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morphology among plants. This approach makes similar use of existing genetic, life history, and ecological data. It complements ongoing discoveries of nucleotide sequence variation and the expression of quantitative traits. It also may allow classification of plant and animal mating systems, using a common evolutionary framework.

TWO PERSPECTIVES ON SEXUAL SELECTION

Darwin (1859/1964) argued convincingly that male combat and female mate choice were the contexts in which sexual differences appeared. Yet Darwin observed too that sexual selection “depends, not on a struggle for existence, but on a struggle between males for possession of the females; the result is not death of the unsuccessful competitor, but few or no off-spring. Sexual selection is, therefore, less rigorous than natural selection” (p. 88). Darwin’s observations on the relative strength of sexual selection raise what can be considered a quantitative paradox (Shuster and Wade, 2003). How can sexual selection seem to be such a powerful evolutionary force, specifically responsible for causing the sexes to become distinct from one another, when sexual selection is less rigorous than natural selection? Stated differently, how can it be that sexual selection is strong enough to counter the opposing forces of male and female viability selection and still cause the sexes to become distinct (Shuster and Wade, 2003)?

Darwin provided an answer to this question when he devoted an entire volume to subject in 1871 (Darwin, 1871, 1874) and identified the specific cause of sexual selection: “if each male secures two or more females, many males cannot pair” (Darwin, 1874, p. 212). This relationship provided a conceptual solution to the quantitative paradox because it identified an evolutionary process responsible for causing the sexes to diverge. Darwin did not develop quantitative aspects of this particular hypothesis further himself but he was clearly aware of its power. Darwin likened the effect of this process to a bias in sex ratio, wherein particular males might disproportionately contribute to future generations, observations that set the stage for the development of the quantitative approaches now used to document sexual selection (Bateman, 1948; Wade, 1979; Wade and Arnold, 1980; Arnold and Wade, 1983; Arnold and Duvall, 1994; Jones et al., 2002; Shuster and Wade, 2003).

We can visualize the evolutionary process Darwin identified by noting, as Darwin did, that when sexual selection occurs, it creates two classes of males, those who mate and those who do not [cf. Wade (1979); Wade and Shuster (2004)]. If we let pS equal the fraction of males in the population who mate, and p0 = (1 −pS) equal the fraction of nonmating males, the average fitness of the mating males is pS(H), where H is the average number of mates per mating male. By the same principle, the average fit-



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