POSTPOLLINATION SELECTION AND MALE FITNESS IN PLANTS

Postpollination events are often described as potential mechanisms for sexual selection in plants. The broadest interpretation of what constitutes sexual selection includes selection on both pollen (analogous to sperm competition in animals) and embryos as possible manifestations of male-male competition and/or female choice. Including selective embryo abortion as a mechanism for sexual selection is controversial because interactions among young progeny and their maternal parent are distinct from interactions among parental plants or their gametes. Nonetheless, I discuss mechanisms for both pre- and postfertilization selection, in part because of the practical difficulties of discriminating between the two. I focus on whether these processes result in differential mating success among pollen donors in natural populations. Evolutionary change occurs when such differences are due to traits controlled by additive genetic variation. Little evidence suggests that postpollination mechanisms affect male reproductive success in existing populations. In part, this situation prevails because few investigators have employed genetic markers to determine whether differences in paternal reproductive success are consistent across recipient plants. Paternal success sometimes varies due to male-by-female interactions, and females may be capable of ''choosing'' mates that produce more vigorous progeny, but this type of female choice does not constitute sexual selection as long as different males are favored by different females. Reasons for avoiding the term sexual selection are discussed. Further studies focusing on variation among pollen donors in both the number and quality of progeny they produce will greatly improve our understanding of the evolutionary implications of postpollination events.