Processes that constrain and facilitate the evolution of sexual dimorphism.

Sexual dimorphism, or differences between the sexes, is pervasive in both plants and animals despite genetic and developmental constraints on its evolution. This special issue of the American Naturalist, which is based on the annual Vice Presidential Symposium, documents how the underlying processes responsible for the presence and extent of sexual dimorphism can be qualified and quantified by a variety of approaches. These include estimates of the G matrix and phenotypic selection, artificial selection, phenotypic manipulation of hormones, estimates of sex-differential sensitivity to maternal effects, among-population and phenotypic plasticity studies, and the mapping of sexual dimorphism onto a phylogeny. The questions addressed in the articles in this issue vary depending on the motivation for the studies and the taxa being investigated, but taken together, they show how the integration of genetic, developmental, physiological, ecological, and phylogenetic approaches can illuminate the processes underlying the evolution of sexual dimorphism.