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Philosophy of Evolutionary Biology: Volume I

The Empirical Nonequivalence of Genic and Genotypic Models of Selection: A (Decisive) Refutation of Genic Selectionism and Pluralistic Genic Selectionism

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Brandon, RN; Nijhout, HF
January 1, 2017

Genic selectionists (Williams 1966; Dawkins 1976) defend the view that genes are the (unique) units of selection and that all evolutionary events can be adequately represented at the genic level. Pluralistic genic selectionists (Dawkins 1982; Sterelny and Kitcher 1988; Waters 1991) defend 278 ROBERT N. BRANDON AND H. FREDERIK NIJHOUT the weaker view that in many cases there are multiple, equally adequate accounts of evolutionary events but that always among the set of equally adequate representations will be one at the genic level. There have been many arguments against these views (e.g., Wimsatt 1980; Brandon 1982; Sober and Lewontin 1982; Lloyd 1988), but the debate continues to animate contemporary philosophy of biology (e.g., Lloyd 2005; Waters 2005). A (perhaps the) reason for this is that the refutations have primarily relied on philosophically contentious views on scientific explanation and causation-views their opponents have not been willing to accept. What both sides in this debate have accepted is that the genic and higher-level accounts are empirically equivalent (but see Brandon and Burian [1984, introduction to part II] and Godfrey-Smith and Lewontin [1993]). This paper will show that that is not the case, that the two accounts give dramatically different, incompatible, predictions in a broad class of cases. The predictions are factually different and the genic models consistently get it wrong. Given that virtually all philosophers and scientists accept the position that scientific theories should agree with known facts, we will refute genic selectionism without resort to anything that is philosophically controversial. 1 1. The Cases. Let us start with the case that has been most discussed in this literature, a case of heterozygote superiority. Let us suppose that there is a single genetic locus with two alleles, A and a. Thus there are three genotypes, AA, Aa, and aa.. By definition the heterozygote Aa is superior in fitness to the two homozygotes. In general the fitness of the two homozygotes need not be equal, but for simplicity we will assume that they are since nothing hinges on that assumption. The standard genotypic model normalizes the fitness of Aa at 1 and assigns the fitness of 1 - s to the two homozygotes (where 1 ~ s > 0). Although the value of s = 1 GENIC AND GENOTYPIC MODELS OF SELECTION 279 is a mathematical, and biological, possibility, for our purposes we cannot focus on that value since it is what Brandon (2005) has termed a value of maximal fitness difference. Fitness values are at the point of maximal fitness difference when some fitness values equal 1 and some equal 0 and there are no intermediary fitness values. Drift is impossible at a maximal fitness differential point. Since we are going to be interested in the interplay of drift and selection, we will need to give s some intermediary value. For now let us assume s = 0.5. This model predicts a stable equilibrium that will be reached in a number of generations (depending on the initial starting point and population size). At this equilibrium the frequencies of the two alleles are both 50'%.

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ISBN

9780754627531

Publication Date

January 1, 2017

Start / End Page

383 / 404
 

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Brandon, R. N., & Nijhout, H. F. (2017). The Empirical Nonequivalence of Genic and Genotypic Models of Selection: A (Decisive) Refutation of Genic Selectionism and Pluralistic Genic Selectionism. In Philosophy of Evolutionary Biology: Volume I (pp. 383–404).
Brandon, R. N., and H. F. Nijhout. “The Empirical Nonequivalence of Genic and Genotypic Models of Selection: A (Decisive) Refutation of Genic Selectionism and Pluralistic Genic Selectionism.” In Philosophy of Evolutionary Biology: Volume I, 383–404, 2017.
Brandon, R. N., and H. F. Nijhout. “The Empirical Nonequivalence of Genic and Genotypic Models of Selection: A (Decisive) Refutation of Genic Selectionism and Pluralistic Genic Selectionism.” Philosophy of Evolutionary Biology: Volume I, 2017, pp. 383–404.
Journal cover image

ISBN

9780754627531

Publication Date

January 1, 2017

Start / End Page

383 / 404