REVISITING THE FISHER-WRIGHT CONTROVERSY

Abstract

INTRODUCTION Between 1918 and 1932, R A. Fisher, J. B. S. Haldane, and Sewall Wright ushered in field of theoretical population genetics. In so doing, they accomplished a formal, or mathematical, synthesis of Darwin's mechanism of natural selection with principles of MendeUan inheritance. Thus began historical period of biology called evolutionary synthesis, spanning roughly 1920s to 1950s. The formal synthesis achieved by architects of theoretical population genetics was not without controversy. Fisher and Wright debated conceptual foundations of population genetics theory from 1928 until Fisher's death in 1962. Wright continued his contributions until his own deadi in 1988. Just over 20 years ago, Provine (1985) published his important paper laying out crucial issues of Fisher-Wright controversy, calling this controversy central, fundamental, and very influential in modern biology (Provine, 1985, pp. 197, 217). The controversy is persistent, too (Skipper, 2002). In this chapter, I revisit Fisher- Wright controversy in spirit of Provine's seminal paper, paying special attention to direction it has taken in past 20 years. In addition, I put a philosophical gloss on structure of controversy in hope of explaining its persistence. I begin with an overview of Fisher's and Wright's general theories, then analyze same three controversies Provine (1985) discussed: (a) controversies over evolution of dominance, (b) their general theories, and (c) evolution of Scarlet Tiger moth, Panaxia dominula. Provine was prescient in choosing these debates from many in which Fisher and Wright engaged. Each has seen considerable continued work, and I consider some of these contributions. It is my view that disagreements about relative significance (Beatty, 1995) form a crucial point of contention in these debates. Recognizing tins dimension helps explain persistence of disagreement. I turn first to Fisher's and Wright's general theories of evolution. FISHER AND WRIGHT ON EVOLUTION Fisher's Natural Selection Theory In 1922, Fisher pubUshed first paper adumbrating a mathematical synthesis of Darwinian natural selection with recently rediscovered laws of Mendelian heredity. As he himself writes, Fisher discusses the distribution of frequency ratio of aUelomorphs of dimorphic factors, and conditions under which variance of population may be maintained (Fisher, 1922, p. 322). His aim was to respond to a set of criticisms about Darwinian natural selection, especiaUy that it cannot be correct explanation for modulation of genetic variation in populations, because genetics of populations are such that there is not enough variation available for selection to act on. In his response, Fisher considered interaction of natural selection, random survival (genetic drift), assortative mating, and dominance. As Provine (1985, p. 199) makes clear, Fisher showed, among other things, that for a single locus with two alleles, natural selection acting to increase one homozygote would lead to eUmination of other alleles. But if selection favored heterozygote, result would be a stable equilibrium of distribution of afieles in population. Fisher also showed survival at low frequency in a population of a rare mutant depended more on chance than selection. Hence, in large populations one would expect, as Darwin ([1859] 1964) had, a higher genetic variability. Fisher (1922) and other work, for example, on evolution of dominance and mimicry, culminated in his book, The Genetical Theory of Natural Selection (1930b). Here, Fisher's aim was to establish particulate inheritance against blending theory of inheritance, then demonstrate how plausibly Darwinian natural selection may be principal cause of evolution in Mendelian populations (cf. …

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