The notion of Darwinian evolution of forms and entities, according to which all organic species descend from a common ancestor through various processes, among which natural selection is the most crucial, forms the framework of current biological investigations. Although biology has undergone deep transformations and conceptual shifts since 1859, the initial formulation of this modern idea of evolution is due to Charles Darwin’s On the Origin of Species. Darwin promoted a new interpretation of major biological concepts (e.g., adaptation or function) and raised new problems for researchers. The notion of biological evolution entails important consequences regarding the idea of time, especially concerning the time of biological processes and forms, as well as the use of temporal processes in biological explanations.
The first section of this entry sketches the historical context of the rise of evolutionism. The second section explains the major concepts of biological evolution, according to the contemporary version of Darwinism. The third section then focuses on some major controversies that recurrently appeared in the history of evolutionary biology, concerning the process and the pattern of evolution.
History of the Idea of Organic Evolution
The idea of an evolution of species traces back far prior to Darwin. However, Darwin is credited as the father of modern evolutionism because he argued not only for a pattern of descent between species—the tree of life—but for a process likely to explain this pattern—natural selection. Among prior tenets of what was called transformism, some 18th-century writers (e.g., Jean-Baptiste Rene Robinet, Benoit de Maillet, and Jean-Baptiste de Lamarck) held versions of evolutionism that explained evolution, yet through rather implausible mechanisms.
Several problems met by naturalists were reasonably stimulating biologists to conceive of a general evolution of species. First of all, whereas big differences among species are obvious, it is not so clear sometimes whether two individuals are of distinct species or of distinct varieties of one species. Therefore lots of classes held to be distinct species, hence distinctively created by God, might turn out to be varieties of one single species. A mechanism would then account for the differences among them. Carolus Linnaeus, author of the most general system of classifying species and proponent of the idea that species were distinctively created by God (fixism), yet faced in this way the problem of hybrids. His contemporary and rival George Buffon defined a concept of species in terms of the ability to interbreed and have fertile offspring, a concept that fared better than a purely morphological concept of species like that of Linnaeus but that was not generally applicable. Moreover, as Immanuel Kant famously explained in the Critique of Judgment, naturalists until the 19th century noticed more and more frequently that the types of various species are often quite similar to one another, as if, for example, all the vertebrates were variations on a same theme—what Johann Wolfgang von Goethe called “original type.” Etienne Geoffroy Saint- Hilaire in 1820 claimed that whereas the two major orders, vertebrates and arthropods (such as crustaceans), seem absolutely separate (an idea held by his rival Cuvier), they were actually variants of a same type, the arthropod living inside its spine and the vertebrate outside of it. Therefore, several thinkers, particularly from the school of Naturphilosophie in 19th-century Germany, conceived of a general evolution of species from the simplest to the most complex forms—though often a logical evolution of forms rather than a historical process—as an explanation of this pattern of similarity.
Religion has obviously been an obstacle to the acceptance of evolution. The Church claimed officially that the earth was only 6,000 years old, which makes a process of transformation of species inconceivable, because at the scale of human civilization no one had witnessed any such change. But this difficulty, which was still precluding Buffon from accepting explicitly the evolution of species (even if he would accept the transformation of varieties) slowly vanished during the 19th century, because, among other reasons, of the discoveries of fossils of unknown animals and of the advancement of geological theories of Earth. Charles Lyell, who was Darwin’s friend, wrote Principles of Geology (1830-1833), which claimed that Earth could have been shaped several million years ago.
For those reasons, a general audience became more familiar with the ideas of evolution. Just before Darwin, Robert Chambers wrote Vestiges of the Creation, a widely read book that sketched a picture of the evolution of organic forms without providing a scientific theory to support it. At the time, philosophers such as Herbert Spencer elaborated general theories of evolution, which in general relied on a formal scheme of complexification. Before him, Lamarckism, the most accomplished transformist theory before Darwin, appealed to two forces: complexification (which explains why, in the same genus, simple forms are likely to give rise to more complex forms) and adaptation to circumstances through inheritance of acquired characters (the latter is generally the version people think of when they refer to Lamarckism).
Evolution by natural selection is one example of simultaneous discovery in the history of science: Darwin, after a long journey on the HMS Beagle, came to this idea through reflections on the geographical distribution of species, on morphology, and on domestication, while at the same time naturalist Alfred Wallace had the same idea. This anecdote indicates that in the 1850s the time was ripe for evolutionary theories. Darwin and Wallace presented their results in 1858 in a joint meeting that anticipated the publication of On the Origin of Species, which Darwin wrote over several years and published in 1859, before revising it extensively in five successive editions. The book is “one very long argument,” and whereas most chapters use arguments from various fields such as embryology, morphology, biogeography, or paleontology to support the idea of evolution by natural selection, several other chapters are devoted to a rebuttal of objections that Darwin foresaw, such as the lack of intermediary forms in the fossil record, complex organs like eyes, and the evolution of instincts.
In general, however, many biologists were convinced by Darwin’s demonstration of an evolution of species. The general audience was more reluctant because of religious reasons. However, the fate of Darwinism is more concerned with the reactions to the process hypothesized by Darwin to account for evolution, that is, natural selection. Darwin was indeed pluralist regarding the processes generating evolution; for example, he accepted Lamarckian inheritance of acquired characteristics. The major question then was the relative impact of those several processes on evolution, and this is perhaps the main issue that biologists who followed Darwin have had to address.
What we call Darwinism might have been born when German biologist August Weissmann, working on heredity, conceived of a separation between somatic cells and germinal cells, that is, the characteristics of an individual (and likely to change) and the characteristics that the individual inherited from his or her parents and passed to his or her offspring. This prevents any inheritance of acquired characters and leaves natural selection as the most plausible general mechanism of evolution.
The history of Darwinism in the 20th century means first the modern synthesis, followed by neoDarwinism in the 1930s. Briefly stated, natural selection, according to Darwin, sorts individuals that vary and have various offspring. The process of selection will occur no matter how the variation is caused, so Darwin’s theory was neutral regarding an explanation of hereditary variation (i.e., why all offspring of a couple of zebras are alike, as zebras, and different, as differing to some extent from their parents). But in the 1900s, Mendelian genetics came into play. At first sight, genetics and Darwinism seem at odds, because Darwin was talking of evolution through selection of small differences, whereas genetics treats combinations of discrete characteristics, which seems nongradual. Hence, a controversy opposed Darwinians and Mendelians until the 1920s. Then, Ronald Fisher, Sewall Wright, and
- B. S. Haldane showed that by devising probabilistic models of the evolution of genetic frequencies in populations (a field called population genetics), far from opposing Darwinism, Mendelian heredity is an explanation of heritable variation that makes natural selection necessary and powerful. Mutation of genes and recombination during meiosis and fecundation (in the case of sexual reproduction) provide the variation upon which selection operates. This synthesis between Mendelism and Darwinism is the origin of today’s biology. Later, such synthesis was extended to systematics (with Ernst Mayr) and paleontology (with George Simpson). However, the preeminence of population genetics in the modern account of the processes of evolution entailed that evolution is now conceived of as the “change of gene frequencies in a population,” whereas the first Darwinians were thinking more loosely, in terms of change of organic forms.
Concepts in Darwinian Evolution
The Concept of Natural Selection
Darwin thought of natural selection as the result of the “struggle for life”: Because resources generally are rare in an environment and because the rate of increase of a population generally exceeds the availability of resources (an idea that he famously took from Thomas Malthus’s Essay on the Principle of Population, 1798), it follows that only the individuals who are better equipped to obtain resources will survive and reproduce, being then likely to pass those abilities to their offspring. The frequency of those traits supporting those abilities increases, and this continued process explains a general change in the species, which finally leads to a novel species (some individuals being so different from the initial ones, or so distant, that they could no longer interbreed with them). More concretely, this competition (mostly between members of the same species) consists in procuring food, escaping predators, and finding sexual partners.
A more abstract formulation of natural selection, less tied to this biological case, can be given. Following Richard Lewontin, any population of individuals satisfying three conditions will undergo natural selection: Those individuals vary regarding some traits (variation); they have offspring that vary from them and from one another, in a way that the variation between offspring and the mean of the population positively correlates with the variation between their parents and the mean of the population (heritabil- ity); and those properties according to which they vary are relevant to the expectation of their having some number of offspring. (If several organisms replicate and vary, but the properties transmitted have no consequences upon their probability of reproduction, there will be no selection at all.) This third property can be called fitness. Whereas still intuitive for Darwin when he talked about “survival of the fittest” (following Spencer, and wanting to reject any intentional connotation of the word selection), fitness is now a technical term that means both survival and expectancy of offspring. Defining the fitness of a trait implies assuming that this trait somehow correlates with the offspring expectancy.
What the selected individuals really are is not relevant in this formulation. Hence there logically can be selection of organisms, which was what Darwin thought, but also of groups, of species, and, at a lower level, of genes. One of the most debated questions in the philosophy of biology, indeed, is the “units of selection” controversy, namely, the question of what, among those kinds of things, are the ones by which selection takes place. In Adaptation and Natural Selection (1966), George Williams argued that selection apparently acting on groups—for example, altruistic behaviors that seem to increase the well-being of the group while threatening the organisms that achieve them—can in fact be explained in terms of selection acting on individuals (organisms or genes). Kin selection, according to William Hamilton, is a selection acting on one gene carried by several individuals: Selection will retain an organism that acts against its “interests” if the consequences can favor one of its kin. On this basis, Richard Dawkins has argued that natural selection indeed acts upon genes rather than organisms. Genes do “replicate”—they are “replicators,” in Dawkins’s language—but the rate at which genes successfully replicate depends upon the interactions among organisms in terms of reproduction. Philosopher David Hull suggested another formulation of selection: Natural selection is the process according to which replicators differentially reproduce due to the interactions of entities named interactors (in the usual case of biology, it is the organisms). This formulation suggests that natural selection can occur among various objects to the extent that they replicate; for example, Dawkins writes of cultural selection, because cultural entities also seem to replicate.
Explaining Through Natural Selection
The diversity of species (in time and in space; e.g., why there are kangaroos in Australia but not in South America) and the adaptation of organisms to their environments are explained through Darwinian evolution. First, the taxonomy of species receives a historical interpretation. The only diagram in On the Origin of Species (Fig. 2) represents a pattern of branching between species, or genera, or any biological taxa. All the taxonomic relations (being part of the same genus, etc.) were reinterpreted by Darwin in terms of history: The closer two species are in a morphological taxonomy, the closer they are in the temporal sequence of evolution.
The traditional explanation for adaptation referred to the Divine Will or a Providence that would account for the fit between organisms and their environment—for example, the beak of certain species of finches that fits to the depth of the holes within which they chase insects. The Darwinian explanation of adaptation is natural selection: Such a process obviously led to the fit of the beaks with the holes, and in the same time, to the divergence of the Galapagos finches into several species, all being specialized in one kind of hole and characterized by one length of beak. This Darwinian example illustrates how natural selection accounts for both diversification and adaptation of species.
Natural selection provides also a way to escape the suspicion of teleology that constantly assaulted biology. Modern natural sciences exclude explanations in terms of intentions or goals, because by principle nature has no desires. However, talking of functions of organic parts means that those parts are here in order to do such and such: Such a proposition seems teleological. Although not really embarrassing for biologists, this problem puzzled philosophers of science—especially when funding science for research on concepts in theology was no longer admitted. Kant devoted half of the Critique of Judgment to the problem of legitimizing teleological judgments in life sciences without appealing to a Creator. Yet in the Darwinian framework, functional statements do not object to naturalism: philosophers Larry Wright and Ruth Millikan suggested interpreting “the function of X is Z” as “X has been selected because it was doing Z,” which does not appeal to any transcendent intention. Natural selection might legitimate the functional talk traditional in biology.
Proximate and Ultimate Causes
The emergence of evolutionism entailed a whole transformation of biology. During the rise of molecular biology, after James Watson and Francis Crick’s discovery in 1953 of DNA as the substance of genes, Mayr reflected upon the synthesis to which he contributed. He argued that causal explanations in biology can take two forms: proximate and ultimate. Proximate causes are the processes and events that cause a feature in the life of organisms; for example, the “genetic program” of a certain bird accounts for its migrating behavior, and the physiology of its muscles accounts for the way it flies. Molecular biology, physiology, biochemistry, and embryology unveil proximate causes and constitute what is called functional biology. Evolutionary biology aims at accounting for why the organism is likely to be the way it is; it searches for the evolutionary history that led to the genetic program embodied by the migrating bird. Those ultimate causes extend far prior to the existence of the organisms or species considered. Paleontology, behavioral ecology, ecology, systematics, and population genetics constitute the main disciplines of this evolutionary biology. Mayr added that whereas proximate causes amount to explanations of the same kind as those usually found in physics and chemistry, ultimate causes require another kind of explanation, as they rely mostly on natural selection and have to integrate knowledge of history as a background condition. This of course makes evolutionary biology the core of biology, upon which relies the specificity of biology regarding other natural sciences. This implies that biology has an irreducible historical dimension, contrasting with natural sciences that search for unhistorical laws or correlations, striving only for what philosophers, following Ernst Nagel, call “nomothetic explanations.” Moreover, most of the conditions of evolution are themselves products of evolution (e.g., the process of fair meiosis assumed by all the Mendelian rules). The science of organic evolution therefore required that scientific explanation consist also in depicting contingent temporal processes.
Form and Function
In 1916, historian Edward Stuart Russell published Form and Function, in which the history of biology is depicted as a fight between two general approaches to living phenomena: a focus on function versus a focus on form. Those features—forms and functions—are obviously both proper and exclusive to living beings (brute matter does not display transmitted forms). Russell saw the famous debate that opposed Cuvier and Geoffroy Saint- Hilaire at the natural history museum in Paris in 1830 as one major episode in this long-standing debate. Geoffroy, arguing in favor of one general type of organisms realized in all orders, was a tenet of form biology, whereas Cuvier was interested primarily in functions and proposed the principle of “conditions of existence,” according to which all functions in an organism must be feasible and compatible. For this reason, because one could not change one function without altering the whole and then making the organism unable to fulfill the principle of conditions of existence, Cuvier opposed Lamarck’s gradual transformism. On the other hand, for Geoffroy the general types are fine-tuned only by local adaptations, but their main morphological rules are something universal in the living realm, hence not connected to specificities of the various environments within which various species came to life: For this reason, adaptation cannot account for those rules. The spine, for example, pervasive across all order of organisms, exemplifies such a formal feature.
In On the Origin of Species, Darwin considers this debate from the viewpoint of evolution by natural selection. He admits that there are two general principles of biology: conditions of existence (Cuvier) and unity of types (Geoffroy Saint- Hilaire). He inquires into their connection, namely, whether functions of organisms, shaped by natural selection, are the main factors accounting for the various features of the species or whether some general formal features, independent of the successive environments of organisms but historically transmitted, are the major constraints on the history of life. He answers that the principle of the conditions of existence redounds to natural selection, whereas unity of type specifies the stable persistence of organic forms. However, because those inherited forms are themselves constituted through evolution by natural selection, those types in the end rely on the action of natural selection and the unity of type gets subsumed under the conditions of existence principle. This means that Darwinian biology subordinates form biology to function biology.
Major Long-Standing Issues
in Evolutionary Biology
Issues Relevant to the Process of Evolution
Even if selection is acknowledged as one of the main causes of evolution—Lamarckian evolution being excluded—other mechanisms have been conceived of by biologists. One classical formulation of the question of evolution in population genetics is analogous to mechanics, as argued by philosopher Elliott Sober in his 1984 work The Nature of Selection. Considering some varying traits in a population, geneticists first write the equations that would rule the change of gene frequencies in a pure Mendelian case with no selection, no mutation, and no migration—this is called the Hardy- Weinberg equilibrium. Then they compare it to the actual case. Any deviation from the expected equilibrium frequencies should have a cause, possibly natural selection. Yet, Sewall Wright in the 1930s showed that in small populations, a sampling error occurs that can lead to the fixation of some genes independently of their selective value. This “random drift” would disappear in infinite populations, but because real populations are always bounded, random drift surely matters in the actual evolution of gene frequencies.
Thus apportioning the causes of evolution, especially selection and drift, is a constant problem faced by population geneticists. Several important controversies in the course of evolutionary biology revolved around such issues. At the origin of the modern evolutionary synthesis, Ronald Fisher argued against Wright about the role of drift. If drift is important, as claimed Wright, then the mean fitness of populations is not always maximized, whereas Fisher opposed a mathematical formula he proved, the “fundamental theorem of natural selection,” meaning that the mean fitness necessarily increases until genetic variance is exhausted. Deciding this point implies an empirical knowledge of the size of natural populations. Later, the question involved an investigation of the reasons of genetic variability—polymorphisms— in populations. Biologists such as Theodosius Dobzhanski and Hermann Müller, among others, forged theories to account for this maintenance. Later, Motoo Kimura vindicated the so-called neutralist theory of evolution, claiming that most evolution at the level of nucleotides (the molecules composing the DNA) is neutral, due to drift, because most of the mutations of nucleotides are selectively neither advantageous nor deleterious. Although theoretically important, this theory does not truly oppose Darwinism because it is concerned only with the molecular level and leaves intact the idea that the evolution of genes themselves and of phenotypes is due to selection. However, the neutralist theory emphasized that evolution is a constant process occurring at many levels and according to various mechanisms.
It is often difficult to consider the evolution of one population of one species, because the ecological fates of several species are tied—each one defining selective pressure for the others. Cases of parasitism and of mutualism (e.g., between figs and wasps, ants and plants, humans and intestinal bacteria) belong to a general study of co-evolution, which accounts for innumerable features of the organic world.
Other important controversies arise when we turn our gaze from microevolution, which concerns rather short timescales and relatively unmodified environments, to macroevolution, or evolution at a higher level, concerning, for example, the appearance of new classes of organisms. It has been debated since the founding fathers of the modern synthesis whether macroevolution is microevolution at a larger scale or whether it requires novel principles. Simpson and Mayr defended the former position, because they thought that an appeal to novel principles (e.g., macromutations) would threaten Darwinism. Biologists such as Stephen Jay Gould, Niles Eldredge, Elisabeth Vrba, and Sean Rice consider natural selection of species, in which case the properties selected (e.g., the geographic range of a species; the genetic variability) are properties of the species itself, unlike the mechanisms accounting for microevolution, all of which have to do with traits of individuals.
But the main challenges concerning this question of macroevolution came recently from the evolutionary theory of development, or Evo-Devo. In microevolution, natural selection shaping adaptations for some functions is plausibly the main cause of evolution. This exemplifies the Darwinian bias in favor of function biology. However, in The Changing Role of the Embryo in Evolutionary Biology (2005), Ron Amundson argues that the adherents of form biology constantly opposed this Darwinian demonstration of the supremacy of function biology. Their challenge gets more consistent because they invoked some novel style of laws of forms that scientific investigation has been pinpointing for 3 decades. Briefly said, neo-Darwin- ians thought that mutation (and recombination) forms the material for natural selection. They separated two ideas traditionally joined: inheritance (i.e., transmission of characters from parents to offspring) and development (i.e., the ontogenetic process of an individual). Selection acts on traits, no matter the process through which the individual came to display those traits, so development seemed relatively external to evolution. Yet, some developmental theorists emphasized that development can both constrain and provide variations by changing the rhythm or order of the process, as Gould summarized in his classic 1977 work Ontogeny and Phylogeny. Evo-Devo researchers contend that the changes relevant to macroevolution, for example, key innovations like the wings of insects, or the thermoregulation system of mammals, involve the effect of developmental constraints and are not understandable solely as effects of natural selection acting on punctual mutations in the DNA. (But for them evolutionary theory is therefore more concerned with explaining across-taxa features, like the vertebrate limb, rather than change within taxa and thus adaptations.) This debate is obviously related to the set of problems posed by the understanding of the phylogenetic pattern in general.
Issues Relevant to the Pattern of Evolution
The tree of life raises three types of questions, all crucial to current evolutionary biology: the shape of evolution, its orientation, and finally its origins.
Gradualism and Discontinuities
For Darwin, evolution of the species was a gradual branching process. Neo-Darwinism emphasized gradualism, as variation would rely on small mutations. For this reason, neo-Darwinists have constantly been puzzled by discontinuous evolution, for example, key innovations. In the 1970s, paleontologists such as Gould and Eldredge challenged the general gradual view of phylogeny, arguing that changes in evolution display a discontinuous pattern: a fast (at the geological timescale) burst of novelty and a very long period of stasis, with only minor modifications possibly due to adaptation to local conditions. This theory of punctuated equilibria, albeit neutral regarding the mechanisms of evolution, fits with theories from Evo-Devo (held by Gould), which contrast very big changes relying on transformations in developmental processes with minor changes due to selection on point mutations. This issue rests mostly on the interpretation of the fossil record: Darwin claimed that its lack of transitional forms was due to geological reasons, whereas adherents of punctuated equilibria claim that the record is, as such, quite reliable and constitutes evidence for the stasis-burst schema of macroevolution. In this case, macroevolution would not be easily reducible to mechanisms of microevolution, and neo-Darwinism in general—since its core is population genetics— would have to be qualified. The emergence of the most general plans of organization (e.g., the one taking place in the Cambrian and explaining most of the extant phyla) is not accountable solely on the basis of micro-evolutionary principles.
In 1995, John Maynard Smith and Eors Szathmary initiated a theory of the major evolutionary transitions. Those are the fundamental events in evolution, through which the forms of inheritance and replication changed. Replicating macromolecules, single cells, multicellular organisms, social organisms, and organisms with language are the steps of this evolution. Each time, natural selection both contributes to the transition and becomes changed through it, because new selectable entities arise. In this perspective, evolution by natural selection is not absolutely tied to genes or life, and extends from molecule to talking beings. Such a globalized theory casts a new light on the problems of discontinuity in the mere history of life. Cooperation between entities (e.g., genes in chromosomes or insects in colonies) is a pervasive pattern of explanation of those transitions and requires understanding how natural selection could favor cooperative behaviors while selfish defection would be at first sight selected for.
A common view of evolution sees it as oriented toward greater perfection and achievement. Yet this is precluded by the very logic of Darwinism, for which natural selection optimizes populations regarding their environments—hence, two organisms of two different species, belonging to different environments, cannot be compared or considered as stages of a continuous improvement. At the same time, it is difficult not to notice in a given branch of the tree of life (e.g., the vertebrates or the mollusks) trends of increasing complexity: diversification of functions, finessing of detecting devices, increase of some quantities related to the size of genes, and so forth. Darwin was ambiguous regarding this issue: Although he opposed the notion of absolute perfection, he was led by common intuitions about progress in evolution. The major conceptual issue, however, is still to figure out a concept of complexity likely to capture those contrasted intuitions. Biologist Dan McShea has shown in recent papers that a “complexity” thought in pure formal terms (diversity of parts with no functional considerations) makes visible some phylogenic trends in increasing complexity. This notion is yet far from the intuitive one, and it seems, then, that no theory could decipher in the history of life this constant progress culminating in human species.
Evolution by natural selection commits one to say, like Darwin, that all forms of life came from one single organism (a position that irritated the defenders of religious orthodoxy). The fact that all species share elements of the same genetic code provides more evidence of this single evolutionary history. From a Darwinian viewpoint, the issue is the genesis of an entity satisfying the conditions of natural selection. This problem involves both chemists and paleontologists—people seeking traces of what happened and people conceiving processes that could have happened. Within evolutionism, questions of origin are crucial: origin of sexual reproduction (why humans pass on only 50% of their genes to the next generation, whereas organisms that reproduce asexually pass on 100%), origin of mind and of culture. Whereas no theory is completely satisfying, natural selection clearly plays a fundamental role in those events.
See also Archaeopteryx; Coelacanths; Darwin, Charles; Dinosaurs; DNA; Evolution, Cultural; Extinction; Extinction and Evolution; Extinctions, Mass; Fossil Record; Fossils, Interpretations of; Fossils, Living; Ginkgo Trees; Haeckel, Ernst; Huxley, Thomas Henry; Life, Origin of; Paleontology; Progress; Saltationism and Gradualism; Trilobites
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