When looks can kill: the evolution of sexually dimorphic floral display and the extinction of dioecious plants

Dioecious plants (with separate male and female individuals) more often have drab, inconspicuous flowers than related bisexual plants. Models indicate, however, that similar conditions favour the evolution of showy floral displays in dioecious and bisexual plants. One difference, however, is that dioecious plants may evolve floral displays that are sexually dimorphic. We show that males are more likely to evolve showy flowers than females in animal-pollinated plants, especially when pollinators are abundant. We demonstrate that this dimorphism places showy dioecious plants at a much higher risk of extinction during years of low pollinator abundance because pollinators may fail to visit female flowers. The higher extinction risk of showy dioecious plants provides an explanation for the fact that dioecious plants that do persist tend to have inconspicuous flowers and are more often wind pollinated. It may also help explain why dioecious plants are less species-rich than related bisexual plants.