LEVITAN, Don/R.; Florida State University: Density-dependent sexual selection and the evolution of sexual dimorphism in external fertilizers

The stunning absence of sexual dimorphism in adult morphological traits among broadcast spawning invertebrates was originally thought to be a consequence of reduced or absent sexual selection among these taxa. However, there is no reason why broadcast-spawning animals should not experience selection for increased mating opportunities. In fact, the high variance in reproductive success noted in these species suggests sexual selection should be intense. Here I examine three alternate hypotheses to explain the lack of sexual dimorphism in broadcast spawning species using the red sea urchin Strongylocentrotus franciscanus. These are (1) sexual selection for mating opportunities is intense but similar for both sexes; (2) sexual selection acts on gamete traits rather than adult traits and (3) sexual selection is manifested as behavioral rather than morphological differences among the sexes. The results indicate that (1) gender differences in the intensity of sexual selection is dependent on population density via it�s influence on sperm availability and that at only a restricted range of densities is there a clear difference in the variance in reproductive success between males and females; (2) Differences in population density can select for differences in gamete traits that are better adapted to either sperm limited or sperm competitive environments and (3) there appear to be gender differences in the costs associated with variation in spawning behavior and the degree of this difference is dependent on population density via it�s influence on the likelihood of male-male competition. These results suggest that the nature of sexual selection in broadcast spawning taxa is highly dependent on population density and that the consequences of this selection may be gametic and behavioral differentiation rather than adult morphological differentiation.