SEXUAL SIZE DIMORPHISM AND SELECTION IN THE WILD IN THE WATERSTRIDER AQUARIUS REMIGIS: LIFETIME FECUNDITY SELECTION ON FEMALE TOTAL LENGTH AND ITS COMPONENTS.

Darwin's fecundity advantage model is often cited as the cause of female biased size dimorphism, however, the empirical studies of lifetime selection on male and female body size that would be required to demonstrate this are few. As a component of a study relating sexual size dimorphism to lifetime selection in natural populations of the female size-biased waterstrider Aquarius remigis (Hemiptera: Gerridae), we estimated coefficients for daily fecundity selection, longevity selection, and lifetime fecundity selection acting on female body size and components of body size for two consecutive generations. Daily fecundity was estimated using females confined in field enclosures and reproductive survival was estimated by twice-weekly recaptures. We found that daily fecundity selection favored females with longer total length through direct selection acting on abdomen length. Longevity selection favored females with smaller total length. When daily fecundity and reproductive longevity were combined to estimate lifetime fecundity we found significant balancing selection acting on total length in both years. The relationship between daily fecundity and reproductive longevity also reveals a significant cost of reproduction in one of two years. We relate these selection estimates to previous estimates of sexual selection on male body size and consider the relationship between contemporary selection and sexual size dimorphism.