beatty - pluralism and panselectionism (1984)

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Pluralism and Panselectionism

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Pluralism and Panselectionism Author(s): John Beatty Reviewed work(s): Source: PSA: Proceedings of the Biennial Meeting of the Philosophy of Science Association, Vol. 1984, Volume Two: Symposia and Invited Papers (1984), pp. 113-128 Published by: The University of Chicago Press on behalf of the Philosophy of Science Association Stable URL: http://www.jstor.org/stable/192500 . Accessed: 06/12/2012 13:15Your use of the JSTOR archive indicates your acceptance of the Terms & Conditions of Use, available at . http://www.jstor.org/page/info/about/policies/terms.jsp

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Pluralism

and Panselectionism' John Beatty

Arizona State University

1. Introduction It is certainly not unreasonable to try to account for patterns of variation in nature in terms of evolution by natural selection. But there are more or less "avid" proponents of this way of understanding nature who are, according to critics, more or less reasonable in inverse proportion. Attempts to understand nature in terms of evolution by natural selection can, according to these be pushed too far. critics, Just how far such pursuits can reasonably be pushed is the central problem of this paper. I will discuss this issue in the context of an episode in the history of evolutionary biology that has been labelled by Stephen Gould, "the hardening of the evolutionary synthesis" (Gould 1980, 1982, 1983). In the course of the hardening of the synthesis, evolutionists attributed a greater and greater role to natural selection, and correspondingly less and less a role to alternative evolutionary agents--minimizing in particular the role of so-called "random drift". Gould-the-evolutionist considers the shift unreasonable, for which reason Gould-the-historian considers the whole episode incomprehensible. John Turner, an evolutionist of rather different persuasions, considers the shift eminently reasonable and likewise historically quite comprehensible (Turner forthcoming). Many of the rest of us who have thought a bit about the problem have conflicting intuitions in these regards. I hope to be able to sort out some of those conflicting intuitions here. 2. The Hardening of the Synthesis Before I get to the disputes at the heart of the hardening of the synthesis, though, I should perhaps first say something very general about disputes in evolutionary biology. Disputes in evolutionary biology, ecology--natural history in general--are rather peculiar affairs, in large part because natural historians are so reluctant to rule out anything altogether. Even the staunchest rivals in natural history are willing to "give or take" a few real exceptions to their

PSA 1984, Volume 2, pp. 113-128 Copyright (D 1985 by the Philosophy of Science

Association

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114 positions, and to admit a few instances where their opponents are in the right. As Gould and Richard Lewontin note, "In natural history, you generally do not support your favoured phenomenonby declaring rivals impossible in theory."'(Gould and Lewontin 1979, p. 585). Most natural historians are, at least in this sense, pluralists with respect to most issues. The snag is, as Gould and Lewontin proceed to point out: pluralism does not preclude polarization. Although natural historians are tolerant of alternative outlooks, they nonetheless tend to relegate all but one to "unimportance". Given a number of alternative outlooks, that is, natural historians often tend to polarize amongst themselves as to which is the "most important" point of view. Exemplary in this regard are disputes concerning the relative importances of the alternative "agents" of evolutionary change: mutation, migration, natural selection, and random drift (to name just a few). Ever since Darwin, evolutionists have haggled over the relative importances of these agents. Darwin and Moritz Wagner argued about the relative importances of migration and selection as agents of evolutionary change (see Sulloway 1979). In the early twentieth century, William Bateson and W.F.R. Weldon argued about the relative importances of mutation and selection as agents of evolutionary change (see Provine 1971). There have even been disputes among the proponents of the importance of selection as to what kind of selection is most important. There was, for instance, a long and complicated controversy between Theodosius Dobzhansky and H.J. Muller as to the predominance of selection in favor of heterozygotes vs. selection in favor of homozygotes (see Lewontin 1974). Representative of the interests of evolutionists in "ranking" the various agents of evolution is the title of a book written by the Hagedoorns in 1921: The Relative Value of the Processes Causing Evolution. These were not (are not) "all-or-none" issues. The disputants defended (defend) the importance of their favorite evolutionary agents without ruling the others entirely out of consideration. Darwin did not altogether deny the evolutionary significance of migration, nor Bateson did Wagner completely overlook the importance of selection. nor did Weldon ignore mutation. certainly did not ignore selection, Dobzhansky always admitted cases of selection in favor of homozygotes, and Muller always admitted cases of selection in favor of heterozygotes. But their pluralism did not (does not) preclude their polarization. In what follows, I will be discussing problems of pluralism and with regard to issues concerning the polarization, specifically relative evolutionary importance of random drift vs. natural selection. That requires that we first consider, briefly (very briefly), the main conceptual differences between evolution by random drift and evolution by natural selection (see Beatty 1984, Sober 1984, and Hodge forthcoming for more thorough analyses of the differences between drift and selection). Perhaps most simply put, evolution by random drift is a "matter of chance", in a sense in which evolution by natural selection is not. That is, we attribute the increase in frequency of a particular trait to natural selection when the

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115 possessors of that trait, because of the possession of the trait, leave a greater average number of offspring than possessors of alternative traits. We attribute the same change to random drift, on the other hand, when it is simply a matter of chance that possessors of the trait in question leave a greater average number of offspring than possessors of alternative traits. Sometimes, for instance, when the survival and reproductive abilities of possessors of different traits are not significantly different, possessors of one trait may just by chance leave more offspring on the average than possessors of alternative traits. So much for the main conceptual differences between evolution by random drift and evolution by natural selection. As for the disputes concerning their relative importances, proponents of the importance of evolution by random drift do not deny the importance of evolution by natural selection, nor do proponents of the importance of selection completely deny the importance of drift, but again, their pluralism does not preclude their polarization (see Beatty 1984 for a review of this controversy). According to Gould, the architects of modern evolutionary theory--the authors of what we call the "synthetic theory of evolution"--were originally genuinely pluralistic with regard to the importances of the various agents of evolution (including drift and selection). But, during the late forties, fifties, and sixties, they and their students came to emphasize more and more the importance of natural selection, to the extent that the other agents, like random drift, became fairly unimportant. Gould has thus far documented this change--which he refers to as the "hardening of the synthesis"--as it occurred in multiple-edition works of two of the most important architects of the synthesis, Theodosius Dobzhansky and George Gaylord Simpson (Gould 1980, 1982, 1983; see also Beatty forthcoming on Dobzhansky). William Provine has documented the same sort of change as it occurred in the writings of another great architect of the synthesis, Sewall Wright (Provine 1983 and forthcoming). Provine also considers briefly the same sort of change of heart as it occurs in the works of Ernst Mayr, yet another important spokesman for the evolutionary synthesis. Gould, and following him, Provine, have shown fairly convincingly that the attitudes of these evolutionists toward alternative evolutionary mechanisms--random drift in particular-became increasing sceptical as they "hardened" in favor of the importance of selection. But Gould, for one, finds it easier to document the hardening of the synthesis than to explain it. Of course, as an advocate of the original, more genuinely pluralistic pluralism, it is understandable that Gould finds the polarization that has since occurred unjustifiable, and at least in th