The Role of Predation in the Evolution of Sexual Dimorphism in Gasterosteus aculeatus

Sexual dimorphism —morphological differences between the sexes, contributes to morphological variation within a species and has been shown to occur in a variety of taxa. One mechanism hypothesized to generate dimorphism is differential niche partitioning between the sexes, which leads to selection acting differently on each sex. Differential predation is one factor that has been suggested to accompany sex-specific niche divergence. However, most studies investigating predation and sexual dimorphism have been conducted in the field, which has prohibited a direct test of the contribution of differential predation. To directly test for the contribution of differential predation to the evolution of sexual dimorphism, I utilized data from a manipulative predation experiment. In the experiment, a highly variable population of hybrid benthic-limnetic stickleback were either exposed to cutthroat trout predation or left in a trout-free control treatment for one generation. Morphological trait distributions were then compared between the two generations to compare the pattern of evolution between the sexes and estimate the strength of selection on sexual dimorphism. When looking at overall dimorphism we did not find a significant effect of predation. However, when looking at specific traits we found a trend towards a significantly greater amount of dimorphism in predation ponds for two traits, anal fin length and caudal peduncle depth. We also found evidence of differential patterns of selection between males and females when exposed to predators, with males experiencing more selection due to predation. These results together suggest that predation might play some role in the evolution of sexual dimorphism in stickleback