Sexual selection in the evolution of flowering phenology

Background/Question/Methods: In dioecious plant species (those with separate sexes), it is frequently observed that males flower earlier in the season than females. However, the generality of this phenomenon has not been quantified, and the reasons for it have not been fully explained. The conventional hypothesis is that female plants require more time for resource acquisition prior to reproduction. An alternate hypothesis is that sexual selection for access to high-quality ovules favors early-flowering males. First, to quantify the prevalence of male precedence (protandry), I compiled published data on flowering phenology of 29 dioecious taxa and compared standardized flowering curves of males and females. I also assembled data on the relationship between plant size and time of flowering onset. Then, to test the role of sexual selection in driving early male flowering, I constructed a model of the evolution of flowering time in a simulated dioecious plant population in which timing of flowering onset is determined by plant size in combination with a sex-linked flowering-time gene, and probability of seed maturation is determined by maternal size and age.

Results/Conclusions: Protandry (earlier flowering by males) is indeed the usual, though not universal, pattern in dioecious plants. In the majority of plant populations growing in natural field conditions (whether dioecious or hermaphrodite), early individuals tend to be larger, and, therefore, more fecund. This seasonal decline in size of potential mates causes males in simulated plant populations to evolve earlier flowering than females. Seasonal declines in resource availability within individual females (another commonly observed pattern) can also select for protandry, though this effect is weaker. Associations between large size and early phenology are widespread in plants and animals and are likely strong contributors to the prevalence of protandry in all taxa, but their importance seems to have been neglected by botanists. I conclude that sexual selection (specifically, male–male competition for access to high-quality ovules) likely plays a more important role in the evolution of flowering phenology than has previously been recognized.