Moving to mate: the evolution of separate and combined sexes in multicellular organisms.

Which conditions favour the evolution of hermaphroditism or separate sexes? One classical hypothesis states that an organism's mode of locomotion (if any) when searching for a mate should influence breeding system evolution. We used published phylogenies to reconstruct evolutionary changes in adult mate-search efficiency and breeding systems among multicellular organisms. Employing maximum-likelihood analyses, we found that changes in adult mate-search efficiency are significantly correlated with changes in breeding system, and this result is robust to uncertainties in the phylogenies. These data provide the first statistical support, across a broad range of taxa, for the hypothesis that breeding systems and mate-search efficiency did not evolve independently. We discuss our results in context with other causal factors, such as inbreeding avoidance and sexual specialization, likely to affect breeding system evolution.