Variation in selection, phenotypic plasticity, and the ecology of sexual size dimorphism in two seed-feeding beetles

Most animals show some degree of sexual size dimorphism. However, the degree and direction of dimorphism vary substantially among taxa and even among populations within species. Major progress has been made in the study of sexual size dimorphism in the last decade. Yet detailed studies on the proximate and ultimate causes of sexual size dimorphism in a single animal taxon are few (e.g. Chapters 9–20 in this volume). In this chapter we examine sexual size dimorphism in two wellstudied species of seed beetle that differ in the direction of dimorphism (female-biased and male-biased) and that show substantial variation in dimorphism among populations within species. Seed beetles are an excellent system for studies of evolutionary biology because of their ease of laboratory rearing, allowing for large-scale studies that are impractical with many other organisms. We review studies on the sources of selection on body size, how this selection varies between species and among populations, and the consequences of this variation for the evolution of sexual size dimorphism.