EVOLUTIONARY THEORY

While the fixity of species was the generally accepted view before Charles Darwin, he was not the first to propose that evolution, understood as the transformation of one species into another, occurred. The ancient Greek philosopher Anaximander maintained that people had evolved from fish, and the zoologist and botanist Jean-Baptiste Lamarck (1744–1829), as well as Darwin's grandfather, Erasmus Darwin (1731–1802), were also proponents of evolution.

Lamarck, for instance, argued, in his Philosophie Zoologique (1809), that life resulted from ongoing spontaneous generation and that each lineage, beginning with simple forms, was driven by an inner tendency to complexity and perfection. On his view, more complex creatures belonged to older lineages, with our own the oldest. Adaptation and diversity was explained by the inheritance of acquired characteristics. Different environments caused organisms to have different needs in response to which they would use or not use their various organs: Use would cause an organ to develop, enlarge, and strengthen, whereas disuse would cause it to shrink, deteriorate, and eventually disappear. Lamarck believed that these changes were inherited by offspring, who would in their turn continue to adapt to their environment, thus leading to transformation of the lineage. The term Lamarckism (or Lamarckianism ) is now used to refer to the idea that a trait that was not inherited, but was acquired within the life of an individual, could be inherited by that individual's descendants. For the most part, this idea has been discredited, but there are cases in which something that satisfies the description occurs.

Darwin's Theory of Evolution

Darwin was not persuaded that evolution occurred by any of his evolutionist predecessors. The true history of his development of his ideas is controversial (Sloan 2005), but there were perhaps four main influences on him in this respect.

One was the Principles of Geology (1931), written by his mentor and friend, the geologist Charles Lyell (1797–1875), which Darwin read at the start of his famous five-year journey on the Beagle (1831–1836). Darwin was profoundly influenced by Lyell's methodological, as well as his factual claims. With respect to the former, Lyell was a uniformitarian. Broadly speaking, uniformitarianism is the view that the laws of nature have always been the same. For Lyell, this meant that geological features are to be explained by natural ("intermediate" not miraculous) processes that can still be observed to be in operation. Since he thought that these tended to bring about only slow and gradual change (e.g., a valley's formation from erosion), Lyell reasoned that the earth must be far older than the biblical 4,000 to 6,000 years. Although not a believer in evolution, Lyell also argued that investigation of the geological layers showed a continual introduction and extinction of species.

A second major influence on Darwin was his observation of the natural world, especially during his journey on the Beagle. His extensive collection of living and fossil animals, taken from many diverse parts of the world, and their analysis by experts in the relevant fields, convinced him (and through him, much of the scientific community) that, contra Lyell, neither fossil findings nor the present geographic distribution of species could be adequately explained other than by evolution. The task, as Darwin saw it, was to explain the evolution of species in a manner that was consistent with Lyell's uniformitarian principles.

At least by his own account (Darwin 1876/1958, p. 120) Darwin had help with this from a third major influence, An Essay on the Principle of Population (1798), written by the parson and social economist Thomas Robert Malthus (1766–1834). Malthus was no evolutionist; he believed that his understanding of population dynamics supported the view that populations could not change much. His concern was the possibility of social improvement, but his social theory was driven by an observation that applied to all species: Unchecked increases in population always outrun the means of subsistence. As Malthus says:

Through the animal and vegetable kingdoms Nature has scattered the seeds of life abroad with the most profuse and liberal hand; but has been comparatively sparing in the room and the nourishment necessary to rear them. The germs of existence contained in this earth, if they could freely develop themselves, would fill millions of worlds in the course of a few thousand years.
(I.I.5, 6th edition)

Malthus's message for the poor was that if they were to reduce their struggle for existence they must reduce their fecundity. According to Darwin, this struggle for existence between members of the same species suggested to him a mechanism by which populations could evolve.

A fourth influence on Darwin that may have been important was his familiarity with the artificial selection of plants and animals for breeding. Such selection showed that differential reproduction could produce a change in the distribution of characteristics in a population. It was believed that this had never produced a new species, and others had used this fact to support the basic fixity of species. However, Darwin argued that if so much change could be produced in the short time since human cultivation began, vastly more change could be produced given vastly more time. Of course, artificial selection involved human intentions; differential reproduction was guided by our choices and design still had a designer. Darwin's task remained that of finding a mechanistic process that could achieve similar, only much more impressive, results.

It is impossible to do justice here to the argument that Darwin assembled in support of his theory, but the main outline of his theory is remarkably simple. It begins with the observation that the individuals of a species vary slightly one from another. Since there are many more offspring born or plants germinated than can possibly survive, there is a struggle for survival within each species. Some of this is direct competition (e.g., for food or mates), but some is indirect (e.g., some individuals are better able to withstand disease or drought). The individuals that have variants that give them an advantage in this struggle will tend to survive longer and leave more offspring. And since offspring tend to resemble their parents, this means that beneficial characteristics will tend to be inherited more frequently than less beneficial characteristics. Over time, this causes a population to be better adapted to its environment and, especially if the environment changes, leads to a gradual change in the character of a population. The relevant periods of time are enormous ("we have almost unlimited time," "millions on millions of generations") so that, eventually, a species can be transformed to such an extent that it would be a new species.

According to Darwin, the main idea for his theory was formed in 1838 when he first read Malthus, but he did not publish his Origin of Species until 1859. Even then he was pushed to publish to avoid being preempted by the self-trained naturalist Alfred Russel Wallace, who in 1858 sent Darwin a letter that proposed a similar theory. Darwin's priority is well established, not only by the circle of scientists to whom Darwin had communicated his ideas but also in a summary of his theory sent in 1857 to the Harvard botanist Asa Gray. Darwin was the first to argue that natural selection was the principal cause of the diversity and adaptedness of organisms, and it was his extensive defense of the claim that evolution had occurred and could occur principally by means of natural selection that revolutionized biology.

Darwin's theory differs significantly from Lamarck's, but the views of the men were less distinct than those now attached to their names. Darwin did not believe in an inner tendency to complexity and perfection, and he argued that life had evolved just once or at most a few times. However, while he did not rate it as important as Lamarck did, he agreed that one mechanism of evolution was the inheritance of acquired characteristics.

Notice that Darwin's theory mentions only processes that can still be observed to be in operation. These processes, as he describes them, are also mechanistic: They do not involve a guiding intelligence. There is, as it is nowadays put, design without a designer.

Darwin's theory is also empirical and not, as sometimes alleged, tautological. The tautology problem was raised because the theory tells us that the fit will tend to leave more viable offspring than the unfit, although an individual's fitness is defined in terms of probable reproductive outcome. However, the theory is not tautological. Individuals within a species might not vary, they might not produce more offspring than can survive, and offspring might not tend to resemble their parents. Moreover, these facts might not lead to evolution, since the outcome also depends on any countervailing forces.

The Modern Synthesis

In modern terms Darwin's main thesis was that when there is heritable variation in fitness within species, species tend to evolve. While Darwin appealed to natural processes that can still be observed to be in operation, he did not adequately explain all such processes: In particular, he did not adequately explain inheritance or the origin of new variation, both of which are crucial to his theory.

The mechanism of inheritance was a problem for Darwin. His (pangenesis) theory involved the idea, popular at the time, that the material responsible for inheritance was blended in offspring. If that were so, an advantageous new variant would be diluted—a popular metaphor here is that it is like a drop of white paint mixed in a can of red—with the result that its benefit, and selection for it, would probably be dramatically weakened. It was Darwin's concern over this that inclined him in his later years to give more credence to Lamarckian inheritance.

Unfortunately, Darwin never knew of the work of the Austrian monk and botanist Gregor Johann Mendel (1822–1884), which provided experimental support for a particulate theory of inheritance. According to Mendel the material responsible for inheritance consisted of discrete units (now known as genes) that could be passed unchanged from one generation to the next. Mendel's work was mostly ignored during his—and Darwin's—lifetime, and it was not until it was rediscovered in 1900 that this major difficulty with Darwin's theory was removed. The combination of Darwin's theory of evolution by means of natural selection, Mendelian genetics, and mathematical population genetics is often referred to as the modern synthesis. (Some major figures in the development of the modern synthesis were T. H. Morgan, Ronald Fisher, Theodosius Dobzhansky, Julian Huxley, and Ernst Mayr.)

Explaining the origin of variation was also important; without a new source of variation, a population cannot change much beyond a redistribution of already existing characteristics. Biologists now understand how, despite a high degree of fidelity, genes are sometimes altered. Biologists construe the word gene in different ways, but a common construal is that a gene is a functional segment of the DNA molecules that constitute chromosomes. Alterations to such genes can occur when there are errors in copying them or when there is a crossing-over of segments of genetic material between matching pairs of chromosomes.

Crucially, the origin of new variation is random, not in the sense that any is as likely to occur as any other, but because whether a given mutation occurs is insensitive to whether it would be adaptive if it occurred. (This leaves open the question of whether there might be selection for an increase in the rate of mutation under some circumstances.) In this sense, mutation is random but selection is not random. Whether there is selection for a characteristic is sensitive to whether or not that characteristic is adaptive. Thus, selection is thought to be mechanistic, but not random or merely a matter of chance.

Darwin's and Mendel's theories form the basis of modern evolutionary theory, but neither has survived without modification. Darwin's support of Lamarckian inheritance has already been mentioned and a number of Mendelian principles have also been revised. For example, Mendel proposed that the units of inheritance were independently sorted during the formation of gametes (sperm and eggs), but it is now known that adjacent genes on a chromosome tend to stay together when gametes are produced (this is known as gene linkage). Since the early twentieth century, however, biology has provided overwhelming confirmation of the dual ideas that evolution occurs by means of (although not exclusively) natural selection and that inheritance involves (although not exclusively) genes that are usually passed unchanged from one generation to the next.

Some developments sometimes touted as radical revisions are better seen as refinements: For example, the theory of punctuated equilibrium, which proposes that long periods of stases in a lineage are punctuated by periods of rapid change, is consistent with Darwin's thesis that evolution occurs primarily through the gradual accretion of small changes: the rapid change of punctuated equilibrium is only rapid relative to the periods of stases: no major saltations are proposed.

Philosophical Issues

No sharp line should be drawn between issues in theoretical biology and philosophy of biology. Some issues of interest to philosophers have already been touched on. The following is an outline of a few others of special interest to philosophers.

The Adaptationism Debate

Biologists agree that natural selection is an important mechanism of evolutionary change, but there has been disagreement over how important it is. The biologists S.J. Gould and Richard Lewontin (1979) accuse some biologists of too readily assuming that every trait has an adaptational explanation (i.e., of assuming that each trait was selected because it was adaptive or contributed to fitness). Although the debate involves certain conceptual issues, and philosophers play a role in clarifying it (e.g., see Sober 1993, chapter 5), it is principally an empirical debate, though with widespread (including methodological) implications.

Evolution (at least genetic evolution) is now said to occur if there is a change in the proportional representation of genes or combinations of genes in a population, counting each individual's genetic makeup just once. Microevolution consists of such change within a species; macroevolution consists of such changes when they result in new species. Biologists agree that much genetic evolution is due to natural selection, but it can also be due to other causes. For example, mutation and migration can bring about a change in frequencies in a population. So can drift.

It is notoriously difficult to define the word drift, but the first thing to note is that both zygote (fertilized egg) formation and the selection operating on the resulting individuals are stochastic (probabilistic nor deterministic) processes, and it is this that makes room for drift. Just as a series of tosses of a fair coin can by chance deviate from a fifty-fifty ratio, genetic drift can occur either as a result of a chance disproportionate sampling of genes during fertilization, or as a result of a chance deviation from probable outcomes in survival and reproduction among the resulting individuals.

The potential for drift is increased when the population is small or the force of selection is weak. So it is, for instance, thought to have special importance in allopatric speciation, in which a small portion of a population becomes geographically isolated from the rest, and competition between almost equally or equally adaptive genes or nongene "junk" DNA (neutral selection). While drift is often spoken of as an alternative to selection, it is an aspect of its stochastic nature (Brandon 2005). Nonetheless, if a trait predominates due to drift alone, it is wrong to say that this was because there was selection of the trait, let alone selection for it.

Besides mutation, migration, and drift, there are other ways in which the evolution of a trait can require explanations other than or besides adaptive explanations. For example, even traits that were selected may not have been selected because they were adaptive. They might have been selected because of their special association with adaptive traits. Gene linkage is a way this can happen. Pleiotropy, in which a single gene has multiple phenotypic effects, is another. When a neutral or maladaptive trait has been preserved or proliferated in a population because of its link to a beneficial trait, it is called a piggyback trait or free rider. There was selection of it, but not selection for it, and only in the latter case are traits considered adaptations (Sober 1984, pp. 97–102).

It is an issue to what extent natural selection has the power to produce ideally adaptive outcomes. How often, for instance, do gene-linked and pleiotropic traits get severed in the long run? To what extent is natural selection playing catch-up with an ever-changing environment? To what extent do developmental and phylogenetic constraints, or the necessity of climbing only local adaptive peaks, restrict its capacity to move around in design-space?

The answers to such questions have interesting methodological implications. Most obviously, if natural selection tends to produce ideally adaptive outcomes, it will be fruitful to try to understand evolutionary products as ideally adaptive solutions to problems posed by a selective regime. In contrast, to the extent that it does not, the fruitfulness of that strategy is more problematic, although the construction of what are known as optimality models could still be useful, for example, in determining to what extent natural selection was involved (Maynard-Smith 1978).

While important questions are engaged in the adaptationism debate, it has often been more rhetorical than substantial. So it is important to stress that behind the heat lays some basic agreement. Contenders agree that natural selection is not the only agent of evolutionary change but they also agree that it is the source of complex adaptive change. As Gould says, when trying to reverse the impression created by his rhetoric:

May I state for the record that I (along with all other Darwinian pluralists) do not deny the existence and central importance of adaptation, or the production of adaptation by natural selection? Yes, eyes are for seeing and feet are for moving. And, yes, again, I know of no scientific mechanism other than natural selection with the proven power to build structures of such eminently workable design.
(1997, p. 35)

The Sociobiology Debate

The main reason the adaptationism debate has been so heated was its connection with attempts to explain human behavior and psychological characteristics by appeal to evolutionary history. A bitter debate over such attempts, one of the biggest scientific controversies of the twentieth century, began after the publication of Sociobiology: The New Synthesis (1975) by the Harvard entomologist Edward O. Wilson, and The Selfish Gene (1976/1989) by the English zoologist Richard Dawkins, which together marked the start of or brought into focus a new push by evolutionary theory into the domain of the social sciences.

Wilson's book discusses the social behavior of a wide range of species, beginning with ants and ending with humans. He suggests we should study ourselves as if we were anthropologists from Mars, bearing in mind evolutionary theory in doing so. He also offers bold and (as he acknowledges, speculative) adaptationist hypotheses regarding gender roles, the causes of war, religion, and such like. Dawkins explicitly distances himself from such claims, emphasizing (as Wilson also does to some extent) the significance of culture in our case. However, Dawkins does not refrain from colorful metaphors that undermine this distancing. In a famous passage, having talked about the origin of replicators in the primordial soup, he says:

Four thousand million years on, what was to be the fate of the ancient replicators? … Now they swarm in huge colonies, safe inside gigantic lumbering robots, sealed off from the outside world, communicating with it by tortuous indirect routes, manipulating it by remote control. They are in you and in me; they created us, body and mind; and their preservation is the ultimate rationale for our existence. They have come a long way, these replicators. Now they go by the name of genes, and we are their survival machines.
(1976/1989, pp. 19–20)

The issues raised relate to what used to be known as the nature versus nurture debate. That debate, put crudely, concerned the extent to which our psychological propensities were due to nature (genes) or nurture (environment). So put, however, the debate is ill conceived, because every trait is necessarily the product of both genes and environment. A better way to understand the debate is that it concerns the extent to which differences among individuals are caused by differences in their genes or in their environments (or both). The suggestion was that we would find psychosocial differences among individuals in our species, as well as between our species and other species, that were due to differences in genes for which there had been selection.

The response was vitriolic. Even sympathizers were often concerned about political implications. Critics blasted Wilson and Dankins—for being adaptationist, for proposing hypotheses that were neither tested nor testable, for being motivated by an ideological defense of the status quo, for being racist and misogynist, and for somehow being against free will and human dignity (e.g., see Rose, Lewontin, and Kamin 1984). They described the application of sociobiology to humans as "biological determinism" and proposed instead a position they called biological potentiality. The latter included the (patently true) claim that all our acts are within our biological potential and, something further, that there are no or virtually no significant task-specific psychological adaptations. On this view, evolution has endowed us with an impressive general-purpose intelligence and a capacity for culture and language, but it has done little else to shape our psychology. The latter is in a way the more absolute position: The claim that some social and psychological characteristics are (let alone may be) genetic adaptations is compatible with the claim that many are not. And those that are in part genetic adaptations might also be shaped by culture.

Evolutionary studies of human social and psychological characteristics now go under other names (e.g., evolutionary psychology). They remain controversial, but universities have increasingly devoted substantial resources to them. Today, the two sides have come together somewhat, with evolutionary theorists stressing the importance of culture, and with less emphasis on the other side on whether certain features are genetic adaptations as opposed to adaptations that can (whatever the basis of their heritability) usefully be understood by means of the concepts and methods developed in the context of evolutionary biology. Gene-culture coevolution has also become an important area of study. There is more discussion of how evolutionary studies should be conducted than whether they should be conducted (for more details, see Laland and Brown 2002). Nonetheless, the criticisms mentioned earlier are still repeated and are worth investigating.

The claim that some sociobiology is unduly adaptationist or inadequately tested is no doubt fair. However, there are poor practitioners in every field. Trivially, one should not too readily assume that a trait is a genetic adaptation, but one should not too readily assume that it is not either. Furthermore, the hypothesis, H1, that a given trait t is a genetic adaptation, competes with the hypothesis, H2, that t is not a genetic adaptation. So if one is not a scientific hypothesis because it cannot be tested, then the same must be true of the other.

Nor does it seem true that we cannot have evidence, one way or another, for such hypotheses. It is hard to assess claims regarding the evolutionary history of social behaviors and psychological characteristics, especially in the human case where ethical considerations constrain experiment more. However, relevant evidence can still be brought to bear. Consider the suggestion that male jealousy is an adaptation to the evolutionary problem posed by fertilization within the female ("Mama's baby, Papa's maybe"). To assess this claim, evolutionary psychologists appeal to analyses of fitness consequences, cross-cultural and cross-species comparisons, and relevant physiological findings (Barkow, Cosmides, and Tooby 1992). All such evidence can be put to poor use but it can also be put to good use. For example, cross-species physiological evidence relating to testes size and sperm competition suggests that human polyandry has been a significant factor. This evidence, while far from conclusive, helps (in combination with other evidence) to confirm rather than disconfirm the hypothesis, since it suggests that, had there been genes that predisposed human males to certain jealous behaviors, (ceteris paribus ) there would have been significant selection pressure favoring those genes.

The criticism that sociobiologists are ideologically motivated attacks the scientists rather than the science. People try to fit new information to their preconceived ideas, and scientists are no exception. However, we can ask if, assuming we do not start from a racist or sexist perspective, an evolutionary study of racial or sexual differences will push us in that direction. Here it helps to distinguish between political consequences and logical implications. The former may be worrisome even if the latter are not, and this can muddy discussion. However, what is clear is that attempts to understand certain social and psychological characteristics as evolutionary adaptations need not be used to defend any racist or sexist status quo.

Suppose it were shown, for example, that women tend, on average, to give more priority to their children than to their careers than men do, and that this difference is in part due to a genetic adaptation. Or suppose it were shown that men tend, on average, to be more competitive and aggressive (even violent) in attempting to acquire power and status and that this difference is in part due to a genetic adaptation. It can be argued from this that men will continue to have more power in the public sphere. However, this is a prediction, not a justification, and it is based on an assumption of nonintervention. One could also argue on the basis of such claims that educators need to consider moderating such difference, that there ought to be more work-based childcare, or that the human race would be better served if less aggressive women held more political power. It is not uncommon nowadays to see theorists from the left employ evolutionary theory to make their arguments (Singer 1999).

Finally, the issue of free will is a large one, but philosophers generally agree that it is a confusion to implicate it in this debate. It is a misunderstanding of the nature of the problem of free will to think that free will is enhanced by environmental as opposed to genetic causes of behavior. The problem of free will arises as soon as human choices are viewed in the context of their causes, whatever the nature of those causes. Nor is it right to see sociobiology or its descendants as committed to determinism. In general terms, determinism is the thesis that every event is causally necessitated by preceding conditions and the laws of nature. Neither sociobiology nor its descendants are committed to this or to variants that might plausibly be described as, more specifically, biological determinism. For instance, they are not committed to the view that if a person possesses a gene that was selected because it predisposes individuals to want multiple sexual partners then someone with that gene will be unable to resist the temptation to have multiple sexual partners. Desires need not be more irresistible for being genetic adaptations as opposed to cultural artifacts.

Concluding Remarks

A general defense of the study of social and psychological characteristics from an evolutionary perspective is not the same as a defense of particular claims about social or psychological adaptation. It is consistent with the claim that such a study will fail to establish that there are any significant social or psychological adaptations. However, many think that this research will provide (and has already provided) valuable insights, relevant to many areas in philosophy. For example, the study of the evolution of altruism and the evolution of emotions are of interest to ethicists, moral psychologists, decision theorists, philosophers of mind, and political philosophers.

Darwinian evolutionary theory has had a profound impact on our understanding of our species and on our worldview, even putting to one side its role in the social sciences. While it is remarkably well confirmed by innumerable findings, and now coheres with our understanding of genetics in innumerable detailed ways, it remains controversial in the public sphere for this reason. Philosophers have played an important role in this debate as well, particularly in discussions of the nature of scientific theories.

This entry has left many issues relating to evolutionary theory untouched. A great many other issues are important as well. For example, what is the role of teleology in Darwinian biology? How does a historical science, like the study of evolution, compare to the other natural sciences? What is the implication of Darwinian evolution for the idea that species are natural kinds? How should living things be classified? How best can the intertwined concepts of selection, fitness, and drift be understood? What is selected in selection? Can memes evolve by means of natural selection? Can cultures? A number of these issues are discussed elsewhere in these volumes.

See also Anaximander; Darwin, Charles Robert; Darwin, Erasmus; Darwinism; Determinism and Freedom; Evolutionary Ethics; Lamarck, Chevalier de; Malthus, Thomas Robert; Paley, William; Philosophy of Biology; Teleological Argument for the Existence of God; Wallace, Alfred Russel; Wilson, Edward O.

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Karen Neander (2005)