When Charles Darwin wrote On the Origin of Species (1859), he issued a challenge to potential critics: “If it could be proved that any part of the structure of any one species had been formed for the exclusive good of another species, it would annihilate my theory, for such could not be produced through natural selection”. Darwin went even further by identifying several traits that seemed especially problematical: the sterile worker castes of social insects and extravagant ornaments that appear to benefit potential predators more than their bearers. It is this latter problem and Darwin’s solution to it—sexual selection—that are the focus of this chapter. I begin by providing a very brief historical overview of sexual selection, focusing on the initial controversies and its resurgence in the 1970s. I then provide an overview of the conditions that lead to sexual selection and the kinds of traits that are favored by it. Sexual selection usually involves evolutionary changes in both males and females. Thus, I first address the evolution of extravagant male traits (it is typically males that exhibit such traits). Since female choice is one of the mechanisms that can lead to the evolution of extravagant male traits, I also address the evolution of female preferences. Finally, I will identify those areas of the field that are the most controversial, unresolved, and promising for future research. This review is, of necessity, brief and selective. I have tried to cite recent reviews rather than the extensive primary literature to provide an easy entry point into the literature. Readers wishing for more detailed treatment would do well by starting with Andersson’s (1994) excellent book. Darwin introduced the concept of sexual selection in On the Origin of Species (1859) and greatly elaborated the idea in The Descent of Man and Selection in Relation to Sex (1871). Darwin defined sexual selection as depending “not on a struggle for existence, but on a struggle between males for possession of the females”.
