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traits. Moreover, because this methodology emphasizes the process of evolutionary change, it is easier to test and interpret than frameworks emphasizing parental investment in offspring and its presumed evolutionary outcomes.

Although sexual dimorphism and sexual differences were well known in his time, Charles Darwin (1859/1964, 1871, 1874) was first to recognize both their selective context as well as their evolutionary cause. Darwin’s initial observations about sexual selection focused mainly on the context in which sexual selection occurred, e.g., during combat involving “special weapons, confined to the male sex” (Darwin, 1859/1964, p. 88), as well as during female mate choice, wherein females “standing by as spectators, at last choose the most attractive partner” (Darwin, 1859/1964, p. 89). Darwin (1871, 1874) later identified the evolutionary process by which sexual selection occurs (see below), but his initial emphasis on context had already taken hold. Considerations of the context in which sexual selection occurs predominate to the present day [see reviews in Jones et al. (2002) and Shuster and Wade (2003)]. One goal of this chapter is to explain why it is time to deemphasize Darwin’s initial focus on context, and continue to develop Darwin’s later, more causeoriented evolutionary approach.

Darwin recognized too that sexual selection is nonubiquitous (Darwin, 1871, p. 208). His statement that “[i]n many cases, special circumstances tend to make the struggle between males particularly severe” demonstrates his intuitive grasp of mating system—the “special circumstances” in which reproduction occurs within individual species. Since Darwin, 2 emphases in mating systems have developed. The first is expressed in terms of the genetic relationships between males and females and is applied mainly to plants and domestic breeding designs. The second emphasis is expressed in terms of mate numbers per male or female, an approach usually applied to animals and mainly for typological classification. Contrary to Darwin’s description of the cause of sexual selection, evolutionary trends within animal mating systems are identified mainly in terms of energetic investment in individual offspring (Bateman, 1948; Williams, 1966; Trivers, 1972).

A second goal of this chapter, then, is to explain how disparate existing approaches to mating system analysis can be combined to yield a quantitative methodology that emphasizes measurement of the sex difference in the variance in relative fitness, as well as genetic correlations underlying reproductive traits arising from the spatial and temporal distributions of fertilizations. Such information predicts the degree and direction of sexual dimorphism within animal species, and may explain variation in floral



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