Lifetime selection on adult body size and components of body size in a waterstrider: Opposing selection and maintenance of sexual size dimorphism

Sexual size dimorphism (SSD), the difference in body size between males and females, is common in almost all taxa of animals and is generally assumed to be adaptive. Although sexual selection and fecundity selection alone hav e often been invoked to explain the evolution of SSD, more recent views ind icate that the sexes must experience different lifetime selection pressures for SSD to evolve and be maintained. We estimated selection acting on male and female adult body size (total length) and components of body size in t he waterstrider Aquarius remigis during three phases of life history. Oppos ing selection pressures for overall body size occurred in separate episodes of fitness for females in both years and for males in one year. Specific c omponents of body size were often the targets of the selection on overall b ody size. When net adult fitness was estimated by combining each individual 's fitnesses from all episodes, we found stabilizing selection in both sexe s. In addition, the net optimum overall body size of males was smaller than that of females. However, even when components of body size had experience d opposing selection pressures in individual episodes, no components appear ed to be under lifetime stabilizing selection. This is the first evidence t hat contemporary selection in a natural population acts to maintain female size larger than male size, the most common pattern of SSD in nature.