Natural Selection and the Sex Ratio: Fisher's Sources

Abstract

which Fisher was certainly familiar. Probably the most celebrated argument in evolutionary The argument, which Fisher had expressed purely verbiology is the explanation of the near equality in the bally, lay fallow from 1930 until population geneticists numbers of males and females in many species that R. A. started to cast it in mathematical form (Shaw and Fisher (1930) gave in The Genetical Theory of Natural SeMohler 1953; Shaw 1958; Bodmer and Edwards 1960; lection. It demonstrated how natural selection operating Kolman 1960; Edwards 1963; see also Edwards 1962). It at the individual level could mold even a population became familiar through its inclusion in the influential characteristic such as the sex ratio, which had been contextbook on population genetics by Crow and Kimura sidered an obvious candidate for group selection; it (1970). showed how sometimes it was necessary to consider three In introducing the argument in 1930, Fisher simply regenerations and not just two in an evolutionary model; it marked (p. 141), ‘‘As is well known, Darwin expressly rewas hailed as the first example of an evolutionarily stable served this problem for the future as being too intricate strategy (ESS) by those who later christened the concept to admit of any solution (Descent of Man, p. 399)’’ and (see Maynard Smith 1982); and it has frequently been he quoted from his source, Darwin’s The Descent of Man clothed in game-theoretic language as a key example by and Selection in Relation to Sex, for which he gave the those to whom such an approach appeals (e.g., Maynard date 1871. In essence, and leaving out questions of paSmith 1978, 1982; Williams 1996b). It even started the rental expenditure for the moment, Fisher reasoned that modern interest in the evolutionary implications of paif in a population there are more males born than ferental expenditure (Hamilton 1964; Trivers 1972; see also males, say, each female will have a greater reproductive Clutton-Brock 1991). It greatly influenced Hamilton’s value than each male, since each sex contributes exactly (1967) paper ‘‘Extraordinary Sex Ratios’’ and, through half the ancestry of future generations. Parents genetiShaw and Mohler (1953), the views of Williams (1966; cally inclined to produce females in excess of the populasee Williams 1996a). Hamilton and Williams have been tion average will therefore make a disproportionately held to be the two authors mainly responsible for the large contribution to the future gene pool, including the modern popularity of the view that natural selection opgenes for producing females in excess. The population erating within populations is the primary mechanism of sex ratio will thus shift from its present imbalance toevolution (Dawkins 1989; Ridley 1996). Both the sexward one-half, which is seen by the argument to be its ratio argument and the concept of parental expenditure equilibrium value. were attributed to Fisher (1930) by Dawkins (1976) in Every commentator quoted above has assumed, and The Selfish Gene. most have firmly stated, that the argument was original *E-mail: AWFE@MEDSCHL.CAM.AC.UK. to Fisher, though he did not claim it to be, nor did he Am. Nat. 1998. Vol. 151, pp. 564–569.  1998 by The University of Chicago. refer to it either before or after 1930 in any of his other 0003-0147/98/5106-0007$03.00. All rights reserved.