The origin of gender dimorphism in animal-dispersed plants: disruptive selection in a model of social evolution.

Dioecy (separate sexes) in plants is associated with animal fruit dispersal, but hypotheses for a role of dispersal in the origin of gender dimorphism have received little support. Here, I present a patch-structured model to explore the conditions that favor dimorphism when dispersal is coupled with sex allocation. The model shows that if the proportion of fruits dispersed from a cosexual plant increases with its allocation to fruits (causing accelerating fitness returns from dispersed fruits), disruptive selection can arise when the cost of dispersal is minimal and the correlation among patchmates (i.e., relatedness) is high. In reality, however, the proportion of fruits dispersed from a plant's patch may decline with further allocation to fruits. Even in this case, novel contexts that lead to disruptive selection on sex allocation are discovered, occurring when dispersal costs are high and relatedness is low, which causes accelerating returns from nondispersed fruits. Hence, surprisingly, gender dimorphism can evolve because female specialists are better able to escape local competition or to succeed in it. Building on the few existing models of disruptive selection on social traits, the mechanisms here show that selection for relaxed local competition (cooperation) can sometimes facilitate diversification and sometimes prevent it.