The theory of evolution, from Darwin to the Modern Synthesis formulation, provided a framework of explanative strategies to explain diversity and adaptation in the living realm. Considered on a large scale, Darwinian science advanced and justified two main claims: the Tree of Life, meaning that all the extant living species are always historical results of common descent, and the Selection hypothesis, meaning that one of the most important mechanisms to account for those transformations is "natural selection.'' Hence, it added to the ancient life sciences

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a new explanandum, e.g., phylogenesis, and a new explanans1—natural selection—which is also an explanatory resource for more traditional kinds of problems.

Of course, the consequences of the two main Darwinian claims were not recognized immediately; people were too much concerned by the two metaphysical issues of evolution versus creationism, and of the animal origins of man. It took a little less than a century to acquire the historical distance that enables one to rightly appreciate the novelty of Darwinism and this happened with the Modern Synthesis. For the Synthesis, population genetics has a central status within evolutionary thinking: historically, two of the founders of the synthesis, Fisher and Wright, were population geneticists, and some fundamental statements of evolutionary biology are enunciated in population genetics (Fisher's theorem, Hardy-Weinberg equilibrium, etc.); conceptually, the definition of evolution, as a change of the gene frequencies in the gene pool, lies in the field of population genetics. This essential feature of the theory was not conceivable in the time of the first Darwinians, since the gradualist view of transformism seemed to contradict the discontinuous vision of organisms as mosaics of traits that Mendelian genetics had presupposed. It is often and truly said that neo-Darwinism unified Darwin and Mendel, thereby superseding such an apparent conflict.2 Weissmann, by separating soma and germen and advocating that there was no transmission of the acquired characters, gave a clear meaning to the difference between Darwinism and Lamarckism and allowed his followers to regard only what is in the germen as the substrate of evolution, enabling the future integration of genetics within evolutionary biology. Moreover, Weissmann proved impossible the theories of heredity and variation maintained by many biologists and Darwin himself, according to which hereditary traits could be produced within the individual organism's cells and flow continuously from them. It then became possible for geneticists to propose mechanisms of heredity and variation in which the Darwinian theory of natural selection had only to assess the facts of heredity and variation, without being in principle committed to any theory of heredity (even if that was what Darwin actually did).

In order to grasp the new kind of epistemological problems brought by the two Darwinian contentions, it is quite useful to recall the features of the earlier biology that they replaced. The main explanans of diversity and adaptation before

1 In the usual vocabulary of the philosophy of science, explanandum means what is to be explained, whereas explanans means what explains the explanandum.

2 For an account of the conceptual transformations which led from Darwin to neo-Darwinism through the successful synthesis of Mendelian genetics and Darwinian hypothesis, see Gayon (1998).

Darwin was, as we know, the divine design, although other hypotheses were being proposed more and more often, especially the evolutionary theory of Lamarck, which was adopted by Geoffroy Saint Hilaire and many morphologists at the beginning of the nineteenth century. This design was invoked to account for some prima facie teleological features of the living world, such as the fine adaptation of organisms to their environment, or the fine tuning of the mechanisms of biological function, or, in the end, the proportions of individuals in various species and the geographical relationships between species. The divine design yielded simultaneously the individual designs of organisms, unlikely to be produced by the mere laws of physics, and the design of the whole nature that Linnaeus called the "economy of nature.'' The Selection hypothesis gave a powerful explanation of those two designs, since adaptations of organisms as well as distributions of species in a population were likely to be understood by appealing to the process of natural selection (even if other mechanisms like Lamarckian ones were also used by the first Darwinians3). Since the result of such a process is a Tree of Life, biologists justify the striking similarity of forms between different species of the same genus, or even different genera of the same family—this fact being an immediate result of the common descent of different members of a same taxon.

However, the rise of Darwinism did not mean a total shift of the relevant questions and tools in biology. Rather than deleting centuries of research in the science of life, Darwinism gave a new and coherent meaning to some admitted facts and descriptions. Instead of rejecting teleology outside science, it provided a way of interpreting teleological phenomena so that they did not depend on nonnaturalistic presuppositions, such as hidden intentions of the organisms or their creator; it kept the result of the traditional taxonomist's effort and conceived the systematic proximities in the classification of species as historical affiliations, as Darwin himself noticed at the end of the Origin of Species (even if, of course, the Darwinian views raised new questions and permitted new criteria and methods for systematists (Ghiselin 1980)).

So, evolutionary theory appears to us as the most successful and integrative framework for research strategies in biology. Before investigating the details of the epistemological challenges raised by the two Darwinian claims, it is therefore useful to situate evolutionary theory within the whole of biology. Here, Ernst Mayr's conception of explanation in life sciences will be of some help. In effect,

3 On the progressive extension of Darwin's theory, and all the slight nuances that made it very different from what we use now and the sharp picture here drawn, see Ghiselin (1969), Ospovat (1981), Ghiselin (1969) and Bowler (1989). Ospovat emphasizes the conditions for Darwinism in the work of morphologists like Geoffroy and Owen.

Mayr distinguished two kinds of causes as different answers to the question "why" (Mayr 1961). When asked: "why does this bird fly along the seashore to the south?,'' you can answer by pointing out its physiology, respiratory system, the diverse pressures on its wings, and the streams of air around it: this indicates the proximate causes of the bird's flight. But you can also answer by emphasizing that the way it takes to go to the south curiously corresponds to the old demarcation of the continents, and you will understand that this is a result of natural selection acting on this species of bird to improve its time of migration. This is the ultimate cause of the bird's trajectory. Notice that the first causes concern exclusively one bird, and each bird is concerned by them in the same way, meaning that they are generic causes. On the contrary, the ultimate causes collectively concern the ancestors of this bird and not the bird itself.

Notice also, and this will be of importance for all epistemological considerations, that the two kinds of causes do not answer exactly the same question: the former answers the question "why does the bird fly along the seashore (rather than being unable to fly)?'', whereas the latter answers "why does the bird fly along the seashore (rather than somewhere else)?'' This distinction, highlighted in another context by Sober (1986), means that the two kinds of causes are embedded in different explanatory strategies. As Mayr would remark, a complete biological explanation of a phenomenon makes use of all those strategies. And the two kinds of causes correspond to two kinds of biological discipline: on the one hand, as sciences of the proximate causes we have molecular biology, physiology, endocrinology, etc. while on the other hand, as sciences of the ultimate causes we have all those disciplines belonging to evolutionary biology: population genetics, ecology, paleontology, etc.

Having characterized evolutionary theory as a specific set of research programs within biology, and those programs being defined by the use of the hypothesis of natural selection, we can present some evolutionary problems raised by evolutionary theory. These will concern essentially the nature and the limits of the explanation by natural selection. Thus, I will reveal these two kinds of problems by picking out, in each category, one or two fundamental and currently debated issues. Then I will stress some important consequences of evolutionary biology upon philosophical theorizing about human nature.

We must nevertheless notice that all those issues involve both biological and philosophical considerations. They will sometimes be closer to theoretical biology and the methodology of biology than to philosophy but will sometimes include apparently pure matters of metaphysics that make no difference in empirical science. However, I claim that there is a set of problems raised by evolutionary theory that is of essential interest for philosophy, but that cannot be handled by the traditional means of a general philosophy of science—and that therefore must constantly appeal to considerations of theoretical biology. The fact that the Modern Synthesis has a unique character compared to other improvements in science (Shapere 1980), is surely one of the reasons for this peculiar status of the philosophical problems raised by evolutionary theory. However, given this special status, authors contributing to the debates are either philosophers of science, like Hull, Sober, Rosenberg, or Kitcher, or sometimes biologists who may have made major contributions to evolutionary biology, like Mayr, Gould, Maynard-Smith, Williams, or Lewontin. Philosophy of biology partly emerged from the dissatisfaction of philosophers of science with the logical positivistic program and their will to find new paths toward unsolved questions, and partly from the need, felt by biologists, of conceptual elucidations of the bases of their practice and of the consequences of their theories.

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