Sexual selection was defined by Darwin in The Descent of Man and Selection in Relation to Sex (1871, p. 256) as a specific form of natural selection that arises “from the advantage that certain individuals have over other individuals of the same sex and species, in exclusive relation to reproduction.” Darwin identified two basic types of sexual selection: (1) intrasexual competition, the process of competing for mates and (2) intersexual selection, the process of choosing mates. A century later an important theoretical contribution was made by Trivers (1972) based on Williams’s (1966) recognition that there were widespread consequences of the differential investment made by each sex in reproduction. Trivers’s parental investment theory is based on the observation that there is typically a difference in the degree of parental investment in offspring favoring females that results in a marked asymmetry in sex roles. Mating competition would be expected to be greater in the lower-investing sex, while mate choice would be expected to be greater in the higher-investing sex. The logic here is simple: if one sex provides a much greater parental investment
they become an important reproductive resource for which the other sex will compete. Thus, almost all examples of sexual selection involve either male-male competition or female choice. Because these forces are thought to account for much of the observable sexual dimorphism across a wide variety of species, let us examine each of them in more detail.