beatty - pluralism and panselectionism (1984)

Pluralism and PanselectionismAuthor(s): John BeattyReviewed 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. 113-128Published by: The University of Chicago Press on behalf of the Philosophy of Science AssociationStable URL: http://www.jstor.org/stable/192500 .

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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 critics, be pushed too far. 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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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 phenomenon by 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 did Wagner completely overlook the importance of selection. Bateson certainly did not ignore selection, nor did Weldon ignore mutation. 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 polarization, specifically with regard to issues concerning the 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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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 that sense, unaccountable. On the other hand, Gould is also a sharp and sensitive historian interested in the hardening of the synthesis as an historical phenomenon. He is right, after all, it is hard to understand. It is hard to see how the arguments usually advanced by those who seek selectionist accounts of everything, those to whom Gould refers as "panselectionists", stand up to the force of the pluralist appeal in natural history.



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The arguments in question concern the ability of selectionists to account for particular evolutionary changes that were originally attributed to random drift. The arguments are based, in other words, on the ability of selectionists to come up with causally significant differences in survival and reproductive capabilities where such differences were previously overlooked. Six or seven such "successes" were invoked by panselectionists in order to make their case for the super-importance of selection (see Beatty forthcoming, and especially Provine forthcoming for extended discussions of the selectionist successes). But it is not clear how these few cases, of all the particular evolutionary changes that had yet (and have yet) to be adequately investigated, could have decided the issue that polarizes panselectionists and the increasingly outnumbered advocates of the importance of drift.

Three such cases, which seemed particularly "telling" at the time, were the selectionist accounts of chromosomal variations in species of the fruitfly genus Drosophila, selectionist accounts of variations in color pattern in species of the snail genus Cepaea, and selectionist accounts of blood-group variations in humans.

Variation in chromosome morphology within Drosophila pseudo- obscura was originally attributed by Dobzhansky to random drift of the frequencies of those variations (Dobzhansky and Queal 1938). He did not originally suspect that the variations in chromosome morphology had any effect on the differential survival and reproductive abilities of their possessors--he did not originally suspect that the variations in chromosome morphology made any recognizable difference. He suspected instead that random drift might be sufficient to account for fluctuations in the frequencies of the chromosomal types. Dobzhansky was much influenced by Wright's theoretical work on drift at this time (e.g., Wright 1931, 1932).

In the early forties, though, Dobzhansky changed his mind about the importance of drift in accounting for the intraspecific frequencies of these chromosomal variations. Beginning in the late thirties, he discovered regular, seasonal fluctuations in the frequencies of the variations, suggesting that the frequencies were not fluctuating randomly, but were under the control of natural selection (see Dobzhansky's first report of these changes in Dobzhansky 1943). He soon obtained strong evidence that possessors of the different chromosomal types indeed had different survival abilities in different seasons (see, e.g., Wright and Dobzhansky 1946). Moreover, the selection pressures in question seemed to be quite large.

The case of the presumed importance of drift in Cepaea is similar in broad outline. Charles Diver, for one, had argued in the very early forties that the frequencies of various banding and color patterns in species of Cepaea were governed mainly by drift (Diver 1940). His and similarly minded views on the subject were severely criticized by A.J. Cain and Phillip Sheppard in the early fifties (Cain and Sheppard 1950, 1954; Sheppard 1951, 1952). They had found that the seemingly innocuous differences in color (yellow vs. brown) and banding pattern (0-5 bands) of the elytra of these snails were



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correlated with differences among the environments in which the snails lived. In particular, they found that the color and banding patterns most frequent in an environment were those that provided more camouflage in that kind of environment, thus better protecting the snails from their bird predators.

The blood group story is similar in certain respects. Wright, whose theoretical work on drift was so influential at the time, had himself suggested early on that blood group frequencies within human populations were governed mainly by drift. Differences between alternative blood groups were, he claimed in the early forties, of no adaptive significance "as far as known" (Wright 1940, p. 179).

The change of attitude that took place with regard to the insignificance of blood group variations was succinctly expressed by the human geneticist William Boyd in the mid fifties:

Realization of the preeminent role of natural selection in evolution has been slow. Until recently it was the fashion . . . to state that characteristics suitable for the classification of Man into races should be non-adaptive, meaning not influenced by selection, and the present author . . . was maintaining in 1940 (Boyd 1940) that the blood groups were suitable for racial classification partly because they were non-adaptive. That point of view has been completely abandoned . . . . (Boyd 1955, p. 37)

Among other things, workers found that possessors of different blood groups were susceptible in different degrees to different diseases. For instance, 0-group individuals were shown to be more susceptible to duodenal ulcers than either A- or B-group individuals, while A-group individuals were shown to be more susceptible to gastric ulcers and cancers of the stomach (Clarke 1961). No straightforward correlations between blood group frequencies and environments were established in the time period in question, but findings that different blood groups had different effects on survival led many investigators to expect that such correlations would be found.

In retrospect, Dobzhansky's selective account of chromosomal inversions in Drosophila has pretty well stood the test of time. But there has been considerable controversy concerning the selective accounts of color patterns in Cepaea and blood groups in humans. Maxine Lamotte has argued forcefully that selection alone is not responsible for Cepaea color patterns--that drift is also largely responsible (e.g., Lamotte 1959). And Luigi Cavalli-Sforza has successfully accounted for differences in blood group frequencies among different populations in northern Italy in terms of drift alone (e.g., Cavalli-Sforza 1969). Let us forget, though, at least for the time being, what has happened in the meantime.

Surely it had something to do with the fact that naturalists had once emphasized the importance of drift in the context of these cases that the opponents of the importance of drift also focussed on these cases. It was particularly important to proponents of the importance of selection in the fifties and sixties to display these cases as



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evidence of the shortsightedness of proponents of the importance of drift (see, e.g., Mayr 1963, pp. 204-214; Ford 1964).

Whether, and/or in what respects, so few cases could really have added-up to a significant case against the importance of drift is a matter that will be discussed in detail later. The fact is that these cases were thought to be devastating to those like Wright, who had strongly promoted the importance of drift. In 1950, Dobzhansky reported to Wright the victorious mood of the British selectionists in particular:

That you and your works have been annihilated in Oxford is well established by testimony of several high authorities. You seem to be the only person who has any doubts about it. (1) E.B. Ford announced it in a triumphant letter to Cain, a very capable (really capable) young Oxford instructor (or whatever they call it in Oxford), who works on snails and who has spent the last winter with Mayr and myself. . . . (2) Julian Huxley in the first draft of his speech which he delivered in Columbus last September has stated that the genetic drift has recently been shown to be an imaginary phenomenon. . . . (3) Darlington has spent a few days with us living in our house and he inquired whether or not I am satisfied that all your stuff is dead. He had no fixed opinion himself, but this is clearly the consensus of those who know in England or at least in the Cambridge-Oxford circles. (Dobzhansky to Wright, November 9, 1950, Sewall Wright Papers, to be deposited at the American Philosophical Library, courtesy of Sewall Wright and William Provine).

Such was the significance attached to the selectionist successes.

It is precisely the significance attached to the selectionist successes that is so incomprehensible to an evolutionist-historian like Gould. From his more pluralistic point of view, a few cases in favor on one or another evolutionary agent do not constitute legitimate grounds for considering that agent all-important. As Gould recites the line of reasoning that he ultimately opposes:

Empirical aspects certainly influenced the hardening. Once the pluralistic version of the synthesis had reemphasized classical Darwinism as a respectable alternative, the search for actual measures of selection and adaptation in nature intensified and succeeded. The British panselectionist school, headed by Ford and Cain (see Ford 1964, for example), presented many examples from butterflies and snails. More importantly, Dobzhansky, the key figure in the transition, discovered that his favorite example of potential nonadaptation needed to be reinterpreted in selectionist terms. In 1937, he attributed differences in inversion frequencies within natural populations of Drosophila to genetic drift, but he then discovered (see Dobzhansky 1951) that these frequencies fluctuate in a regular and repeatable way from season to season, and decided (with evident justice) that they must be adaptive. Still, we surely cannot attribute such a major change as the hardening of the synthesis entirely to induction from a few empirical cases.



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After all, no pluralist had doubted that selection regulated many examples, so the elegant display of a few should not have established a generality. (Gould 1983, p. 89)

How different is the recent assessment of the hardening of the synthesis by John Turner, an evolutionist in the British panselectionist tradition, who is also interested in the history of science. Consider the parable that Turner offers in defense of the hardening of the synthesis:

The empirical approach of the panselectionists to Wright's theory regarding the importance of drift had been what we could call the "fairies in the garden" argument. A man who maintains that there are fairies in the garden may rightly be challenged to take an independent observer to Julian Huxley's garden to watch for them for a few evenings. If their non-appearance leads him to defend his proposition on the grounds that this garden is ecologically unsuited to them, but that they may be in J.B.S. Haldane's garden, and if similar watches in Haldane's, Ford's and Fisher's gardens still fail to produce the little people, the general proposal is no more disproved than the infinite number of other fanciful hypotheses. Most neutral commentators would be inclined to think however that it could be for all practical purposes dismissed, pending a positive sighting. Likewise, repeated observations of rather strong selective effects, too strong to allow significant drift, or producing clear adaptive differences between local populations, leave an onus of proof on the supporters of Wright's theory. The rest of the world tends to take the issue as provisionally answered in favour of selection and to go out cultivating its own fairyless garden. (Turner forthcoming)

Of course, Turner loads the deck by associating pro-drift hypotheses with the ridiculous-sounding fairy hypothesis. But beyond that there is something appealing about his line of reasoning, which is disconcerting, because there is also something appealing about Gould's line of reasoning in support of the opposite conclusion.

3. Panselectionist Pursuits and the Evolutionary Community

There is indeed something reasonable about the hardening of the synthesis, though also something quite unreasonable. In order to distinguish between the respects in which the hardening of the synthesis was justified and unjustified, I will rely on two distinctions of an epistemological sort. These are, first, the distinction between pursuing an hypothesis and accepting it, and second, the distinction between what is reasonable/rational as far as individual scientists are concerned vs. what is reasonable/rational as far as the scientific community is concerned. It was, I will argue, rational for at least some individual scientists to pursue exclusively selectionist accounts on the basis of the selectionist successes of the forties, fifties, and sixties, although it was not rational for any of them to accept the hypothesis of the all-importance of selection on those meager grounds. Moreover, although it was reasonable for at least some individual evolutionists to invoke the



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selectionist successes of the time in pursuit of further selectionist accounts, it was not reasonable for the entire community of evolutionists to pursue exclusively selectionist accounts on those grounds.

Following Larry Laudan, we may distinguish between the "pursuit" of an hypothesis, and the "acceptance" of an hypothesis, roughly as follows (Laudan 1977, pp. 108-114). To pursue an hypothesis is to seek evidence in its behalf by trying to understand the world in terms of it. To accept an hypothesis is to assert its truth on the basis of the evidence already gathered in its behalf. One may accept the hypotheses one pursues, but one may very well pursue an hypothesis without ever accepting it.

As far as grounds for acceptance are concerned, degrees of evidential support--measured by some in terms of probabilities--seem the most relevant criteria. Acceptable hypotheses are just those whose degree of support or probability is higher than some particularly high degree. Hypotheses with less support or lower probability are unacceptable and perhaps even rejectable.

What makes an hypothesis worthy of pursuit is not the same as what makes it worthy of acceptance. Presumably, for instance, it would be rational to pursue an hypothesis prior to accepting it, in order to determine whether it is actually worthy of acceptance. There are assuredly a number of different criteria by which to judge whether an hypothesis is worthy of pursuit. Listing them and evaluating their relative merits is not my concern here. Rather, I want to consider just one criterion that may be pertinent to our story. Consider the sort of distinction upon which Laudan relies in order to differentiate grounds for acceptance from grounds for pursuit--namely, the distinction between overall degree of empirical problem-solving success and rate of empirical problem-solving success. Degree of empirical problem-solving success is something like degree of support, the most important criterion for acceptance. Rate of empirical problem-solving success, on the other hand, is very different. An hypothesis with an, as yet, low degree of problem-solving success (hence low degree of support) may nonetheless enjoy a considerable rate of success, and may be worth further pursuit on those grounds. For example, an hypothesis that has only been pursued for a short period of time may not yet have solved all that many problems, but may have solved a lot considering the short amount of time that it has been pursued. In that time, it may have proved worthy of further pursuit, not in virtue of any particular number of empirical successes, but in virtue of its rate of success.

At least in the case where two rival hypotheses are about equal in overall empirical problem-solving success, the one with the greater rate of success would seem to be the one with the greater empirical "promise", and at least for that reason the one most worthy of further pursuit. In such a case, the burden would seem to be on those who continue to pursue the hypothesis with the lower rate of success to explain their doggedness in doing so. (As we shall see later, though, there is something to be said in their favor.)



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Hopefully, at this point, it is more understandable why, or in what respects, panselectionism appealed to so many evolutionists in the late forties, fifties, and sixties. It was not because panselectionism proved itself the acceptable alternative during that period, but because it seemed to be the alternative most worthy of pursuit.

As far as grounds for acceptance of the panselectionist hypothesis were concerned, it is not at all clear how the selectionist successes of the forties, fifties, and sixties could have raised the degree of support or probability of that hypothesis enough to render it acceptable as true. Or, to put it another way, it is not at all clear how the successes of the selectionists could have lowered the degree of support or probability of hypotheses favoring the importance of drift enough to render them rejectable as false. Until some enterprising Bayesian convinces me otherwise, the successes of the selectionists during the period in question were too few, relative to the number of patterns of variations yet to be understood in terms of selection, to have changed significantly the degree of support or probability of the panselectionist hypothesis. Gould's incredulity at the hardening of the synthesis is understandable enough when the hardening is understood in terms of the general acceptance of the panselectionist hypothesis.

The case for the increased pursuit of selectionism is, however, another matter altogether. By explaining how patterns of variation previously attributed to drift could be better attributed to selection, panselectionists turned their own previous empirical failures into successes, and turned their opponents' successes into failures. Their rate of empirical problem-solving success thus increased at the expense of their opponents'. In fact, not only did their opponents' rate of empirical problem-solving success decrease, but their rate of failure increased. If anything (as Turner suggests), the burden would seem to have been on those who continued to pursue drift hypotheses--that is, it would seem to have been incumbent upon them to explain their faith in that particular pursuit (though, as I hinted above, there is something important to be said in their behalf).

Taking into consideration the distinction between acceptance and pursuit, then, we might say that Gould's assessment of the hardening of the synthesis is fairer than Turner's in one respect, and Turner's is fairer than Gould's in another respect. That is, to the extent that the hardening of the synthesis consisted in the general acceptance of the panselectionist hypothesis, Gould is right to criticize it and Turner wrong to applaud it. To the extent that the hardening of the synthesis consisted in the increased pursuit of the panselectionist hypothesis, on the other hand, Gould is wrong to dismiss it as entirely unreasonable.2

This sort of compromise is, however, only partly satisfying. Even though there seems to have been something right about the increased pursuit of panselectionism in the forties, fifties, and sixties, there still seems to have been something wrong with it. More specifically, although it seems reasonable enough that some or even



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many evolutionary biologists became committed to the pursuit of panselectionism in the time period in question, it does not seem reasonable that they practically all became committed to that pursuit at that time. In other words, the selectionist successes of the forties, fifties, and sixties may have been sufficient grounds for the increased pursuit of panselectionism, but those successes were not sufficient grounds for the exclusive pursuit of panselectionism. At least some--perhaps even many--evolutionists should have been encouraged to continue pursuing the importance of drift in spite of the selectionists' successes.

Before defending this intuition, though, it is worth distinguishing this intuition from another similar intuition. What the two intuitions in question have in common is the appeal of pluralism--in particular, the appeal of pursuing more than one kind of evolutionary account. But there is more than one way to promote pluralism in this regard. One can promote the idea that each and every evolutionist should pursue multiple evolutionary accounts. Or one can promote the idea that the community of evolutionists should pursue multiple evolutionary accounts. The difference is that the community can pursue multiple accounts all the while each of its members is pursuing one or another particular account.

As Hussain Sarkar has recently pointed out, there is an important difference between what is rational as far as individual scientists are concerned and what is rational as far as communities of scientists are concerned (Sarkar 1982). Sarkar himself suggests that we have given too much attention in the past to the rationality of individual scientists when it is the rationality of the community of scientists that should interest us most. Science is, after all, a community affair. Of particular relevance to the problems at issue here, Sarkar suggests that pluralistic directives ought to be aimed at communities, not individuals. Arguments for pluralism (such as the ones we will be considering shortly) constitute sounder cases for the promotion of community pluralism than they do for increasing the number of jack-of- all-trades scientists.

The intuition to be defended here, then, is not the idea that each and every evolutionist of the time period in question should have reserved some time for the pursuit of pro-drift hypotheses, but that the community of evolutionists should have encouraged the continued pursuit of pro-drift hypotheses by some of its members.

There are a couple of lines of reasoning--by now standard--that make pretty good sense of community pluralism, and that apply as well to the case in question. The first begins with the commonplace observation that the fortunes of scientific hypotheses change considerably over time, from bad to good and from good to bad. The most acceptable hypothesis at one time may be the least acceptable at some later time. Similarly, the hypothesis least worthy of pursuit at one time may be most worthy of pursuit later. The proponents of an underdog position may be so clever and persistent in interpreting phenomena in terms of their hypothesis that their position may eventually overtake the previously favored position with respect to



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instead to alternative selection hypotheses. "The" alternative to any particular selection hypothesis is not, after all, a drift hypothesis. Among the alternatives to any particular selection hypothesis are an uncountable number of other specific selection hypotheses, among which a selectionist may choose a new one to pursue.

So if the relative evolutionary importances of drift and selection are really at issue, then someone ought to be looking for drift, or else the issue will be resolved quite unjustifiably--by default, as it were--in favor of the all-importance of selection.

This is how I construe the pluralistic methodological directive implicitly invoked by Gould in his critical assessment of the hardening of the synthesis, and by Gould and Lewontin in their critique of present-day panselectionist tendencies (Gould and Lewontin 1979). That is, when Gould and Lewontin complain about the prevalence of selectionist pursuits in evolutionary biology, I consider the target of their complaints to be for the most part the community of evolutionary biologists. Individual evolutionary biologists are not so much their targets, but are instead the only kinds of instances to which they have recourse in order to point out the selectionist excesses of the community. Their point is, in other words, not that it is unreasonable for an individual scientist to be a stalwart selectionist, but that it is unreasonable for the community of evolutionary biologists to put all its eggs into the selectionist basket. The community of evolutionary biologists should support a variety of evolutionary pursuits--not just, selectionist pursuits. At least in this respect, the hardening of the synthesis was, as Gould suggests, unfortunate.3

4. Conclusion

I am about to stop now, just where the going gets tough, leaving almost completely untouched some major problems that arise in connection with the issues I've discussed. Having decided that the pursuits of the community of evolutionary biologists should be divided, we then face difficult decisions concerning how exactly to divide those efforts. In an unpublished manuscript on a related topic, Philip Kitcher has raised two specific questions that need to be answered in this regard: 1) "What is an optimal division of labor within a scientific community?", and 2) "How might the optimal division be maintained?" (Kitcher manuscript).

To give you an idea about what the first question involves, consider a tentative answer proposed by Kitcher. If the probability of one hypothesis is p and the probability of a rival hypothesis is 1-p, then p and 1-p are the proportions of the scientific community that ought to be pursuing the respective hypotheses. This unfortunately makes overall degree of empirical support the key criterion for deciding the extent to which an hypothesis is worth pursuing--and we have already considered reasons for not measuring worth of pursuit in that currency. But this solution at least illustrates the nature of the problem at issue.



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rate of problem-solving success, and even with respect to overall degree of success (e.g., Laudan 1977).

The point is that we should be careful not to dismiss underdog hypotheses "prematurely", i.e., prior to the time when they might have overtaken their rivals with regard to rate and even overall degree of problem-solving success. Underdog hypotheses should thus be pursued for some period of time even when they are not the hypotheses most worthy of pursuit. A scientific community can see to the continued pursuit of an underdog hypothesis by encouraging some of its members to take up that cause.

It is, of course, difficult to say just how long an underdog ought to be pursued before giving-up on it. It is difficult, for instance, to say just how long pro-drift hypotheses should have been pursued in the face of the selectionists' successes. Similarly, it is difficult to say just how many members of a scientific community ought to be committed to the pursuit of an underdog hypothesis--hence difficult to say just how many evolutionists should have been committed to the pursuit of pro-drift hypotheses during the forties, fifties, and sixties. But surely it would not have been reasonable for the entire community of evolutionists to have given-up on the pursuit of pro-drift hypotheses solely on the basis of the small number of selectionist successes during the time period in question. Surely a plurality of pursuits, at least at the community level, would have been a reasonable way to avoid the premature dismissal of the importance of drift.

Another reason why it might be to the scientific community's benefit to support the pursuit of hypotheses other than those most worthy of pursuit is that the rivalry might help enforce the standards of assessment applied to the favored hypothesis. Better cases are required for accounts that have rivals. Thus, for instance, panselectionists are not as likely to "get away" with meagerly substantiated selective accounts when proponents of the importance of drift are present to challenge those accounts.

This virtue of pluralism has been discussed at length by Paul Feyerabend, who has even formulated a principle of rationality embodying this line of reasoning: "Proliferation of theories is beneficial for science, while uniformity impairs its critical power" (Feyerabend 1975, p. 35). As Sarkar points out, though, the appeal of such a principle is diminished somewhat by the fact that it does not specify the level at which it should apply. The principle might thus be mistaken for a principle of individual rationality rather than as a principle of community rationality. But as long as the community supports a plurality of pursuits, there is no obvious reason why each individual scientist need proliferate his or her pursuits.

Perhaps the simplest and most compelling argument in favor of the pursuit of drift by at least some (if not many) members of the community of evolutionary biologists, though, is that drift will not be found otherwise. A community of evolutionary biologists sincerely committed to the pursuit of selection hypotheses will not entertain drift hypotheses when their selection hypotheses fail, they'll turn



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The second problem, of how to maintain the optimal division of cognitive labor, would perhaps be less of a problem if something like Adam Smith's theories about the labor force applied to the scientific labor force as well. It might, in other words, be the case that the optimal division of cognitive labor could be maintained naturally, simply as a result of scientists pursuing whatever lines of research they perceive to be in their best interests. Kitcher is exploring this possibility. If this "laissez-faire" solution fails, then a sort of "regulatory policy" might be required. Scientific associations and funding agencies might have to exert whatever influences are necessary to promote the proper division of labor (as if they didn't do that already).

These are difficult problems, to be sure. But they must eventually be discussed. For the way in which a community divides-up the pursuit of alternative hypotheses among its members is crucial to that community's success. Episodes like the hardening of the synthesis suggest that pursuits may not be naturally divided-up in any particularly optimal way. We must, then, be able to recognize when one type of account is being pushed too far, so that we can take steps to achieve a better division of labor. But this will require a good deal more thought about the functioning of scientific communities.

Notes

I am grateful to Ronald Giere, Philip Kitcher, Ernst Mayr, William Provine, and John Turner for helpful comments and criticisms. This work was sponsored in part by a fellowship from the Center for Interdisciplinary Studies at the University of Bielefeld, in Bielefeld, West Germany.

2The distinction between acceptance and pursuit is not unknown to scientists, though they use different terms to mark it. Scientists often distinguish, in particular, between accepting an hypothesis as true, and employing an hypothesis as a "working hypothesis". For instance, in their selectionist account of insect mating strategies, Randy Thornhill and John Alcock are careful to point out that they are treating the panselectionist hypothesis in terms of which they try to understand all insect mating strategies as a working hypothesis rather than one they have accepted as true (Thornhill and Alcock 1983).

3Various other interpretations of Gould's and Lewontin's critique have been outlined by Daniel Dennett (Dennett 1983; see also Lewontin's 1983 response). Two of the other speakers in this session, Ernst Mayr and Alexander Rosenberg, have also addressed Gould's and Lewontin's critique (Mayr 1983 and Rosenberg 1985).



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beatty - pluralism and panselectionism (1984)

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