Spatial variation in selective regimes: sexual selection in the water strider, Gerris odontogaster

Abstract

A general reluctance of females to mate allows for intersexual selection for ability to subdue reluctant females in males. Female reluctance to mate decreases with increasing population density, suggesting that sexual selection should be weaker in high density populations. Three different populations with large differences in population density were studied. A number of traits including parasite load, body mass, body size and male abdominal process length experienced significant sexual selection. Populations differed considerably with regard to the total strength of selection on the measured traits and the form of selection on single traits. In general, males in the population with the highest density experienced the weakest selection for grasping ability. This pattern is ascribed to density-related alterations of female mating behavior. Selection for male grasping ability, as reflected by selection on male abdominal process length, is reduced in high-density populations where reluctant females are more easily subdued. The studied populations differed significantly in mean phenotype and phenotypic variance for male abdominal process length. Interpopulationals differences in selective regimes may generate local adaptations with respect to male abdominal process length, and gene flow may contribute to maintenance of the high genetic variation in this trait. -from Author