SEXUAL DIMORPHISM, SEXUAL SELECTION, AND ADAPTATION IN POLYGENIC CHARACTERS

Conspicuous sexual dimorphism is a feature of many species of higher animals. The genetic basis of variation in metrical characters, including that in sexual dimorphism between families or lines, is usually polygenic (Falconer, 1960; Frankham, 1968b; Wright, 1968, 1977; Bird and Schaffer, 1972; Ehrman and Parsons, 1976). Genetic experiments on mice, birds and Drosophila flies indicate that artificial selection practiced on a character of one sex causes not only a direct response of the character in the selected sex, but also a correlated response of the homologous character, if any, in the opposite sex (Shaklee et al., 1952; Harrison, 1953; Korkman, 1957; Becker et al., 1964; Eisen and Legates, 1966; Frankham, 1968a, 1968b; Eisen and Hanrahan, 1972). Such correlated selective responses are attributable to pleiotropy (and linkage) of genes affecting the characters of both sexes, that is, correlations between the additive effects of genes as expressed in males and females. The genetic correlation between homologous characters of the sexes is often quite high (op. cit.). As will be shown, this greatly restricts the rate of evolution of sexual dimorphism relative to that for the average phenotype of the two sexes. Sexual dimorphism may result from natural and/or sexual selection. Darwin (1874, Part 2) elucidated how natural selection operating differently on males and females arises from their distinctive roles in reproduction, or from competition between the sexes for resources such as food, leading to adaptive sexual dimorphism. He also reasoned that intrasexual contests for mates and intersexual mating preferences exert sexual selection, usually on the males, producing sexual dimorphism which is maladaptive with respect to natural selection. Comparisons within and between closely related species led Darwin to conclude that adult males typically are more modified than adult females or juveniles of either sex, but that females have often acquired male characters by "transference. " It was difficult for Darwin to believe that sex-limitation of characters could evolve by selection, but Fisher (1958, Ch. 6) outlined how divergent selection on the two sexes could accumulate genes with different effects in males and females, causing a character at first expressed equally in both sexes to become sexually dimorphic and finally sex-limited. The strength of sexual selection is enhanced by a polygamous mating system, but the possibility of sexual selection in monogamous systems of mating exists due to male competition for early-breeding females, and mate choice exercised by these females (Darwin, 1874; Fisher, 1958; O'Donald, 1977). Systems of mating are often thought to evolve in response to ecological pressures (reviewed by Selander, 1972; Brown, 1975; Emlen and Oring, 1977), although mating preferences may be self-reinforcing (Fisher, 1958; O'Donald, 1967, 1977; Lande, unpubl.). Darwin and Fisher described qualitative methods by which an observed sexual dimorphism could be attributed mainly to either natural or sexual selection. To assign natural selection as the primary cause requires ecological observations that males and females follow different ways of life and employ the dimorphic character(s) adaptively in their distinct modes of survival or reproduction. Darwin presented several such examples, mostly among the lower classes of animals. Selander (1972) 292