Pattern, Process and the Evolution of Sexual Dimorphism in Three-Spine Stickleback Fish Gasterosteus Aculeatus from Vancouver Island, British Columbia

Males and females of a species often differ from one another in phenotype—a phenomenon known as sexual dimorphism. Sexual dimorphism is most often attributed to sexual selection. However, sexual dimorphism also derives from natural selection when resource competition generates disruptive selection favoring specialization on different resources, resulting in niche partitioning and the evolution of dimorphism in traits related to competition. Sexual dimorphism may be an important route to increased individual fitness (i.e., more resources, more survival). For example, ecologically driven divergent selection between sexes may derive from differences in their environments, resulting in the evolution of dimorphism in traits related to habitat use and resource competition. Such differences between sexes should reduce intersexual competition. First, an existing dataset was used to quantify the existence of sexual dimorphism in 32 populations of Three-spine Stickleback fish, comprised of 16 paired lake-stream populations. We tested for shared and unique aspects of dimorphism within and across lake-stream pairs and asked whether variation in dimorphism could be explained by variation in the environment. We found a significant effect of sex, showing that dimorphism exists in the lake-stream pairs, implying that female sticklebacks are repeatably different from males. Next, using Three-spine Stickleback, we ran a 5-week enclosure experiment with 119 cages containing either same-sex or opposite-sex pairs, using body condition and survival to measure dimorphism's effects on competition. We found that male-male pairs ended with better body conditions than female- female and male-female pairs. Individually, males also had slightly better survival rates than females. This suggests that our evidence is mixed and that natural selection may be driven by patterns of sexual selection in the evolution of sexual dimorphism.