect is our view into long past geological ages, that we only see that the forms of life are now different from what they formerly were.

Perhaps recognizing the ease with which such language can be misconstrued, Darwin ( 1868 ) later wrote that “The term ‘Natural Selection’ is in some respects a bad one, as it seems to imply conscious choice; but this will be disregarded after a little familiarity.” Unfortunately, more than “a little familiarity” seems necessary to abandon the notion of Nature as an active decision maker.

Being, as it is, the simple outcome of differences in reproductive success due to heritable traits, natural selection cannot have plans, goals, or intentions, nor can it cause changes in response to need. For this reason, Jungwirth ( 1975a , b , 1977 ) bemoaned the tendency for authors and instructors to invoke teleological and anthropomorphic descriptions of the process and argued that this served to reinforce misconceptions among students (see also Bishop and Anderson 1990 ; Alters and Nelson 2002 ; Moore et al. 2002 ; Sinatra et al. 2008 ). That said, a study of high school students by Tamir and Zohar ( 1991 ) suggested that older students can recognize the distinction between an anthropomorphic or teleological formulation (i.e., merely a convenient description) versus an anthropomorphic/teleological explanation (i.e., involving conscious intent or goal-oriented mechanisms as causal factors; see also Bartov 1978 , 1981 ). Moore et al. ( 2002 ), by contrast, concluded from their study of undergraduates that “students fail to distinguish between the relatively concrete register of genetics and the more figurative language of the specialist shorthand needed to condense the long view of evolutionary processes” (see also Jungwirth 1975a , 1977 ). Some authors have argued that teleological wording can have some value as shorthand for describing complex phenomena in a simple way precisely because it corresponds to normal thinking patterns, and that contrasting this explicitly with accurate language can be a useful exercise during instruction (Zohar and Ginossar 1998 ). In any case, biologists and instructors should be cognizant of the risk that linguistic shortcuts may send students off track.

Source Versus Sorting of Variation

Intuitive models of evolution based on soft inheritance are one-step models of adaptation: Traits are modified in one generation and appear in their altered form in the next. This is in conflict with the actual two-step process of adaptation involving the independent processes of mutation and natural selection. Unfortunately, many students who eschew soft inheritance nevertheless fail to distinguish natural selection from the origin of new variation (e.g., Greene 1990 ; Creedy 1993 ; Moore et al. 2002 ). Whereas an accurate understanding recognizes that most new mutations are neutral or harmful in a given environment, such naïve interpretations assume that mutations occur as a response to environmental challenges and therefore are always beneficial (Fig.  2 ). For example, many students may believe that exposure to antibiotics directly causes bacteria to become resistant, rather than simply changing the relative frequencies of resistant versus non-resistant individuals by killing off the latter Footnote 13 . Again, natural selection itself does not create new variation, it merely influences the proportion of existing variants. Most forms of selection reduce the amount of genetic variation within populations, which may be counteracted by the continual emergence of new variation via undirected mutation and recombination.

Typological, Essentialist, and Transformationist Thinking

Misunderstandings about how variation arises are problematic, but a common failure to recognize that it plays a role at all represents an even a deeper concern. Since Darwin ( 1859 ), evolutionary theory has been based strongly on “population” thinking that emphasizes differences among individuals. By contrast, many naïve interpretations of evolution remain rooted in the “typological” or “essentialist” thinking that has existed since the ancient Greeks (Mayr 1982 , 2001 ; Sinatra et al. 2008 ). In this case, species are conceived of as exhibiting a single “type” or a common “essence,” with variation among individuals representing anomalous and largely unimportant deviations from the type or essence. As Shtulman ( 2006 ) notes, “human beings tend to essentialize biological kinds and essentialism is incompatible with natural selection.” As with many other conceptual biases, the tendency to essentialize seems to arise early in childhood and remains the default for most individuals (Strevens 2000 ; Gelman 2004 ; Evans et al. 2005 ; Shtulman 2006 ).

The incorrect belief that species are uniform leads to “transformationist” views of adaptation in which an entire population transforms as a whole as it adapts (Alters 2005 ; Shtulman 2006 ; Bardapurkar 2008 ). This contrasts with the correct, “variational” understanding of natural selection in which it is the proportion of traits within populations that changes (Fig.  2 ). Not surprisingly, transformationist models of adaptation usually include a tacit assumption of soft inheritance and one-step change in response to challenges. Indeed, Shtulman ( 2006 ) found that transformationists appeal to “need” as a cause of evolutionary change three times more often than do variationists.

Events and Absolutes Versus Processes and Probabilities

A proper understanding of natural selection recognizes it as a process that occurs within populations over the course of many generations. It does so through cumulative, statistical effects on the proportion of traits differing in their consequences for reproductive success. This contrasts with two major errors that are commonly incorporated into naïve conceptions of the process:

Natural selection is mistakenly seen as an event rather than as a process (Ferrari and Chi 1998 ; Sinatra et al. 2008 ). Events generally have a beginning and end, occur in a specific sequential order, consist of distinct actions, and may be goal-oriented. By contrast, natural selection actually occurs continually and simultaneously within entire populations and is not goal-oriented (Ferrari and Chi 1998 ). Misconstruing selection as an event may contribute to transformationist thinking as adaptive changes are thought to occur in the entire population simultaneously. Viewing natural selection as a single event can also lead to incorrect “saltationist” assumptions in which complex adaptive features are imagined to appear suddenly in a single generation (see Gregory 2008b for an overview of the evolution of complex organs).

Natural selection is incorrectly conceived as being “all or nothing,” with all unfit individuals dying and all fit individuals surviving. In actuality, it is a probabilistic process in which some traits make it more likely—but do not guarantee—that organisms possessing them will successfully reproduce. Moreover, the statistical nature of the process is such that even a small difference in reproductive success (say, 1%) is enough to produce a gradual increase in the frequency of a trait over many generations.

Concluding Remarks

Surveys of students at all levels paint a bleak picture regarding the level of understanding of natural selection. Though it is based on well-established and individually straightforward components, a proper grasp of the mechanism and its implications remains very rare among non-specialists. The unavoidable conclusion is that the vast majority of individuals, including most with postsecondary education in science, lack a basic understanding of how adaptive evolution occurs.

While no concrete solutions to this problem have yet been found, it is evident that simply outlining the various components of natural selection rarely imparts an understanding of the process to students. Various alternative teaching strategies and activities have been suggested, and some do help to improve the level of understanding among students (e.g., Bishop and Anderson 1986 ; Jensen and Finley 1995 , 1996 ; Firenze 1997 ; Passmore and Stewart 2002 ; Sundberg 2003 ; Alters 2005 ; Scharmann 1990 ; Wilson 2005 ; Nelson 2007 , 2008 ; Pennock 2007 ; Kampourakis and Zogza 2008 ). Efforts to integrate evolution throughout biology curricula rather than segregating it into a single unit may also prove more effective (Nehm et al. 2009 ), as may steps taken to make evolution relevant to everyday concerns (e.g., Hillis 2007 ).

At the very least, it is abundantly clear that teaching and learning natural selection must include efforts to identify, confront, and supplant misconceptions. Most of these derive from deeply held conceptual biases that may have been present since childhood. Natural selection, like most complex scientific theories, runs counter to common experience and therefore competes—usually unsuccessfully—with intuitive ideas about inheritance, variation, function, intentionality, and probability. The tendency, both outside and within academic settings, to use inaccurate language to describe evolutionary phenomena probably serves to reinforce these problems.

Natural selection is a central component of modern evolutionary theory, which in turn is the unifying theme of all biology. Without a grasp of this process and its consequences, it is simply impossible to understand, even in basic terms, how and why life has become so marvelously diverse. The enormous challenge faced by biologists and educators in correcting the widespread misunderstanding of natural selection is matched only by the importance of the task.

For a more advanced treatment, see Bell ( 1997 , 2008 ) or consult any of the major undergraduate-level evolutionary biology or population genetics textbooks.

The Origin was, in Darwin's words, an “abstract” of a much larger work he had initially intended to write. Much of the additional material is available in Darwin ( 1868 ) and Stauffer ( 1975 ).

See Gregory ( 2008a ) for a discussion regarding the use of the term “theory” in science.

Ridley ( 2004 ) points out that Darwin's calculations require overlapping generations to reach this exact number, but the point remains that even in slow-reproducing species the rate of potential production is enormous relative to actual numbers of organisms.

Humans are currently undergoing a rapid population expansion, but this is the exception rather than the rule. As Darwin ( 1859 ) noted, “Although some species may now be increasing, more or less rapidly, in numbers, all cannot do so, for the world would not hold them.”

It cannot be overemphasized that “evolution” and “natural selection” are not interchangeable. This is because not all evolution occurs by natural selection and because not all outcomes of natural selection involve changes in the genetic makeup of populations. A detailed discussion of the different types of selection is beyond the scope of this article, but it can be pointed out that the effect of “stabilizing selection” is to prevent directional change in populations.

Instructors interested in assessing their own students' level of understanding may wish to consult tests developed by Bishop and Anderson ( 1986 ), Anderson et al. ( 2002 ), Beardsley ( 2004 ), Shtulman ( 2006 ), or Kampourakis and Zogza ( 2009 ).

Even more alarming is a recent indication that one in six teachers in the USA is a young Earth creationist, and that about one in eight teaches creationism as though it were a valid alternative to evolutionary science (Berkman et al. 2008 ).

Strictly speaking, it is not necessary to understand how evolution occurs to be convinced that it has occurred because the historical fact of evolution is supported by many convergent lines of evidence that are independent of discussions about particular mechanisms. Again, this represents the important distinction between evolution as fact and theory. See Gregory ( 2008a ).

http://www3.niaid.nih.gov/topics/antimicrobialResistance/Understanding/history.htm , accessed February 2009.

One should always be wary of the linguistic symptoms of anthropomorphic misconceptions, which usually include phrasing like “so that” (versus “because”) or “in order to” (versus “happened to”) when explaining adaptations (Kampourakis and Zogza 2009 ).

It must be noted that the persistent tendency to label the inheritance of acquired characteristics as “Lamarckian” is false: Soft inheritance was commonly accepted long before Lamarck's time (Zirkle 1946 ). Likewise, mechanisms involving organisms' conscious desires to change are often incorrectly attributed to Lamarck. For recent critiques of the tendency to describe various misconceptions as Lamarckian, see Geraedts and Boersma ( 2006 ) and Kampourakis and Zogza ( 2007 ). It is unfortunate that these mistakenly attributed concepts serve as the primary legacy of Lamarck, who in actuality made several important contributions to biology (a term first used by Lamarck), including greatly advancing the classification of invertebrates (another term he coined) and, of course, developing the first (albeit ultimately incorrect) mechanistic theory of evolution. For discussions of Lamarck's views and contributions to evolutionary biology, see Packard ( 1901 ), Burkhardt ( 1972 , 1995 ), Corsi ( 1988 ), Humphreys ( 1995 , 1996 ), and Kampourakis and Zogza ( 2007 ). Lamarck's works are available online at http://www.lamarck.cnrs.fr/index.php?lang=en .

One may wonder how this misconception is reconciled with the common admonition by medical doctors to complete each course of treatment with antibiotics even after symptoms disappear—would this not provide more opportunities for bacteria to “develop” resistance by prolonging exposure?

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24 Charles Darwin and Natural Selection

In the mid-nineteenth century, two naturalists, Charles Darwin and Alfred Russel Wallace, independently conceived and described the actual mechanism for evolution. Importantly, each naturalist spent time exploring the natural world on expeditions to the tropics. From 1831 to 1836, Darwin traveled around the world on H.M.S. Beagle , including stops in South America, Australia, and the southern tip of Africa. Wallace traveled to Brazil to collect insects in the Amazon rainforest from 1848 to 1852 and to the Malay Archipelago from 1854 to 1862. Darwin’s journey, like Wallace’s later journeys to the Malay Archipelago, included stops at several island chains, the last being the Galápagos Islands west of Ecuador. On these islands, Darwin observed species of organisms on different islands that were clearly similar, yet had distinct differences. For example, the ground finches inhabiting the Galápagos Islands comprised several species with a unique beak shape (Figure 1).

The species on the islands had a graded series of beak sizes and shapes with very small differences between the most similar. He observed that these finches closely resembled another finch species on the South American mainland. Darwin imagined that the island species might be species modified from one of the original mainland species. Upon further study, he realized that each finch’s varied beaks helped the birds acquire a specific type of food. For example, seed-eating finches had stronger, thicker beaks for breaking seeds, and insect-eating finches had spear-like beaks for stabbing their prey.

Wallace and Darwin both observed similar patterns in other organisms and they independently developed the same explanation for how and why such changes could take place. Darwin called this mechanism natural selection. Natural selection , or “survival of the fittest,” is the more prolific reproduction of individuals with favorable traits that survive environmental change because of those traits. This leads to evolutionary change.

For example, Darwin observed a population of giant tortoises in the Galápagos Archipelago to have longer necks than those that lived on other islands with dry lowlands. These tortoises were “selected” because they could reach more leaves and access more food than those with short necks. In times of drought when fewer leaves would be available, those that could reach more leaves had a better chance to eat and survive than those that couldn’t reach the food source. Consequently, long-necked tortoises would be more likely to be reproductively successful and pass the long-necked trait to their offspring. Over time, only long-necked tortoises would be present in the population.

Natural selection, Darwin argued, was an inevitable outcome of three principles that operated in nature. First, most characteristics of organisms are inherited, or passed from parent to offspring. Although no one, including Darwin and Wallace, knew how this happened at the time, it was a common understanding. Second, more offspring are produced than are able to survive, so resources for survival and reproduction are limited. The capacity for reproduction in all organisms outstrips the availability of resources to support their numbers. Thus, there is competition for those resources in each generation. Both Darwin and Wallace’s understanding of this principle came from reading economist Thomas Malthus’ essay that explained this principle in relation to human populations. Third, offspring vary among each other in regard to their characteristics and those variations are inherited. Darwin and Wallace reasoned that offspring with inherited characteristics which allow them to best compete for limited resources will survive and have more offspring than those individuals with variations that are less able to compete. Because characteristics are inherited, these traits will be better represented in the next generation. This will lead to change in populations over generations in a process that Darwin called descent with modification. Ultimately, natural selection leads to greater adaptation of the population to its local environment. It is the only mechanism known for adaptive evolution.

In 1858, Darwin and Wallace (Figure 2) presented papers at the Linnean Society in London that discussed the idea of natural selection. The following year Darwin’s book, On the Origin of Species, was published. His book outlined in considerable detail his arguments for evolution by natural selection.

It is difficult and time-consuming to document and present examples of evolution by natural selection. The Galápagos finches are an excellent example. Peter and Rosemary Grant and their colleagues have studied Galápagos finch populations every year since 1976 and have provided important evidence of natural selection. The Grants found changes from one generation to the next in beak shape distribution with the medium ground finch on the Galápagos island of Daphne Major. The birds have inherited a variation in their bill shape with some having wide deep bills and others having thinner bills. During a period in which rainfall was higher than normal because of an El Niño, there was a lack of large hard seeds of which the large-billed birds ate; however, there was an abundance of the small soft seeds which the small-billed birds ate. Therefore, the small-billed birds were able to survive and reproduce. In the years following this El Niño, the Grants measured beak sizes in the population and found that the average bill size was smaller. Since bill size is an inherited trait, parents with smaller bills had more offspring and the bill evolved into a much smaller size. As conditions improved in 1987 and larger seeds became more available, the trend toward smaller average bill size ceased.

CAREER CONNECTION

Field biologist.

Many people hike, explore caves, scuba dive, or climb mountains for recreation. People often participate in these activities hoping to see wildlife. Experiencing the outdoors can be incredibly enjoyable and invigorating. What if your job entailed working in the wilderness? Field biologists by definition work outdoors in the “field.” The term field in this case refers to any location outdoors, even under water. A field biologist typically focuses research on a certain species, group of organisms, or a single habitat (Figure 3).

One objective of many field biologists includes discovering new, unrecorded species. Not only do such findings expand our understanding of the natural world, but they also lead to important innovations in fields such as medicine and agriculture. Plant and microbial species, in particular, can reveal new medicinal and nutritive knowledge. Other organisms can play key roles in ecosystems or if rare require protection. When discovered, researchers can use these important species as evidence for environmental regulations and laws.

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Natural Selection: Definition, Darwin's Theory, Examples & Facts

The concept of natural selection was first proposed formally at a biology conference of the Linnean Society. On July 1, 1858, a joint paper on the subject was presented and subsequently published. It included contributions from Charles Darwin and Alfred Russel Wallace .

Both men wrote about the idea that natural selection contributed to earth's evolution through the survival of organisms most suited to their environment . Scientists at the time realized that evolution took place but did not know how species evolved.

After this introduction of natural selection, Darwin elaborated on the subject with his theory of evolution and his book, On the Origin of Species , published in 1859. His work with Darwin's finches and his ideas on survival of the fittest explained the mechanism of natural selection and how it could lead to a proliferation of many different kinds of organisms.

Natural Selection Definition

Evolution is the cumulative change in the characteristics of an organism or a population over the next generations. It is sometimes summarized as descent with modification . Natural selection is one of the mechanisms that drives evolution.

To be an active characteristic or trait causing natural selection to take place, the trait has to have the following features:

• Heritability.  A trait can only influence evolution through natural selection if it is passed on from parents to descendants.
• Functionality.  The trait must have a function. Traits must do something for natural selection to take place.
• Advantage.  To be selected for passing on to descendants, the trait must confer an advantage on the organism that has it, or make the organism more fit for survival in its environment.
• Origin.  The trait must have caused the organisms to evolve because it made the organisms that had it more fit for survival. If the organisms changed due to another mechanism, such as genetic mutation, it was not due to natural selection.

Natural Selection and Darwin's Theory of Evolution

Based on the fossil record, it is clear that species change over time and new species develop while others die off. Before Darwin, there was no explanation of how such changes could take place.

The theory of evolution describes what happens as the characteristics of some individuals of a species become predominant and natural selection describes how this predominance comes about.

Darwin studied natural selection in finches. Even when another mechanism such as mutation changes a population, if the mutation does not confer a natural advantage, it may die out due to natural selection.

How Natural Selection Works

Within a species, a typical population includes individuals with varying traits because they receive half their genetic code from the father and half from the mother. For traits with a genetic basis, this combination of genes from parents results in a wide variety of characteristics in the individuals of the population.

The combination of traits in some individuals gives them an advantage in looking for food, reproducing or withstanding predators or disease. Other individuals receive traits that place them at a disadvantage.

The advantaged individuals will live longer and produce more descendants. Their descendants will mostly receive genes that result in the advantaged traits. Over time, most of the population will evolve with the advantaged traits, and the traits giving a disadvantage will disappear. Natural selection has selected the individuals with positive characteristics .

Darwin's Voyage on the Beagle

In 1831, the British navy sent survey vessel the HMS Beagle on a mapping expedition around the world. Charles Darwin came on board as the naturalist assigned to observe local fauna and flora. The expedition took five years and spent a lot of time along the Atlantic and Pacific coasts of South America.

Upon leaving South America for the Pacific crossing to New Zealand, the ship spent five weeks exploring the Galapagos Islands. As he did everywhere, Darwin took extensive notes about the characteristics of the plants and animals he found. Eventually these notes would form the basis for his development of the concept of natural selection and his theory of evolution.

Darwin's Finches Demonstrated Survival of the Fittest

Back in England, Darwin and an ornithologist associate examined Darwin's notes on the finches of the Galapagos Islands. Apparently the islands were home to 13 different species of finches while the nearest South American land mass 600 miles away had only one species. The main difference between the species was the size and shape of the beaks .

Darwin's analysis of his notes led him to draw the following conclusions:

• The finches had different beaks because they lived on different islands in different environments .
• The environment did not cause the differences in beaks because there was no mechanism for such an influence.
• The different beak characteristics must have all been present in the original finch population.
• As the finches from the original population settled on an island, the finches with the beaks best adapted to the local food supply would have an advantage.
• The finches with beaks best suited to the food source on their island would survive in greater numbers than the less adapted finches.
• Eventually, over many generations, the finches on an island would form a distinct species with a distinct beak size and shape because finches with those beaks would be the fittest for their environment.

With these conclusions, Darwin explained the evolution of the finch beaks in the Galapagos Islands by proposing the mechanism of natural selection . He summarized this mechanism as survival of the fittest , where fitness was defined as reproductive success .

Darwin's Work Relied on Three Observations

For his conclusions, Darwin relied on his notes, his own observations and his interpretation of the writings of Thomas Robert Malthus . Malthus was an English scholar who, in 1798, published his theory that population growth will always outpace the food supply. The corollary is that, in any population, many individuals will die off due to competition for a limited supply of food.

The three observations that allowed Darwin to develop his theory of evolution and natural selection were:

• The individuals in a population display a variation in traits such as color, behavior, size and shape due to genetic variation.
• Some of the traits are passed down from parents to descendants and are heritable.
• The parents in a population overproduce offspring so that some will not survive.

Based on these observations, Darwin proposed that those individuals with traits that made them fitter would be the ones to survive while the least fit would die off. Over time, the population would be dominated by individual with the traits that made them fitter.

Natural Selection Examples: Bacteria

Populations of bacteria exhibit very strong natural selection because they can multiply rapidly. They usually multiply until they reach a constraint such as lack of food, space or other resources. At that point, those bacteria best suited to their environment will survive while the rest will die off.

One example of natural selection in bacteria is the development of antibiotic resistance . When bacteria cause an infection and the individual is treated with antibiotics, any bacteria that have the antibiotic-resistance trait will survive while all others will die off. The proliferation of antibiotic-resistant bacteria is a major medical problem.

Natural Selection Examples: Plants

Plants evolve to become suited to their environment through natural selection. Some plants evolve flower colors to attract pollinators of a specific kind and develop special mechanisms to spread their seeds. They have to adapt to more or less sunlight and fight off pests.

Cacti are an example of natural selection in plants. In the desert where they live, there is lots of sunlight, little water and occasionally an animal that would love a juicy bite.

As a result, cacti have developed compact bodies or small, succulent leaves with thick skins to protect against the strong sun and minimize water loss. They can also store water and have sharp spikes to discourage animals. The cacti with these traits were the fittest, and they are still evolving.

Another example is the change in the field mustard plant caused by the drought in Southern California. To survive a drought, plants must grow, flower and distribute their seeds quickly. The Southern California field mustard plants that flowered early became dominant while those flowering later died out.

Natural Selection in Animals

Animals have more scope for influencing their survival because they can engage in complex behavior patterns. Traits that can determine fitness fall under three main categories. The ability to find enough food through hunting or foraging is a key for survival.

Most animals have predators , and specific traits allow them to avoid being eaten. Finally, the ability to find and attract a mate allows them to pass their positive traits on to offspring.

Typical characteristics that influence natural selection include:

• Movement.  The ability to run, swim or fly fast determines whether an animal can hunt successfully or escape predators.
• Camouflage.  If an animal can hide successfully, it can evade predators or ambush prey.
• Immunity.  Some animals will be more resistant to a disease than others and will survive.
• Strength.  Competing for a mate often involves tests of strength with other members of the same species.
• Senses.  Animals that can see, smell or hear better may have a better chance of survival.
• Sexual characteristics.  Natural selection in animals depends on successful reproduction after attracting a mate.

Animals evolve continuously, first to better adapt to a given environment and then, if the environment changes, to the new environment. Natural selection can cause evolutionary changes in existing populations and can also favor one species over another if two species are competing for the same space and resources.

Natural Selection Examples: Animals

Natural selection in animals is best seen when the environment changes in some way, and animals with specific characteristics become better suited and soon become dominant.

For example, the peppered moth in London was light-colored with dark spots. During the industrial revolution, buildings became darkened with soot. Birds could easily see the light-colored moths against the dark background, and soon only dark-colored moths were left. Natural selection favored the moths with more and larger dark spots.

In another example, say some insects become resistant to a chemical pesticide very quickly. Even if only a few individuals are resistant, the rest will die off, and the resistant insects will survive. Insects typically produce large numbers of offspring, so the insects with the resistant genes will rapidly take over.

In an example of reproductive preference, female peacocks choose mates based on the size and brightness of their tails. After the effects of natural selection , almost all peacock males today have large, brightly colored tails.

While Darwin is best known for his publications on the theory of evolution, it is natural selection that powers change and adaptation in species. Charles Darwin's 1858 paper, with contributions from Alfred Russel Wallace whose paper was published at the same time, forever changed how people viewed evolution and the natural changes in plants and animals that continuously took place around them.

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Bert Markgraf is a freelance writer with a strong science and engineering background. He has written for scientific publications such as the HVDC Newsletter and the Energy and Automation Journal. Online he has written extensively on science-related topics in math, physics, chemistry and biology and has been published on sites such as Digital Landing and Reference.com He holds a Bachelor of Science degree from McGill University.

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Definition of Natural Selection

Simple illustrations from the world of plants and animals, simple illustrations from the world of people.

Charles Darwin revolutionized scientific world in the middle of the nineteenth century. He cast doubt on the idea of divine origins of the world. He also stated that people were nothing more than animals that managed to adapt better than the rest of species. Of course, his theory was like a bomb.

However, in the course of time people accepted this bold theory. Some people still do not understand why people did agree with Darwin’s assumptions. The answer to this question is on the surface. Darwin managed to provide really simple and meaningful examples that illustrated his theory.

His “Natural Selection” is one of those works that convinced people of his being right. Darwin provides a brief but comprehensive analysis of his theory in this work. His theory of natural selection is easily proved as the scientist provides examples which can be understood by everyone. Therefore, Darwin’s theory of natural selection was soon accepted by people as it was well-grounded and it was perfectly explained in simple terms by the scientist.

In the first place, Charles Darwin (2011) provides a very simple definition for his term: “This preservation of favourable variations and the rejection of injurious variations, I call Natural Selection” (p. 81). The definition is really brief but comprehensive. The researcher manages to reveal the essence of his theory in a few words.

Thus, natural selection is based on two notions: preservation of important features and elimination of unfavorable ones. This explanation speaks to everyone as people often choose objects that have certain ‘favorable’ features, and do not choose things which have some unnecessary characteristics.

This comprehensive definition makes the theory more credible. Admittedly, people tend to believe in things which can be explained in simple terms. Unlike some scientists who use difficult terms and obscure explanations, Darwin gives a transparent definition for his term. He shows that natural selection is something really easy and logical. He shows that it is the fact that people have to acknowledge.

Apart from the simple definition, Darwin provides simple examples to prove his theory. For instance, the scientist mentions that “man can produce and certainly has produced a great result by his methodical and unconscious means of selection” (Darwin, 2011, p. 82).

Therefore, the scientist addresses all those who have known that people do resort to selection in farming. For instance, farmers have always tried to notice some favorable features in some plants. Those plants have been used for sowing. Thus, farmers expect that they can soon get plants which have the necessary features.

The scientist mentions that people often try to breed cattle as well. However, the author also states that the man “begins his selection by some half-monstrous form; or at least by some modification prominent to catch his eye” (Darwin, 2011, p. 82). Darwin states that nature is more precise and thoughtful. The scientist points out that various external factors influence the development of species to make them fit the world around them.

Furthermore, the researcher also provides a simple example from the world of people. Thus, Darwin (2011) notes that

all the inhabitants of each country are struggling together with nicely balanced forces, extremely slight modification in the structure or habits of one inhabitant would often give it an advantage over others. (p. 82)

This is a very precise illustration of natural selection manifested in human societies. Admittedly, people have learnt a lot about various countries and societies. There have been many examples of changes in countries which took place after some external factors (e.g. climate) had changed.

Many people had to leave their homeland because they could not adapt to new conditions. On the contrary, the area could be invaded by those who were accustomed to such conditions. Admittedly, when some people acquired certain features favorable for living in certain areas, these people had an advantage over others.

To sum up, Darwin’s “Natural Selection” is a brief but precise explanation of one of the major principles of the theory of evolution. The scientist introduces a new term (Natural Selection) and explains it in simple words. Darwin provides illustrations which are easy to understand.

Thus, people accepted the theory because they already had certain experience in selection and breeding. People noticed that they could modify certain species to fit certain purposes. Therefore, it is but natural that every species in this world is subjected to some kind of selection. People modify some species to make them fit human’s needs. Likewise, people are modified to fit various external factors like climate.

Darwin did change the world as he made people understand that everything in this world is undergoing continuous change. The greatest achievement of the scientist is that he managed to explain his revolutionary theory in simple terms so that people could understand and accept it. Nowadays many researchers lack this ability to explain their ideas in simple words.

Darwin, C. (2011). Natural selection. In R. DiYanni (Ed.), Fifty great essays (pp. 80-91). New York, NY: Longman.

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Natural Selection

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Differences in the propagation of genes in a population as a result of survival and reproduction of organisms carrying those genes.

Introduction

Charles Darwin described natural selection in private essays in 1842 and especially 1844 (Darwin 1909 ; Glik and Kohn 1996 , pp. 90–96). He then drafted a large manuscript on natural selection, which he left unfinished. It was subsequently overlooked until recently (Darwin 1975 ). Eventually in 1858, the Linnean Society published a version of his 1844 essay in conjunction with a communication from Alfred Russell Wallace. Wallace presented some related ideas, but not natural selection as we now understand it (Bulmer 2005 ). Soon afterward there appeared On the Origin of Species / By Means of Natural Selection, / or the / Preservation of Favoured Races in the Struggle for Life (Darwin 1859 ), which developed the concept in detail.

Introducing natural selection on the first few pages, Darwin emphasized the importance of variation among...

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Wiley, R.H. (2021). Natural Selection. In: Shackelford, T.K., Weekes-Shackelford, V.A. (eds) Encyclopedia of Evolutionary Psychological Science. Springer, Cham. https://doi.org/10.1007/978-3-319-19650-3_2095

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UC MUSEUM OF PALEONTOLOGY

Understanding Evolution

Your one-stop source for information on evolution

The History of Evolutionary Thought

Natural selection: charles darwin & alfred russel wallace.

Pre-Darwinian ideas about evolution

It was Darwin’s genius both to show how all this evidence favored the evolution of species from a common ancestor and to offer a plausible mechanism by which life might evolve. Lamarck and others had promoted evolutionary theories, but in order to explain just how life changed, they depended on speculation. Typically, they claimed that evolution was guided by some long-term trend. Lamarck, for example, thought that life strove over time to rise from simple single-celled forms to complex ones. Many German biologists conceived of life evolving according to predetermined rules, in the same way an embryo develops in the womb. But in the mid-1800s, Darwin and the British biologist Alfred Russel Wallace independently conceived of a natural, even observable, way for life to change: a process Darwin called  natural selection.

The pressure of population growth

Interestingly, Darwin and Wallace found their inspiration in economics. An English parson named  Thomas Malthus  published a book in 1797 called  Essay on the Principle of Population  in which he warned his fellow Englishmen that most policies designed to help the poor were doomed because of the relentless pressure of population growth. A nation could easily double its population in a few decades, leading to famine and misery for all.

When Darwin and Wallace read Malthus, it occurred to both of them that animals and plants should also be experiencing the same population pressure. It should take very little time for the world to be knee-deep in beetles or earthworms. But the world is not overrun with them, or any other species, because they cannot reproduce to their full potential. Many die before they become adults. They are vulnerable to droughts and cold winters and other environmental assaults. And their food supply, like that of a nation, is not infinite. Individuals must compete, albeit unconsciously, for what little food there is.

Selection of traits

In this struggle for existence, survival and reproduction do not come down to pure chance. Darwin and Wallace both realized that if an animal has some trait that helps it to withstand the elements or to breed more successfully, it may leave more offspring behind than others. On average, the trait will become more common in the following generation, and the generation after that.

As Darwin wrestled with  natural selection  he spent a great deal of time with pigeon breeders, learning their methods. He found their work to be an analogy for evolution. A pigeon breeder selected individual birds to reproduce in order to produce a neck ruffle. Similarly, nature unconsciously “selects” individuals better suited to surviving their local conditions. Given enough time, Darwin and Wallace argued, natural selection might produce new types of body parts, from wings to eyes.

Darwin and Wallace develop similar theory

Darwin began formulating his theory of natural selection in the late 1830s but he went on working quietly on it for twenty years. He wanted to amass a wealth of evidence before publicly presenting his idea. During those years he corresponded briefly with Wallace (right), who was exploring the wildlife of South America and Asia. Wallace supplied Darwin with birds for his studies and decided to seek Darwin’s help in publishing his own ideas on evolution. He sent Darwin his theory in 1858, which, to Darwin’s shock, nearly replicated Darwin’s own.

Charles Lyell  and Joseph Dalton Hooker arranged for both Darwin’s and Wallace’s theories to be presented to a meeting of the Linnaean Society in 1858. Darwin had been working on a major book on evolution and used that to develop  On the Origins of Species , which was published in 1859. Wallace, on the other hand, continued his travels and focused his study on the importance of biogeography.

The book was not only a best seller but also one of the most influential scientific books of all time. Yet it took time for its full argument to take hold. Within a few decades, most scientists accepted that evolution and the descent of species from common ancestors were real. But natural selection had a harder time finding acceptance. In the late 1800s many scientists who called themselves Darwinists actually preferred a Lamarckian explanation for the way life changed over time. It would take the discovery of  genes  and  mutations  in the twentieth century to make natural selection not just attractive as an explanation, but unavoidable.

• More Details
• Read more about  the process of natural selection  in Evolution 101.
• Go right to the source and read Darwin's  On the Origin of Species by Means of Natural Selection .
• Explore the  American Museum of Natural History's Darwin exhibit  to learn more about his life and how his ideas transformed our understanding of the living world.

Discrete Genes Are Inherited: Gregor Mendel

Early Evolution and Development: Ernst Haeckel

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Natural Selection

Darwin's theory of evolution by natural selection provided the first, and only, causal-mechanistic account of the existence of adaptations in nature. As such, it provided the first, and only, scientific alternative to the “argument from design”. That alone would account for its philosophical significance. But the theory also raises other philosophical questions not encountered in the study of the theories of physics. Unfortunately the concept of natural selection is intimately intertwined with the other basic concepts of evolutionary theory—such as the concepts of fitness and adaptation—that are themselves philosophically controversial. Fortunately we can make considerable headway in getting clear on natural selection without solving all of those outstanding problems.

1. Natural Selection and Evolutionary Theory

2. natural selection and fitness, 3. common selective environments, 4. does natural selection require differential reproduction, 5. does natural selection require heritable variation, 6. natural selection and drift, 7. bibliography, other internet resources, related entries.

The theory of evolution by natural selection forms a central part of modern evolutionary theory. There is some controversy among biologists as to just how important natural selection is compared to other processes producing evolutionary change, but there is no controversy over the proposition that natural selection is important. Some good might come of the efforts to produce a general selection theory that would include the natural selection that occurs as a part of the evolutionary process as a special case (e.g. Hull 2001), but here the focus will be solely on the evolutionary process.

Biology starts when reproduction begins. Stars may be said to evolve, but they do not reproduce and so biological theory does not apply to them. Biological evolution requires reproducing entities that form lineages. It is these lineages that evolve. Thus it is only within such lineages that we will properly apply the term natural selection. A kindergarten class may certainly select among different colored candies, but since those candies are not part of self-reproducing lineages, we should not confuse this selection process with natural selection.

Natural selection is a causal process. Distinguishing it from other processes in evolution is one of major conceptual and empirical problems of evolutionary biology. The bare bones of Darwin's theory of evolution by natural selection are elegantly simple. Typically (but not necessarily) there is variation among organisms within a reproducing population. Oftentimes (but not always) this variation is (to some degree) heritable . When this variation is causally connected to differential ability to survive and reproduce, differential reproduction will probably ensue. This last claim is one way of stating the Principle of Natural Selection (from here on PNS). The PNS goes beyond the causally neutral statement that is sometimes listed as the third of what are often called “Darwin's Three Conditions”, viz., different variants sometimes reproduce at different rates. That statement leaves open the question of whether or not the variation in question is causally responsible for the differential reproduction. It leaves open the question of whether a qualitatively similar outcome would result from repeated iterations of this set-up. It leaves open the question of whether this process is natural selection or drift (see below). It—the causally neutral statement—does not suffice to state Darwin's causal theory. Darwin clearly recognized this (see, for example 1871) as did Lewontin (1978); although many contemporary commentators fail to see this.

Why is it that some variants leave more offspring than others? In those cases we label natural selection, it is because those variants are better adapted , or are fitter than their competitors. Thus we can define natural selection as follows: Natural selection is differential reproduction due to differential fitness (or differential adaptedness) within a common selective environment (see next section). This definition makes the concept of natural selection dependent on that of fitness, which is unfortunate since many philosophers find the concept of fitness deeply mysterious (see e.g., Ariew and Lewontin 2004). But like it or not, that is the way the theory is structured. And, fortunately, we can make considerable headway in understanding natural selection without solving all of the philosophical problems surrounding the concept of fitness.

As a causal theory natural selection locates the causally relevant differences that lead to differential reproduction. These differences are differences in organisms' fitness to their environment. Or, more fully, they are differences in various organismic capacities to survive and reproduce in their environment. When these differences in capacities are heritable, then evolution will (usually) ensue. This sort of case must be carefully distinguished from cases where the causes of differential reproduction are not located in the organisms, but rather in their different environments. Let us make this distinction more concrete. Imagine two genotypes of an annual plant that grows in dense patches. Sunlight is at a premium and taller plants shade shorter plants thus garnering more energy for growth and reproduction. One genotype, G 1 , grows more quickly at germination than the other, G 2 . Thus G 1 s are initially taller than G 2 s and this difference persists throughout the growing season due to G 1 s increased energy stores. The consequence of this is that G 1 s produce more flowers, more pollen and more seeds than G 2 s. This is natural selection at work.

In contrast, imagine two genotypes of the same species, G 3 and G 4 that do not differ in germination date, growth rate, resource allocation between growth and reproduction or any other factor relevant to reproduction when grown in a common environment. Now imagine that seeds of these two genotypes are distributed randomly across a patchwork of two quite different soil types, call them E 1 and E 2. E 1 and E 2 are identical except that E 1 contains high levels of lead whereas E 2 does not. Lead dramatically and equally reduces growth and reproduction in both G 3 and G 4 . Finally imagine that this random distribution process results in a correlation between genotype and environment—i.e., G 3 s are disproportionately found in E 1. In consequence of all of this, G 4 s produce more flowers, more pollen and more seeds than G 3 s. But natural selection is not occurring here. (Indeed this is a type of drift, see below.)

In both cases differential reproduction occurs, and in both cases I have already sketched causal explanations of this. In neither case is differential reproduction mysterious (although chance does play a role in the second explanation, but not the first). But only the first case could result in adaptive evolution. (See Brandon 1990, chapter 2 for fuller discussion.) Biologists have long recognized the importance of the difference discussed above, thus the importance of so-called “common garden” experiments in experimental evolutionary genetics. (In common garden experiments, the environments in which different genotypes are placed are controlled as far as possible. Furthermore, when environmental control is imperfect, statistical techniques, in particular the analysis of variances and covariances, are employed to separate out the effects of genotype vs. environment.)

Clearly the two cases sketched above are highly simplified, and it might be objected that nature is unlikely to contain many examples that either model exactly applies to. This objection has both epistemological and ontological sides. The epistemological side of the objection has, I think, been met. First, the statistical techniques mentioned in the last paragraph mitigate the force of it. Even in messy cases biologists are fairly successful in separating out environmental causes from genetic, or organismic, causes of differential reproduction. (It should at least be mentioned here that this sort of analysis also often yields genotype by environment interactions, G × E . This occurs when, in contrast to case 2, the relative performance of different genotypes differs in different environments. Although common in nature, and very important, we can ignore that here.) Second, the experimental techniques and conceptual resources developed by Antonovics et al. (1988), Brandon (1990) and Brandon and Antonovics (1996) allow for precise measurements of environmental heterogeneity in real life populations.

Ontologically the objection is this: when measured precisely enough each organism lives in a unique environment. Thus if natural selection requires multiple organisms competing in a common selective environment, then it never really occurs. That, if true, would be a serious objection to the picture of natural selection I am presenting here. But I think we have very good reason to believe it is not true. Unfortunately, here I can present only the briefest sketch of it. What natural selection explanations require is consistent ordinal relations in fitness of the competing types—they do not require precise agreement of absolute fitnesses of the competing types. Lots of empirical studies of natural selection in the wild have shown consistent ordinal fitness relations (see Endler 1986). Furthermore, the fact of adaptive evolution, the consistent and persistent increase of one type over others in evolutionary history, requires these consistent ordinal relations, or what I have termed “common selective neighborhoods”. Thus I think the ontological objection has been met as well.

The short answer to this question is “Yes”. I have already offered arguments in favor of that answer. However the longer more complete answer is “No, no, but yes”. I will explain.

There are two reasonable arguments that suggest a negative answer to our question. I will review them both. In the beginning of this essay I stated that considerable progress could be made in articulating a clear and adequate conception of natural selection without solving all of the philosophical problems associated with the notion of fitness. However, in this section I will have to make certain assumptions about fitness in order to address the issues to be raised. The assumptions are quite plausible, but not defensible in a short essay of this sort.

One negative answer is based on a radical rethinking of the problem nature presents to evolving entities. Mainstream evolutionary biology measures fitness in terms of reproductive success. (Exactly how it defines fitness will not be addressed here.) Survival is important exactly insofar as it contributes to reproduction. Evolutionary success is reproductive success. But suppose we turn the relation between reproduction and survival on its head and think of reproduction as important only insofar as it contributes to lineage survival. The fundamental evolutionary problem is persistence, and reproduction is but one means of achieving that (Bouchard 2004). Compare two lineages, one composed of short-lived organisms that reproduce every year, the other composed of organisms that live for 1,000+ years, but reproduce only rarely. At the end of a thousand year period the first may have gone extinct, while the second persists in virtue of simply surviving without reproduction. Isn't this radical view just as defensible as the more mainstream view? If so, then differential persistence would count as natural selection, and differential reproduction would not be necessary for natural selection to occur.

Interesting as this view is, I reject it because, with Dawkins (1982), I believe that reproduction, in particular going through a single cell bottle neck from whence the developmental process is started anew, is necessary for the evolutionary process of adaptation. Only by restarting the developmental process each generation can fundamental alterations of that process be achieved. Consider a grove of aspens that grows vertically (in what we intuitively think of as trees) and horizontally by underground runners that produce more “trees”. The phenotype of the whole grove can change through time—growing vigorously here, not growing there. But consider a genetic mutation that would fundamentally alter Aspen phenotype. It may occur in a multicellular runner. If so, it will be incorporated in the resultant cell lineage. But that cell lineage will be one of many in the next “tree” produced, since the next tree grows not from a single cell, but from a multi-cell runner. Thus the resultant “tree” will be a chimera, and not fundamentally different from the others in the grove. In contrast, were the runners single-celled at any cross-section, then a somatic mutation would be incorporated in the whole of the downstream “tree”. This, according to Harper (1977) and Dawkins (1982), would count as reproduction, not growth, because this could fundamentally rearrange Aspen development. (But that is not the way Aspens grow.) Accordingly, the evolution of complex adaptations, fundamental rearrangements of development, requires differential reproduction.

A second reason to answer our question in the negative comes from a (quite appropriate) focus on the ecological process of selection. For practical reasons, many studies of natural selection in the wild focus on only a small part of the life cycle. For instance, in the most famous such study H. B. D. Kettlewell (1955, 1956) marked different morphs of the peppered moth, Biston betularia , released them (into polluted woods in one treatment, non-polluted woods in another) and then recaptured them three days later. The difference in the proportion of dark to light forms in the recaptured vs. the released groups was taken to measure natural selection in action. As Rudge (1999) points out, Kettlewell had preformed numerous auxiliary studies to justify this inference, but for present concerns what is crucial is that Kettlewell looked at only a small portion of the entire life cycle, and in particular a portion that did not involve reproduction. It is beyond the scope of this essay to critically examine Kettlewell's actual work, but let us use it as a basis to construct two hypothetical examples.

The first example mirrors the situation with which Kettlewell was actually dealing. Let us suppose that there are two genetically distinct morphs in a population of moths, call them Light and Dark. In survivorship through the larval stage both forms are identical. They are also identical in fertility and fecundity. They differ only in survivorship during the adult stage prior to mating. This difference is due to a difference in their conspicuousness to birds as they rest on the trunks of trees. On dark trees the Darks are less conspicuous than Lights, and vice versa on light trees. In a polluted wood, most of the tree trunks are dark. A biologist marks equal numbers of Darks and Lights, releases them into the polluted woods and recaptures them three days later. She recaptures twice as many Darks as Lights. Based on her knowledge of when pollution was introduced into the woods, the frequency of the two morphs in woods that have not been polluted, the underlying genetics of the two morphs, and her observed selection differential between them, she builds a population genetic model of the situation that predicts a relative frequency of the two morphs for the present. That prediction fits the observed frequencies. She concludes that her mark-release-recapture experiment has captured natural selection in action. I, and the vast majority of evolutionary biologists, would agree with this conclusion.

In contrast, imagine a second example. It is just like the first except that in the larval stage the Lights out survive the Darks by a two to one margin. This difference in survival in the larval stage exactly counterbalances the difference in survival in the adult stage, leading to no difference in reproductive success between the two morphs. Were this going on we couldn't explain the match between frequencies observed in different woods and the selective processes occurring in the adult stage. But that is not crucial to our question. The question for us is: Are the two processes observed in the mark-release-recapture studies in our hypothetical cases both examples of natural selection?

If you think, as has been suggested, that natural selection requires differential reproduction, then you must say that the second example is not one of natural selection. But one might object to this conclusion as follows. The ecological process of birds preying on moths based on their relative conspicuousness is exactly the same in the two examples. That ecological process was identified as natural selection in the first example; therefore it must be natural selection in the second example as well.

Although quite compelling I think the above should be resisted in the following way. Fitness attaches to the whole life cycle, not some subpart of it. Why? Again the short answer relies on a commitment to the fundamentality of reproduction. Reproduction is a reproduction of the entire life cycle. It is true that fitness is often measured, as in Kettlewell's case or in our hypothetical examples, by looking at only a part of the life cycle. But from an evolutionary point of view we are interested in this only when relative performance in this part of the life cycle actually influences relative reproductive success. Thus the first, but not the second, is a case of natural selection. (Consider how Kettlewell's studies would have been received by the evolutionary community if they had mirrored our second hypothetical example rather than the first.)

And so natural selection does require differential reproduction.

In the preceding section we had to draw some fairly subtle distinctions, but our conclusion is one with which the vast majority of evolutionary biologists would agree. There is no such answer to the question of this section. Many biologists define natural selection as differential reproduction of heritable variation (see, e.g., Endler 1986). Many other biologists follow the tradition of quantitative genetics and draw a sharp distinction between the ecological process of selection and the evolutionary response to selection (see e.g., Falconer 1981). There are a large number of advantages to the second approach, and I will follow it here. Thus we will arrive at a negative answer to the question of this section.

First we must get clear on the relevant sense of heritability. Recall § 2 above where we stated “Darwin's Three Conditions”. They are: variation; heredity; and differential reproduction. The notion of heritability relevant here is the purely phenotypic, purely statistical notion developed by Francis Galton (1869). That notion identifies heritability with the regression of the offspring phenotype on the parental (or biparental mean in the case of sexual reproduction), where both phenotypes are presented as z -scores (i.e., set to mean = 0 and standard deviation = 1). Intuitively the heritability is a measure of how closely offspring deviation from the (offspring) mean phenotypic value matches that of the parental deviation (from the parental mean). That is, it measures the degree to which, for example, taller than average parents produce taller than average offspring. A value of 1 represents a perfect correlation between offspring and parental deviations from their respective means, while a value of 0 means there is no correlation, so that, for example, the offspring of tall parents would not differ statistically from those of short.

Unfortunately many people think that some sort of genetic definition has supplanted this basic concept of heritability. In particular, many would say that the Galtonian notion corresponds to the “narrow sense” of heritability ( h 2 ), h 2 = V A / V T , where V A is the additive genetic variance and V T is the total variance. ( V T is usually decomposed into V A , V D —the variance due to allelic dominance, and V e —the environmental variance. The latter is in fact a statistical catch-all, so it includes not just variation due to environmental differences, but everything else.) Even though this equation is very important and we will revert to it shortly, we cannot accept this as a definition of heritability for at least two reasons: (1) Even for diploid sexually reproducing organisms that equation holds only under certain genetic conditions (Roughgarten 1979); and (2) It certainly is not applicable to pre-genetic or non-genetic systems. But that would mean that the theory of evolution by natural selection would not be applicable to the early stages of life on this planet, nor to epigenetic inheritance, nor to cultural transmission, nor to life elsewhere in the universe. Surely such consequences are unacceptable and entirely unnecessary. Thus the Galtonian notion of heritability is fundamental.

The motivation for saying that natural selection requires heritable variation is clear. Only selection acting on heritable variation will have evolutionary consequences. And since we are interested in the concept of natural selection from a purely evolutionary point of view (recall the introduction to this piece), we don't count selection acting on non-heritable variation as natural selection . (This seems analogous to the argument in the last section saying that selection acting in a part of the life cycle that is exactly counterbalanced later in the life cycle should not count as natural selection.)

One response to this line of reasoning is to insist on the importance of drawing a sharp distinction between the ecological process of selection and the evolutionary process of response to selection. The ecological process of selection, the domain of ecological genetics, is complicated and difficult to study. The evolutionary response to selection is the domain of population genetics. It too is complicated and difficult. From a purely strategic point of view it would seem wise not to conflate these two complex processes into one.

I think most would agree with this bit of wisdom, but some would counter by saying that it does not require a definition of natural selection that is neutral on the heritability of the variation in question. We can define ‘natural selection’ one way or another and still agree on all of the facts. The argument for one definition over the other will have to be made in terms of simplicity, elegance, or some other non-empirical virtue of theoretical constructs.

Let us illustrate the last point with a simple example, an example that has been commonly used in the philosophical literature on units of selection. The example is of heterozygote superiority, though any example where the allelic effects on phenotype are non-additive would do. (As would any number of cases involving non-additive interactions among multiple genes.) There are two alleles at a locus, A and a , and thus three genotypes, AA , Aa , and aa . For simplicity suppose that the two homozygotes are lethal, i.e., they die before reproducing. Thus all the matings are Aa x Aa . Mendelism results in offspring of genotypes AA , Aa and aa , in the ratio of 1:2:1. In a single generation the allele frequencies equilibrate at 50:50 and stay there unless the fates of homozygotes changes.

Although we criticized the genetic “definition” of heritability as being derived and not general; it is useful for simple genetic models like the one just described. In this example, once the stable equilibrium is reached, the heritability, h 2 , is zero. This is true because the additive genetic variation is zero. (To apply the Galtonian notion of heritability to this example, we would need to complicate it by specifying a genotype-phenotype mapping, since that notion relates phenotypes. We could do this, and provided V e is not zero the Galtonian value would be zero. But the point presently under discussion would not be made stronger by this extra complication.) So here we have a case where there is phenotypic variation caused by genotypic variation, but the heritability of this variation is zero because of the non-linear relationship between genotype and fitness. How do we categorize the differential reproduction that occurs in this example? Those who think that natural selection requires the differential reproduction of heritable variation would say there is no natural selection here. Those following the quantitative genetics tradition would say that natural selection is occurring, but that since h 2 is zero the response to selection is zero. (In accordance with the fundamental formula of quantitative genetics, R = h 2 S , where R is the response to selection and S is the selection differential.) But, it seems, there is no empirical difference between these two points of view, so we need to decide between them on the basis of some non-empirical reasons.

The seeming empirical equivalence between the two accounts offered of our example is, in fact, illusory. Godfrey-Smith and Lewontin (1993) have pointed out that there is an empirical difference between an account of this case that says there is selection vs. one that says there is no selection. (They make this point in the context of discussing genic selectionism, which is committed to saying that there is no selection in this case since the allelic fitnesses are the same. For further discussion of this see Brandon and Nijhout 2006.) The empirical difference is that the first, but not the second, can account for the within generation change in genotype frequencies. At the start of each generation the three genotypes are represented in the 1:2:1 ratio discussed above. Later in the generation the homozygotes, both AA and aa , die, so that 100% of the population is composed of Aa s. However, this difference is not an evolutionary difference, since evolution is trans-generational change. Our focus is on evolution. And it turns out that the two models are not empirically equivalent with respect to evolutionary predictions. The standard account, which says that selection occurs each generation against the homozygotes, predicts a stable equilibrium—one actively maintained by strong selection. But the account that says that there is no selection in this case has no basis to predict the same sort of stable equilibrium. This is not apparent if one only considers selection, because the genic selectionist recognizes the existence of selection once one perturbs the population from its equilibrium and so, it would seem, both models predict the same stable equilibrium. However, once one brings in considerations of the interaction of drift and selection (see next section) it has been shown that the models do not give empirically equivalent predictions (Brandon and Nijhout 2006). This is because small perturbations from equilibrium will have very different likelihoods of drift, because they will experience quantitatively quite different regions within which drift vs. selection is likely to dominate.

This example has a number of consequences (again see Brandon and Nijhout 2006 and Brandon 2006). But for us the consequence is this. To say that selection requires heritable variation is factually wrong. When applied to a broad class of cases it makes the wrong evolutionary predictions. And so we must reject that view, and conclude that natural selection does not require heritable variation.

Why should an entry on natural selection contain a section on drift? One good reason is that natural selection and drift are co-products of the same process, namely a probabilistic sampling process (Brandon and Carson 1996, Matthen and Ariew 2002, Walsh et al. 2002). Thus, although it is of crucial importance to separate selection and drift, one cannot do so on the basis of process alone (contra Millstein 2002), one must do so on the basis on outcomes (Brandon 2005). Why is this? If we think of fitness as a probabilistic propensity, then, as we have seen, differential fitness is a necessary condition for natural selection. Thought of this way, natural selection is a probabilistic sampling process. We can characterize a continuum of all possible fitness differences starting with maximal fitness differences at one end of the continuum (i.e. where all fitness equal either 1 or 0, with at least some of each value), and minimum fitness differences on the other (i.e., an equiprobable distribution). The two endpoints are exceptional with respect to the relation of selection and drift. At the maximal fitness difference end, one unlikely to occur in nature, drift is impossible and selection is necessary. (See Figure 1.) At the finite set of minimal fitness differences at the other end of the continuum, corresponding to absolute neutrality of the traits under consideration, selection is impossible, because there are no fitness differences. And drift is maximally likely, but not necessary. Why not? Because the sample may be in accord with the probabilistic expectations and thus no drift occur. (Imagine a population with two alleles, A and a , absolutely selectively neutral with respect to each other, and at a 50:50 ratio. The next generation may also contain the two alleles at the same ratio, if so, no drift has occurred.

Figure 1. The heavy horizontal line, with dotted center section, represents the infinite number of possible fitness distributions from Maximal Probability Differences (MPD—all fitness = 0 or = 1, with some of both) on the left to the Equiprobable Distribution (EP—all fitness the same) on the right. The arrows emanating from the different descriptions of the modalities of selection and drift indicate the areas of the distribution falling under these descriptions.

But these two endpoints of this continuum are exceptional. More common is surely the vast middle area where drift and selection are both possible. In this middle ground we cannot say until after the fact whether or not drift occurs, nor quantify the degree to which it occurs. Prior to the fact we can only quantify the relative likelihoods of selection and drift—they vary according to the crucial quantity 4 N s . (Where N is the effective population size and s is the selection coefficient, i.e. the strength of selection. For discussion of 4 N s see any standard textbook on population genetics, e.g., Roughgarden 1979, see also Brandon and Nijhout, 2006.) The larger that quantity is, the larger N and the larger s, the greater is the likelihood that selection will dominate drift. And vice versa for small N and small s. Only after the fact can one tell whether or not the probabilistic sampling has gone in accord with the probabilistic expectations, or not. To the extent that it has not, drift has occurred. Drift simply is the deviation from probabilistic expectation. And since selection itself is a probabilistic process there can be no purely process-derived distinction between selection and drift—attractive as that idea may be.

A more fundamental reason for a discussion of drift here in an entry on natural selection is the view that drift is the zero-force background against which evolutionary forces, including natural selection, act (Brandon 2006). Just as in Newtonian mechanics one could not properly understand the notion of gravitational force, without understanding Newton's 1 st Law, similarly one cannot understand natural selection in evolutionary biology without understanding the background against which it operates. Put in other terms, drift provides the appropriate null hypothesis against which to test any selection hypothesis. Unfortunately this is not always well understood. Indeed this way of thinking about evolution stands some canonical versions of evolutionary theory on their heads. For instance, those who would take the Hardy-Weinberg Law as a Zero-Force Law of evolution (see, e.g., Ruse 1973, and Sober 1984, but also many standard textbooks in evolution) view stasis as the default state of evolutionary systems, with some evolutionary force needed to move (i.e. evolve) them. Without a net force, no net change. But all modern methodology in molecular evolution is predicated of the truth of just the opposite idea, namely that left alone evolutionary systems drift. Drift is the default state. So that, for instance, neutral alleles in different populations differentiate from each other. But this molecular truth is iterated throughout the biological hierarchy. Once speciation occurs, species differentiate (drift apart) as a null expectation. Which is not to say that natural selection cannot produce evolutionary change. Of course it can. But if we are to properly recognize it, we must be able to recognize the signature of selection and differentiate it from drift's signature (see, e.g., Bamshad and Wooding 2003). Change in evolution is a heterogeneous category.

Stasis, on the other hand, is largely homogeneous. Long-term stasis can only occur by natural selection.

• Antonovics, J., Ellstrand, N. C., and Brandon, R. N., 1988, “Genetic variation and environmental variation: Expectations and experiments,” in Plant Evolutionary Biology, (eds.) L. D. Gottlieb and S. K. Jain, pp. 275–303. London: Chapman and Hall.
• Ariew, A. and Lewontin, R. C., 2004, “Confusions of fitness,” British Journal for the Philosophy of Science 55: 347–363.
• Bamshad, M. and Wooding, S. P., 2003, “Signatures of natural selection in the human genome,” Nature Review Genetics 4: 99–111.
• Bouchard, F., 2004, Evolution, Fitness and the Struggle for Persistence, Ph. D. dissertation, Duke University.
• Brandon, R. N., 1990, Adaptation and Environment, Princeton: Princeton University Press.
• Brandon, R. N., 2005, “The difference between selection and drift: A reply to Millstein,” Biology and Philosophy 20: 153–170 .
• Brandon, R. N., 2006, “The principle of drift: Biology's first law,” Journal of Philosophy 103(7): 319–335.
• Brandon, R. N., and Antonovics, J., 1996, “The coevolution of organism and environment,” in Concepts and Methods in Evolutionary Biology, R. N. Brandon, pp. 161–178. Cambridge: Cambridge University Press.
• Brandon, R. N., and Carson, S. 1996, “The indeterministic character of evolutionary theory: no ‘no hidden variable proof’ but no room for determinism either,” Philosophy of Science 63: 315–337.
• Brandon, R. N. , and Nijhout, H. F. 2006, “The empirical non-equivalence of genic and genotypic models of selection: a (decisive) refutation of genic selectionism and pluralistic genic selectionism, ” Philosophy of Science 73: 277–297.
• Darwin, C., 1871, The Descent of Man, London: John Murray.
• Dawkins, R., 1982, The Extended Phenotype, Oxford: Freeman.
• Endler, J. A., 1986, Natural Selection in the Wild, Princeton: Princeton University Press.
• Falconer, D. S., 1981, Introduction to Quantitative Genetics, New York: Springer-Verlag.
• Galton, F., 1869. Hereditary Genius, London: Macmillan and Co.
• Godfrey-Smith, P., and Lewontin, R. C., 1993, “The dimensions of selection,” Philosophy of Science 60: 373–395.
• Harper, J. L., 1977. The Population Biology of Plants, London: Academic Press.
• Hull D. L., 2001, Science and Selection, Cambridge: Cambridge University Press.
• Kettlewell, H. B. D., 1955, “Selection experiments on industrial melanism in the Lepidoptera,” Heredity 9: 323–342.
• Kettlewell, H. B. D., 1956, “Further selection experiments on industrial melanism in the Lepidoptera,” Heredity 10: 287–301.
• Lewontin, R. C., 1978, “Adaptation,” Scientific American 239 (9): 156–169.
• Matthen, M. and Ariew, A. 2002, “Two ways of thinking about natural selection,” Journal of Philosophy 49(2): 55–83.
• Millstein R. L. 2002, “Are Random Drift and Natural Selection Conceptually Distinct?” Biology and Philosophy 17: 33–53.
• Roughgarden, J. 1979, Theory of Population Genetics and Evolutionary Ecology: An Introduction. New York: Macmillan Publishing Company, (Reprinted 1987 by Macmillan, and in 1996 by Prentice Hall).
• Rudge, D. W., 1999, “Taking the peppered moth with a grain of salt,” Biology and Philosophy 14: 9–37.
• Ruse, M., 1973, The Philosophy of Biology , London: Hutchinson Publishing Group, (Reprinted 1998 by Prometheus Books).
• Sober, E., 2004, The Nature of Selection: Evolutionary Theory in Philosophical Focus , Cambridge: MIT Press.
• Walsh, D., Lewens, T. and Ariew, A., 2002, “Trials of life: natural selection and random drift,” Philosophy of Science 69: 452–473
• Evolution 101: How It Works , at the Understanding Evolution website, maintained at the University of California/Berkeley.
• Natural Selection: How Evolution Works , at ActionBioscience.org, maintained by the American Institute of Biological Science.

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• v.25(2); 2012 Apr

Darwinian natural selection: its enduring explanatory power

Evolutionary theory has never had a stronger scientific foundation than it does today. In a short review I hope to portray the deep commitment of today's biologists to Darwinian natural selection and to discoveries made since Darwin's time. In spite of the scientific advances in the century and a half since the publication of On the Origin of Species , Darwin still remains the principal author of modern evolutionary theory. He is one of the greatest contributors of all time to our understanding of nature.

An awesome gulf divides the pre-Darwinian world from ours. Awesome is not too strong a word…. The theory of natural selection revolutionised our understanding of living things, furnishing us with a comprehension of our existence where previously science had stood silent. –Helena Cronin ( 1 )

The deluge continued day after day on the tiny island of Daphne Major in the Galápagos Islands, 600 miles off the coast of Ecuador. Dusty soil from years of drought washed in torrents down the steep volcanic slopes into the surrounding sea. Plants began to sprout that had lain dormant for years, and vines grew up the tent poles of the researchers on the only flat ground high up near the extinct volcano's rim. Some plants producing large seeds were smothered by the prolific vines, and others flourished. The finches on the island celebrated by “going crazy,” in the words of one researcher—the males sang, established territory, and mated. The young grew fast on the insects that appeared all over the island, and they began mating at an unusually young age. The findings from this unusual year provided stunning evidence that natural selection was working on every generation of ground finches, changing the calculus of reproductive success and the composition of alleles in the gene pool of the species.

The biologists Peter and Rosemary Grant began studying Darwin's finches in 1973, and their research has continued full-time ever since ( 2 , 3 ). It is the longest field study in biology other than that of Jane Goodall, who has studied chimpanzees in Tanzania since 1962. Younger biologists have assisted the Grants in their study, so that the ground finches of Daphne Major have been studied in great detail every year since 1973.

Daphne Major is a volcanic cone with a central crater; the island is only one half mile long (Figure ​ (Figure1 1 ) . No tourists visit the island because there is no place to land. Steep cliffs encircle almost the entire perimeter, some with reverse slopes and all with waves battering their sides. Embarkation onto the slopes involves maneuvering a small boat next to an area of relatively flat volcanic surface and jumping onto the surface as the wave hovers briefly at the right level. For researchers, this means negotiating the hair-raising landing while carrying tents, food, and research equipment.

An extinct volcanic cone forms the tiny island of Daphne Major in the Galápagos, home of one of the longest studies of natural selection acting on single generations in the wild. Reprinted with permission from Grant PR, Grant BR. How and Why Species Multiply: The Radiation of Darwin's Finches . Princeton, NJ: Princeton University Press, 2008.

This inaccessibility has made the island an ideal place for the isolated study of animals that have arrived by water or by air and have established a foothold and reproduced. Island species are free from the competition of innumerable mainland species, but are faced with the challenge of how to exploit the sparse resources of their small world.

Ground finches on the island are tame, letting researchers walk up to them at times and even landing on their arms as they are measuring the beak size of one bird with calipers. Because they don't migrate, they are available for study year round. There is no obstructing vegetation to hamper observations with binoculars. There are no tourists to disturb the fiches or the researchers. For these reasons and more, the island has been described as a natural laboratory.

In 2008 the Grants, who teach biology at Princeton, published a scientific volume about their study. Their findings would have been stunning to Charles Darwin, who believed that evolutionary changes brought about by natural selection would become evident only after long periods of time. Instead, every generation of ground finches has produced evidence of changes in morphology and allele frequencies in the population of one ground finch, Geospiza fortis . The birds and their genes were changed by the severe selection pressures of the years of harsh drought; small seeds were scarce, and those individuals with smaller beak depth and smaller body size died. Evolution placed a meaning on death . Through the death of individuals less fit in the prevailing environment, alleles coding for less useful variations became less common in the gene pool. This is nothing less than evolution occurring in real time, measurable in only months, and brought about only by natural selection—the differential survival of alleles that code for more useful traits.

The beak of finches is their secret for manipulating seeds. In his superb book about the Grants’ research, The Beak of the Finch , Jonathan Weiner reminded us ( 4 ): “Beaks are to birds what hands are to us. They are the birds’ chief tools for handling, managing, and manipulating the things of this world…. Each beak is a hand with a single permanent gesture.” Beaks are continually reshaped to maximize their efficiency in crushing seeds of specific sizes and shapes and can be compared to pliers and wrenches (Figure ​ (Figure2 2 ) .

Bird beaks are like pliers and wrenches, each adapted to its own narrow task, and are constrained in their size and shape by the demands of the ongoing environment in which the bird lives and reproduces. Reprinted with permission from Grant PR, Grant BR. How and Why Species Multiply: The Radiation of Darwin's Finches . Princeton, NJ: Princeton University Press, 2008.

Torrential rains came to the Galápagos in 1983 during the most severe El Niño event in 400 years, as documented in the coral reef fossil record. Research data from this 1 year on Daphne Major required still another year for entering it all into a computer. The final analysis was stunning: birds with large bodies and deeper beaks were dying; small birds with less deep beaks were thriving. Natural selection had reversed its direction . Now on the island small seeds were abundant, and trees producing large seeds were choked by vines. Death of the less fit became an evolutionary “force,” and the gene pool of G. fortis changed again. So did the morphology of the birds, which were now smaller in average body size, with a more pointed beak than in the 1970s. Generation by generation, natural selection could be monitored as it occurred.

These findings are robustly documented by elaborate analyses involving 1) beak and body measurements of thousands of birds on the island, 2) observations of behavior, 3) studies of embryonic development, and 4) genetic sequencing of both nuclear and mitochondrial DNA. The issue of fundamental complexity is thus addressed: morphology, behavior, and the genetic code itself changed pari passu with selection pressures. One may argue that this is only correlation, but it is such consistent and remarkable correlation that causation is the only reasonable conclusion. There is no contender for causation other than natural selection. Over the years since these early studies, findings have enabled testing through predictions, in which the correlation has remained true.

Natural selection is no more, no less, than the changing representation of alleles that code for traits selected for by the environment. It is not a “force,” although “evolutionary force” is an expression that is often used to describe it. It is just the differential survival of alleles in succeeding populations. The environment may be natural or artificial; we know that our artificial environment of antibiotics provides a selective force for alleles in microorganisms that contribute to antimicrobial resistance. There is no fundamental difference in the dynamics of natural and artificial selection. Darwin knew this and began his major opus with a long discussion of the domestication of animals and plants as an excellent analogy to natural selection in the wild.

The term “islands” refers not only to oceanic islands, but also to freshwater lakes separated from each other (in which innumerable fish species have evolved, for example, the African cichlids), and even to human bodies, in each of which HIV-1 evolves into a smorgasbord of “quasispecies” variants over the course of infection. The field of biological science that addresses geographic diversity is called biogeography . Geographic isolation enables a population to evolve without the intermixing of genes from other populations. Sometimes that proceeds to speciation, or the creation of a new species—reproductively isolated from other species. At other times it may go part of the way, with the creation of variants or subspecies.

When I recently visited the White Sands National Monument in Arizona, I learned of a striking example of natural selection on the “islands” of extremely white sand dunes, which are made of gypsum (hydrated calcium sulfate) sand crystals. The dunes are so white that they resemble a snowscape. Three small diurnal (day-active) lizards live in the dunes, having recently evolved from closely related species that live in the brown soils of the surrounding Chihuahuan Desert. The White Sands species are no longer brown but almost white, perfectly mimicking the color of the sands (Figure ​ (Figure3 3 ) . When mating, they demonstrate a preference for white color morphs if given a choice in laboratory tests. Researcher Erica B. Rosenblum of the University of California at Berkeley has found a genetic basis for this color change, stemming from mutations in the melanocortin-1 receptor gene, which has a key role in producing melanin in vertebrates ( 5 ). She explained to me that the change is caused by allelic variants conferring adaptive coloration, not by epigenetic gene silencing or by phenotypic plasticity (variable phenotypic expression without genetic change). It thus represents true genetic differentiation brought about by natural selection operating in a relatively new environment. The fast-track evolution reminded me of the Galápagos finch study; in fact, the White Sands newspaper sported the headline, “The Galápagos Islands of North America!”

The bleached earless lizard, which lives only in White Sands National Monument, New Mexico, has evolved in only 2000 to 6000 years from a darker form living in the surrounding sands of the Chihuahuan Desert. The color change, along with changes in several other traits, has been mapped to gene mutations favored by natural selection. Photograph by Greg and Mary Beth Dimijian.

Natural selection is one of the pillars of contemporary evolutionary theory. Nevertheless, there are other causes of biotic evolution, some of which were unknown to Darwin. These are addressed after a further elaboration on natural selection.

MORE ABOUT NATURAL SELECTION

Bricolage is a wonderful French word, the best English translation being “tinkering.” It was first used by François Jacob in 1977 to describe how evolution uses “whatever he [a tinkerer] finds around him whether it be pieces of string, fragments of wood, or old cardboards” to fashion new structures or behavior coded for by genes. Jacob explained: “Evolution does not produce novelties from scratch. It works on what already exists…. The appearance of new molecular structures during much of biological evolution must, therefore, have rested on alteration of preexisting ones” ( 6 ).

Biological structures are thus palimpsests , with layers upon layers of history, like an old scroll erased and written over many times. One example is the vertebral column, which has been tinkered with and modified many times in vertebrate history. In the evolutionary history of whales, there is a stunning discovery: the pelvis becomes detached from the spine, as it is no longer needed to support hind limbs. The whale's range of spinal motion is thus increased, and tiny hind limbs appear in the soft-tissue areas of fossils as relics of ancestors, destined to disappear completely in modern whales.

Other structures in animals are rendered obsolete, such as eyes in some cave-dwelling fishes. The term vestigial has been used to describe these structures; they are remnants of organs once useful to an evolutionary ancestor. Genes are no exception; innumerable examples of vestigial genes, some “rusting away” like submarines on the ocean floor, have been uncovered in animal genomes.

For bird lovers, a striking example of bricolage and co-opting of earlier structures is the avian feather. In the past decade paleontologists have found hundreds of fossils of feathered dinosaurs, some with fluffy down, some with simple barbs, still others with hollow filaments. The transition from scales to feathers may have hinged on a relatively simple genetic switch. What adaptive benefits might feathers have conferred on dinosaurs? The same that they confer on birds today: warmth, cryptic coloration, showy patterns used in courtship, and possibly gliding to the ground from low platforms. One chicken-sized dinosaur had feathers on arms, legs, and toes. Even though these feathered dinosaurs were not capable of flight, protofeathers and true feathers may have paved the way for true flight millions of years later. It's an example of “exaptation,” the assignment of a new adaptive function to a structure that evolved under different selective pressures in an earlier environment.

The evolution of vertebrate limbs from the fins of fish is yet another example of a new assignment (by natural selection) to an earlier structure. Fossil finds have recently come one after the other. Tiktaalik , a 375-million-year-old fossil found in 2004, is the colorful name given to a fish skeleton with gills, the first neck, and the first front limbs; the limbs consisted of a functional wrist, elbow, and shoulder—the owner could “do push-ups.” More recently, in 2011, came the discovery of pelvic-fin muscles in the first fishes to emerge on land ( 7 ). Here was evidence of a weight-bearing pelvis, hindlimbs, and their associated musculature—and the “rear-wheel drive” strategy that characterizes terrestrial locomotion in most vertebrates. Play the fossil frames in a movie sequence and you see the emergence of fishes onto land.

Even the abrupt Cambrian “explosion” of life 541 million years ago is yielding up its secrets. There is growing evidence from molecular sequences, molecular clocks, and developmental histories that most of the Cambrian fauna originated tens to hundreds of millions of years before the onset of the Cambrian, leaving a clear fossil signature only in the Cambrian ( 8 ). Darwin has been vindicated in his prediction that this apparent anomaly would some day be resolved with evidence of ancestral lineages leading up to the explosive appearance of fossils in the Cambrian.

Paleontologists stress that it is time to move past the simplistic question, “Where are the missing links in the fossil record of life?” Instead, it is time to accept that 1) the fossil record is now extraordinarily rich, and 2) a seamless record is an impossible goal. Any transition between fossils will always be a “missing link.”

If you think the above examples of bricolage are amazing, get ready for this one. The stapes (or stirrup, the innermost of the three middle-ear bones) originated as the hyomandibular bone in fishes, supporting the gills. It later migrated to the hard palate, which it braced against the cranium in jawed fishes and the earliest tetrapods. It made a third change to become the columella in the middle ear of birds and the stapes of the middle-ear bones in mammals. Now a hearing aid, it was once a feeding aid and even earlier a breathing aid . And there is this: When an immature opossum is born, it climbs into its mother's pouch with its future ear bones still articulating its jaws. The stapes will migrate to the middle ear as the embryo develops. There is hard anatomical evidence supporting these anatomical transitions ( 9 ). What better example is there of a “fossil record” in development?

Remember that for natural selection to act, there must be 1) genetic variation in a population, 2) occasional mutations, and 3) mixing of genetic entities, either during reproduction (as in eukaryotic sexual reproduction) or in horizontal gene flow (as in bacteria and viruses).

Crypsis (hiddenness) is a relatively simple case of natural selection. It refers to camouflaged body color or shape, and to behavior that enhances concealment. We have discussed crypsis in lizards in the White Sands National Monument. Behavioral crypsis is obvious in the immobility and squinted eyes of the Scops Owl on the bare tree branch in the Okavango Delta of Botswana (Figure ​ (Figure4 4 ) . It is useful for hiding from predators (if you are potential prey) or for remaining unseen by potential prey (if you are a predator). The role of natural selection is inferential, but no other explanation comes close. Alleles arising by chance mutations, which cause crypsis, render an animal less visible to predators or prey. Such alleles are more successful than competitor alleles in getting into the next generation, by virtue of the benefits they confer. The mutations may be random, but natural selection is anything but random.

Only 8 inches tall, the Scops Owl is almost invisible on the bare branch that is its home, in the Okavango Delta of Botswana. Its extraordinary camouflage includes anatomy and behavior: its breast feathers resemble tree bark, and it remains immobile during the day, keeping its eyes closed so that it is less likely to be spotted by Africa's diurnal birds of prey. Photograph by Greg and Mary Beth Dimijian.

Mimicry is another relatively simple example of natural selection. If one animal is toxic to predators, and predators learn to avoid it, another animal will benefit from mimicking the same disguise. A hawkmoth caterpillar in a Costa Rican cloud forest displays conspicuous eyespots (its real eyes are tiny) and a soft, fake stinger (Figure ​ (Figure5 5 ) .

Munching on a plant stem in Costa Rica's Monteverde Cloud Forest Reserve, this Xylophanes caterpillar exhibits fake eyes and stinger. Its real eyes are so tiny that you would need a hand lens to see them. Photograph by Greg and Mary Beth Dimijian.

Do plants ever “lie”? Consider the passionfruit vine, often parasitized by butterfly eggs that hatch into caterpillars. The caterpillars feed on the leaves. If mutations occur in the plant that produce light-colored spots on the leaves (Figure ​ (Figure6 6 ) , they might just resemble eggs laid by Heliconius butterflies. Experiments have shown that these butterflies are less likely to lay eggs on host plants that have eggs or egglike plant structures ( 10 ). Again, natural selection is the only candidate explanation.

Do plants tell lies? Passionflower vine leaves in Costa Rica do, preserving mutations that produce spots closely mimicking eggs of Heliconius butterflies. Plants with the spots are protected from caterpillar predation because butterflies choose to lay their eggs on other plants. Photograph by Greg and Mary Beth Dimijian.

What about bacteria and viruses? Even though they don't reproduce sexually, they both enjoy high levels of horizontal gene transfer (“parasexual reproduction”) and maintain populations with high genetic diversity. There is thus ample variation for selection to act on. Under an antibiotic regime, selection occurs exactly as in Darwin's finches on the Galápagos. Differential death is the great reaper, eliminating the less fit.

Antibiotic resistance occurs not only in modern medicine but also in nature, where microbes, plants, fungi, and insects make their own antimicrobials. It is no surprise to find that these natural antimicrobials must keep evolving in the universal host-pathogen arms race. A study in 2011 demonstrated antibiotic resistance genes comprising part of fossil bacterial DNA 30,000 years old. Those same genes are found today in modern bacteria, where they encode resistance to beta-lactam, tetracycline, and glycopeptide antibiotics ( 11 ).

Just think: before Darwin, essentialism was the prevalent view of nature. Each species had an “essence” that was as unchanging as chemical elements in the periodic table. Each plant and animal species was believed to have originated in the same form as we see it today.

DOMESTICATION OF ANIMALS

Domestication is not just an excellent analogy of natural selection. It's also a good experiment. –Richard Dawkins ( 12 )

The best experiment ever made in animal domestication ( 13 , 14 ) is the ongoing study of silver foxes, initiated in the 1950s by the Russian geneticist Dmitry K. Belyaev (Figure ​ (Figure7 7 ) . On a Siberian fur farm, Belyaev raised silver foxes, Vulpes vulpes , and observed the young of each litter. Without prompting, he and his coworkers noted which juveniles were friendly and which avoided human contact. The friendly “tame” ones were later mated with tame members of other litters, and this mating selection was performed generation after generation. Only tameness was selected for. Now, over 50 years later, the result is a breed of foxes never imagined before: friendly from birth, begging for attention, and with striking anatomical changes: a piebald coat color (with a white patch on the top of the head, seen in border collies, pigs, horses, and cows), short legs, a curled-up tail, and floppy ears. Charles Darwin, who loved dogs and spent much of his life studying domestication, would have been stunned. These changes, which occurred over only 40 generations, reflect changed timing of developmental processes . Childlike traits prolonged into adulthood are an example of neoteny —neo-, “new,” and -teny, “holding onto.” Belyaev's unique experiment compressed into decades an ancient process that unfolded over centuries. Instead of foxes, wolves are believed to be the ancestral canids that were domesticated into the hundreds of dog breeds that have become our “best friends” (Figure ​ (Figure8 8 ) .

A silver fox pup shows tame and affectionate behavior, which results from selective breeding in the longest scientific study of domestication ever made, conceived by the Russian geneticist Dmitry K. Belyaev in the 1950s. Reprinted with permission from Trut LN. Early canid domestication: the farm-fox experiment. American Scientist 1999;87( 2 ):160–169.

Over the past 10,000 to 15,000 years, humans have domesticated the Eurasian wolf and used its natural genetic variation to create hundreds of breeds of domestic dogs. Reprinted with permission from Ellegren H. Genomics: the dog has its day. Nature 2005;438(7069):745–746.

Dog fossils have been found at archeological sites dating from 11,500 to 15,500 years ago ( 15 ). It is not surprising that dogs were domesticated long ago. They have served humans as close companions, guard dogs, police dogs, herding dogs, hunting dogs, sled dogs, military dogs, seeing dogs for the blind, and olfactory search dogs. Have dogs domesticated us as well? They may have secured equally important services from us, from feeding to family membership. There is, however, at least one example of a serious disservice we are guilty of: the bulldog's craniofacial malformation, in which facial shortening has created severe medical problems (Figure ​ (Figure9 9 ) . In the bulldog's unfortunate outcome, domestication differs from natural selection. Such a defective phenotype would quickly be eliminated from the reproductive pool by natural selection.

The bulldog skull before 1890 (top) and its unfortunate fate through selective breeding in 1935 (bottom). Craniofacial malformation has created serious medical problems, which we would attempt to correct if they occurred in humans. Reprinted with permission from Thomson KS. The fall and rise of the English Bulldog. Amer Scientist 1996;84:220–223.

Are we domesticating ourselves ? Consider the following features of modern life:

• Sanitary sewerage disposal
• Clean water
• Refrigeration
• Climate control
• Modern medical care
• “Assisted reproduction”—in vitro fertilization, preimplantation genetic diagnosis, intracytoplasmic sperm injection

A more troubling question is: Are we eliminating alleles for “robust” traits from the human gene pool?

ENDOGENOUS VIRAL ELEMENTS

Viruses, especially bacteriophages (“phages,” viruses that infect bacteria), may be the most numerous and ubiquitous genetic entities on the planet. Genetic sampling techniques show that seawater is a soup of viruses. Bacterial turnover on Earth occurs daily through the most common predator-prey relation known, that of phages and bacteria. Whether or not you choose to consider viruses as living entities, they are visible to natural selection, just as cellular genetic entities are.

There is an “archeological record” of past infections by viruses that inserted their genes seamlessly into our DNA ( 16 ). These genes are recognized by their sequence similarity to present-day viruses. Many have been found to be degraded, some more than others. Endogenous viral elements (EVEs) constitute a significant portion of our genome—as much as 8%. That's over 6 times more DNA than is found in all of our 20,000 protein-coding genes. They are replicated in Mendelian fashion every time a cell divides.

Most EVEs are retroviruses (which, like HIV-1, convert their RNA to DNA and insert it directly into our genome), but some are nonretroviral, such as Ebola-like and herpesvirus-like sequences. Retroviral EVEs are called ERVs (endogenous retroviruses) and HERVs (human endogenous retroviruses). The age of endogenous viruses can be estimated by molecular clock techniques, because they are confined to a host genome and therefore “frozen” in a slower mutational state than freely existing viruses.

EVEs constitute direct evidence that modern viral lineages have very ancient roots. Lentiviruses are 2 to 4 million years old; filoviruses, 12 to 30 million years. The science writer Matt Ridley has said: “If you think being descended from apes is bad for your self-esteem, then get used to the idea that you are also descended from viruses” ( 17 ).

PALEOANTHROPOLOGY

During only the past decade, fossil discoveries in Africa, Asia, and the Near East have provided an extraordinary sequence of the transition from arboreal to terrestrial locomotion in early hominins. One of the defining characteristics of hominins is bipedalism, and we are fast approaching an almost seamless fossil record of skeletal adaptations progressing through intermediate stages to fully bipedal, with the requisite changes in foot, ankle, knee, pelvis, vertebral column, upper extremities, and forward placement of the foramen magnum. These changes occurred as forests in East Africa were changing into a more open habitat, typical of the “wooded grasslands” of the East African savannah today. Bipedal locomotion enabled a huge increase in efficiency for traveling long distances in search of food and a new habitat, especially when carrying children.

The most stunning finding of paleoanthropology, however, has been this: in only 3 million years the hominin body size doubled and the brain tripled in volume to its present size, violating the usual “rules” of allometry. Typically in mammals, if body weight doubles (× 2 1 ), brain weight increases not by 2 1 but by about 2 3/4 or about 1.7 times. Instead, our intracranial volume increased 3 times, with the neocortex expanding the most. More sophisticated tools, long-distance trade, language, and the earliest art accompanied the encephalization. There can be no better evidence of natural and sexual selection, even though the evidence is “only” inferential and cannot be verified experimentally. Once the stage was set—with hands free to manipulate objects, brain structures capable of complex language, and an omnivore's gastrointestinal tract providing more efficient energy extraction from a diet of plants and animals—brain expansion progressed steadily and inexorably. Cooperation among kin and tribal members may have contributed significantly to survival of children, who—with their early birth and large brain—required a long period of upbringing.

PROCESSES OTHER THAN NATURAL SELECTION THAT CONTRIBUTE TO EVOLUTION, SOME UNKNOWN TO DARWIN

Natural selection is not the only process by which life evolves. I have listed other processes and mechanisms below in a short outline.

• Sexual selection . Proposed by Darwin and rejected by Alfred Russel Wallace, sexual selection is distinct from natural selection and involves mate choice (intersexual selection) and competition between members of the same sex (intrasexual selection). Even though sexual selection is “natural,” it is not the same as natural selection and may even oppose natural selection, as in the case of male ornaments and bright colors that make the male more vulnerable to predation.
• Endosymbiosis . “Endo,” or inside , and “symbiosis,” or living together , refer to the incorporation of a microscopic organism such as a bacterium into a larger cell, such as the protoeukaryotic cell. Mitochondria and chloroplasts have all the identifying traits of bacteria, and they perform crucial functions today (ATP synthesis and photosynthesis, respectively). Most of their genes have migrated to the host cell nucleus and are integrated into the nuclear genome, seamlessly joined in a now obligate partnership—one of the most critical events in the history of the eukaryotic cell. Endosymbiosis is an example of inheritance of acquired characteristics , i.e., Lamarckism. Surprisingly, it is entirely compatible with Darwinian natural selection acting on each partner independently; as with other mutualisms, it confers benefits upon both partners.
• Major extinctions . Both fit and unfit have perished together in Earth's great mass extinctions, the latest of which—the “Anthropocene” (also dubbed the “Homogenocene”)—has been proposed as being underway. (Who would doubt this?)
• Genetic drift in small populations . Without the buffering effect of large population size, accidents eliminate fit and unfit alike, and gene frequencies would thus change in the population, some at random.
• “Accelerated evolution.” An increased mutation rate appears to have occurred in some gene regions of humans—one in neurons playing a key role in the developing cerebral cortex, and another in the FOXP2 gene, involved in human speech. This accelerated mutation rate seems also to occur in some bacterial populations subjected to stress. Something is “tampering” with mutations, providing a surplus when they are needed for a diversity of lottery tickets. The mechanism of this acceleration is unknown, but it sounds as if it may be adaptive—and thus visible to natural selection.
• Neutral protein polymorphisms . Different structural forms of a protein that have little or no effect on the phenotype are invisible to natural selection in some environments.
• Epigenetics and gene regulation . See discussion immediately below.

EPIGENETICS

At the interface of gene and environment, epigenetics (epigenomics) addresses heritable changes in gene expression that cannot be explained by changes in DNA sequence. In eukaryotes and prokaryotes, epigenetic changes can activate, reduce, or completely disable a gene's activity. Epigenetic “marks” control access to DNA by different mechanisms, one of which is methylation of cytosine. Small noncoding RNAs (“noncoding” meaning not coding for proteins) are believed to be another agent of epigenetic change. The terms “epigenetic” and “epigenome” are still somewhat fluid and subject to change.

Early in the embryonic development of multicellular organisms, undifferentiated stem cells develop into the many different cells of the developing organism, through the silencing of genes. These changes, also called “epigenetic,” usually last for a lifetime, so that a liver remains a liver. A cancer cell, however, may undergo epigenetic reprogramming, and “epimutations” may contribute to aging.

Some epigenetic marks change as the organism responds to environmental change, such as starvation, stress, or disease, and some of these marks may persist for several generations (and are thus called “transgenerational epigenetic inheritance”). Mapping the epigenome has become increasingly important as we realize that the genome holds only a fraction of the information needed to understand development and disease.

Genome-wide association studies are uncovering evidence of polygenic (many-genes) predisposition to specific diseases ( 20 ). Many of these genetic predispositions involve noncoding DNA that regulates gene expression. Many so-called “genetic” diseases may have their origin in such epigenetic changes.

Does epigenetics change our understanding of evolution? Two studies in 2011, one in the plant Arabidopsis thaliana ( 18 ) and the other in the nematode Caenorhabditis elegans ( 19 ), showed epimutations that changed the phenotype for only a few generations. The changes, though inherited, were unstable over short time periods. Such cycling is not characteristic of genomic DNA, which remains relatively stable over time.

Epigenetically silenced alleles seem to be taken out of the selection pool for short periods of time. This could affect evolution by natural selection on short time scales, but seems unlikely to be the basis of adaptations that are stable over long time periods.

Epigenetic silencing of genes appears to be a key defense against transposons, the “jumping genes” discovered by Barbara McClintock. Transposons may be the ultimate “selfish” elements in our genome. A stunning 50% or more of the human genome is derived from retrotransposons , a category of transposons that copy and amplify themselves through RNA intermediates. Retrotransposons pepper our genome, moving to future generations in egg and sperm. Many originate from viruses, and most are strongly mutagenic , inserting themselves inside genes or adjacent to genes. Some 70 human genetic diseases are strongly correlated with mutations caused by the “gymnastics” of these mobile genetic elements. The relevance of epigenetics became apparent when it was found that retrotransposons are heavily methylated and silenced epigenetically, possibly as a defense against their continuous onslaught ( 21 ).

In summary, epigenetics is of paramount importance in cellular differentiation, disease, and our defenses against endogenous and freely circulating viruses. But our understanding of its full importance in evolution is in its infancy.

EARLY LIFE EVOLUTION

The ponderous gap between amino acids on the one hand, and cellular organelles, cell membranes, and self-replicating macromolecules on the other, is too great for our current theories. We are very much in the dark about the origin of life.

Stanley Miller's famous experiments in the 1950s with electrical discharges, ammonia, methane, hydrogen, water vapor, and hydrogen sulfide were discounted in the 1990s, but came into favor again in 2008 when heat from hydrothermal vent ecosystems was considered. One current hypothesis states that RNA served as a hereditary template and catalyst, and that the ribosome evolved as a “machine” for building proteins, as it does today. Research suggests that a mineral in common clay may have played a role in the synthesis of RNA. Nevertheless, early life researchers are engaged in formalized guesswork.

Darwin thought that the “tree of life” had a last universal common ancestor , now known by the acronym LUCA . Today we believe that the trunk of the tree was a heterogeneous mix of genetic entities that traded genes wantonly by horizontal gene transfer. Vertical inheritance would evolve later. Curiouser and curiouser .

INFORMATION

When DNA was found to carry the genetic code, it was realized that the information it bears is its only function . This was hard for some biologists to swallow, as it didn't sound like biochemistry. No one had ever suspected that one organic molecule could code for others, eschewing a chemical function.

A digital code was clearly at the root of life. Whereas the English alphabet has 26 letters and the Greek 24, the DNA alphabet has 4 letters. Those letters spell out 3-letter words (codons) which tell the ribosome which amino acids to assemble into proteins. Was it significant—or a stunning historical accident—that binary computer science developed at the same time that we discovered the digital code of life?

Here is the heart, the pulsing core of complexity: the informational code that runs the engine of life, the complex calculus that changes under the steady beat of natural selection. The complexity of life is hardly irreducible—we hold it in our hands, and we are learning to manipulate it at the molecular level.

With selective death as a portal, evolution changes the information in the gene pool of a species, setting the stage for reproductive isolation and the origin of new species.

CONCLUSIONS

Across biological disciplines, natural selection has become accepted as a powerful and peerless explanatory principle. It is constantly scrutinizing the smallest differences among competing alleles and their phenotypic expression and has had ample time over Earth's history to shape the life forms we see around us and in the deep fossil record. The death of the individual has been its portal for changing the gene pool of a species. Through bricolage, or tinkering, it uses old parts and constructs new machines on the palimpsest of its canvas. Biology's informational code underlies the complex dynamics of life and has only recently yielded secrets that were undreamed of by Charles Darwin. Utterly without the knowledge we have gained since he published On the Origin of Species in 1859 , Darwin gave us one of the most profound explanatory principles in the history of science.

Acknowledgments

The author is grateful to George M. Diggs, Jr., PhD, Professor of Biology, Austin College, and to Kyle E. Harms, PhD, Associate Professor of Biological Sciences, Louisiana State University, for their helpful comments and recommendations.