Male-Male Competition During Pollination in Plant Populations

Variation in mating success in plant populations may reflect differences in performance during pollination as well as nonrandom events occurring after pollen deposition. For studies of evolution by sexual selection, it is especially important to determine the degree to which variation in mating success is dictated by environmental versus genetic variation. It is extremely difficult to discriminate between male-male competition and female choice during postpollination interactions, but here I argue that male-male competition commonly occurs during pollination itself. Male-male competition will reduce average male fertility, but asymmetrical competition should be evolutionarily advantageous to some individuals and lead to the evolution of traits that confer a competitive advantage during pollination. Contrary to a popular assumption, pollen-producing plants can compete for pollinator visits and access to receptive stigmas, even when average seed set in the population is not pollen limited. Variation in male mating success should depend on the abundance and distribution of pollinator visits and fertilizable ovules, the potentially limiting resources. A computer simulation study suggests that preemptive competition for rare fertilization opportunities or consumptive competition for pollinators can increase variation in male fertility. When pollinators are abundant, synchronous floral presentation is likely to favor attractive plants that export pollen more rapidly than their competitors. In contrast, gradual floral presentation should select for high pollen production and greater efficiency of pollen delivery.