Limits of selectionist explanation The debate on adaptationism

Even if selectionist explanation is capable of rendering intelligible many nonbio-logical facts, no matter how far this capacity will be proved to extend, there

25 Sober (1988a).

remains the preliminary question of its limits within the field of evolutionary biology. Darwin said that the Tree of Life (first principle of Darwinism) was partly explained by natural selection (second principle), but that there were other mechanisms at work in its production.26 So, I now turn to the actual limits of selectionist explanation in explaining both the form of the Tree of Life and the peculiar features of organisms.

The question of the limits and conditions of selectionist explanation has been approached in what has been called the controversy about adaptationism. In a very influential paper, Gould and Lewontin described and criticized a too pervasive method in evolutionary biology, which they called "adaptationist program.'' In short, adaptationism means to think that the majority of the most important features of the living realm are explainable by natural selection (Sober 1994a).

There have been many attempts to clarify this adaptationism (Sober 1994a; Amundson 2001; Godfrey-Smith 2001b; Lewens forthcoming) 25 years of spandrels. In the cited title, this program, in summary, consists of atomizing an organism into discrete traits, and then building a selective history which establishes how each trait appeared as an adaptation to solve a peculiar problem. The authors contend both that we cannot atomize an organism in any way we want and that each trait allows the reconstruction of a selective history which should be testable. Too often, biologists, be they ecologists, paleontologists, or ethologists, create "just so stories,'' e.g., stories that invent a plausible scenario of the resolution of a supposed antique problem—the trouble being that there is no way to prove that such a problem existed.

I have no interest here in deciding the fate of adaptationism. In fact, the most salient consequence of the spandrels paper is the necessity of clarifying the implicit assumptions in the research on adaptations, leading to a real formulation of an adaptationist program, and forcing scientists to take sides on the question.27

But, if the consequence of the controversy concerns the extension of the theory of natural selection to human psychology and sociology, this was addressed

26 ''I am convinced that natural selection has been the main but not exclusive means of modification." (1859 p 6). First of all, sexual selection, that is here left aside. For its complex relationship with natural selection see Mayr (1965a) and then the current research on the evolution of sex (Williams 1975; Maynard-Smith 1978).

27 Maynard-Smith (1984), Dennett (1995) profess adaptationism with some reservations; in contrast see Gould (1980), Wake (1991). Orzack and Sober (2001) contain illuminating essays on the testability and the meaning of the adaptationist program. Lauder and Rose (1996) turned to an investigation of the meaning of adaptation in various fields of evolutionary biology. Dupre (1987) provides illuminating essays with a focus on the adaptationist commitments in evolutionary anthropology. Walsh (forthcoming) tries to make sense of the fate of the spandrels paper 25 years after its publication.

primarily through the questions of its limits within biology. And here the major concept pointed out by Gould and Lewontin is ''constraint.'' By this word, people mean various things and state of affairs, so some clarifications are needed.28 Constraints can be physical, such as the size of the genome, which entails some impossibilities for rapid metabolism within a cell in salamanders (Wake 1991); or, more obviously, an elephant cannot have thin feet. They can be of genetical order, for instance, when two genes are too close to be separated by crossing over during meiosis. They can be phylogenetic, meaning that selection acts on entities that come from a determinate history and have then inherited features difficult to change. For example, selection cannot adapt a respiratory system of vertebrates by creating a perfect respiratory device but has to modify the preexisting devices in fishes. A constraint is recognized by comparison across several species or clades: the fact that giraffes, like all mammals, have seven neck vertebrae like mice, indicates that the number of such vertebrae is a constraint, since we would expect number of vertebrae to be more proportioned to size (therefore more adapted). Moreover, phenotypes undergo genetic constraints, since there are epistasies and pleiotropies which entail that a trait will, in any case, be accompanied by another trait which has no adaptive relationship to it. Or constraints can be ''developmental''—this word needs some further comment. Those meanings, unfortunately, are not easy to distinguish in fact. But let us keep in mind that this issue of the limits to the power of natural selection (of a given trait) facing constraints is tightly bound to the other issue raised by Gould and Lewontin, namely the impossibility of atomizing living beings into discrete traits. The set of constraints in the end gives the conditions for a kind of form untouched by selection but always slightly altered and reshaped by it, which after the German morphologists Gould and Lewontin named Bauplan.

However, the emphasis on constraint should be best understood with reference to the recent evolutionary theory of development.29 Selection acts on variants, but not all variants are able to develop from a given gene pool. The evolutionary theory of development unveils the constraints on the rise of those variants upon which selection is about to act. For example, Wake (1991 pp 547549) showed that in all species of plethodontidae, the feet have got four toes instead of five in the ancestor from which they derived by miniaturization. This happened in unrelated lineages, as an alternative state of developmental mechanisms sharply distinguished with the five-toes producing state. Adaptive processes

28 See Raff (1996), Arthur (1997), Amundson (1994) Griffiths and Sterelny (1999) as introductory text.

29 Raff (1996), Arthur (1997), Gilbert et al. (1996), Gehring (1998).

select for size, and developmental constraints switch from five to four toes, independently of the lineage.

This example has nonetheless been challenged by Reeve and Sherman (1993) in one of the most convincing defenses of the adaptationist program. Their argument is rather simple: one can always appeal to selection even in Wake's case, since it is possible that there is a selection at an embryonic stage that eliminates variants that have more than four toes. So, the case for developmental constraints is not so easy to defend in the face of elaborated and differentiated concepts of natural selection.

Wimsatt elaborated the helpful concept of generative entrenchment,,30 meaning that, no matter whether or not selection acts on some feature, the fact that it has been built into the developmental program of a species at a rather early stage implies that it is easier, less costly, and more probable for selection to modify traits that appear later in development. Since to modify a very entrenched trait entails modification of numerous connected traits that are built on it, this modification is very likely to be nonadaptive, hence disregarded by selection. The more relative to the early formation of the organizational plan of a species a trait is, the more entrenched it is, so the less probable it is that selection will act upon it and modify it, hence it can be considered as a constraint for selection.31

Clarification of this debate has been provided by Amundson (1994,2001), by arguing that in the end, developmentalists and selectionists do not ask the same question. Selection is appealed to in order to explain why such and such variants arose and spread in the gene pool among a given set of variants; but developmen-talists, on the other hand, try to answer the question of the nature of this set of variants: why are there these variants and no other variants, and to what extent is the emergence of some variants unlikely or impossible? This, in fact, is not exactly a constraint on selection because selection is an explanans to a different expla-nandum than the one developmentalists are interested in.

This recognition of pluralism within the various explanatory strategies in evolutionary biology is likely to eliminate the false problems created by the adaptationist debates and leaves philosophers and biologists with the task of formulating and evaluating what could count as an adaptationist research program. Following Godfrey-Smith (2001b) and Lewens (in press), it is useful to define two large categories of adaptationists, the empirical one, who makes assertions on the pattern of the Tree of Life and the actual mechanisms of evolution, and the methodological variety of adaptationism, who contends that

30 Wimsatt and Schank (1988).

31 Reservations are made about the generality of entrenchment by Raff (1996); a general critique of the concept is to be found in Sterelny (2000 p 377).

biologists have to suppose, first, the presence of adaptations, even if they recognize later that in fact the predefined adaptational optima are not reached and that constraints exist.

However, notwithstanding conclusions about the compared values of the many adaptationist programs or hypotheses, there is a larger fundamental issue to be addressed as a background to this question, namely the conditions under which we are likely to recognize the effects of selection and its place in relation to the other causes of evolution. I will first address the question of phylogenetic inertia related to selectionist explanation, and then I will turn to the question of the status of genetic drift.

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