Truth in Advertising: The Kinds of Traits Favored by Sexual Selection

Abstract

Ever since sexual selection was first described by Darwin (1871), biologists have recognized the importance of the phenomenon but have had difficulty in understanding how it operates. Sexual selection is that special form of natural selection which is responsible for the evolution of traits that promote success in competition for mates. It is particularly important in polygamous breeding systems in which individuals of one sex, usually female, invest many more resources in gametes and often also in parental care, and individuals of the other sex, usually males, compete to mate with these females (Bateman 1948; Trivers 1972). Sexual selection can take two forms. On the one hand it can favor the evolution of traits, such as those related to fighting ability, which increase success in direct competition with other males for mates. On the other hand, it can lead to the evolution of characteristics, such as those involved in courtship displays, which make males more attractive to females. Although strikingly sexually dimorphic characteristics, such as the antlers of deer, the plumes of birds of paradise, and the elaborate displays of bowerbirds have been attributed to sexual selection, the precise mechanisms responsible for the evolution of these traits have remained the subject of much debate. There are two main hypotheses. Fisher's (1930) runaway selection model proposes that sexual selection will favor females that choose mates on the basis of any trait that confers an initial survival or reproductive advantage. This process will continue, resulting in simultaneous exaggeration of the trait and enhanced female choice, until the trait reduces survival of males to the point that this outweighs the advantages of increased reproductive success. This model assumes that the advantages and disadvantages conferred by the trait are confined to the sex (male) that expresses it, and that sexual selection is a special form of directional selection that ultimately tends to oppose natural selection for traits related to survival. Zahavi's (1975) handicap hypothesis also proposes that sexual selection favors the exaggeration of traits which promote male reproductive success at the expense of survival. Females choose mates with such traits because their demonstrated ability to survive despite such a handicap indicates the overall fitness of their genotypes. Under this model both male and female offspring tend to inherit