MODE OF POLLINATION AND SELECTION ON MATING SYSTEM: A COMMENT ON AIDE'S PAPER.

Abstract

Aide (1986) has demonstrated the interesting result that the distribution of plant outcrossing rates is strongly influenced by pollination system. The distribution for wind-pollinated species is bimodal, as predicted by our model for the joint evolution of inbreeding depression and mating system (Lande and Schemske, 1985), while that for animal-pollinated species is not. He suggests that the greater unpredictability of animals as pollen vectors leads to greater interpopulation variation in outcrossing rates for animal-pollinated species. We concur with this interpretation and had pointed out in our paper on empirical observations (Schemske and Lande, 1985) that the extreme interpopulation variation in outcrossing rates observed in some species was due to variation in pollinator activity. Aide has carried this further by noting that wind-pollinated species show less interpopulation variation in outcrossing rates than animal-pollinated species. We do not agree with Aide's conclusion that the close fit of wind-pollinated species to the predictions of our model and the poor fit for animal-pollinated species suggest that selective forces on outcrossing rates differ depending on the pollination syndrome. The implication of his statement is that animal pollination results in selection for mixed mating systems. While animal pollination can greatly increase the amount of phenotypic variation in outcrossing rates that is attributable to the environment, this may act only to reduce the rate of response to selection on the mating system and to increase the relative importance of random genetic drift and of local extinction and colonization. However, selective pollination affecting the mating system through differential fecundity is also a possibility. At present, the selective forces on mating system are poorly understood. We feel that the distinction between the selective force on a character and the evolutionary response to selection is an important one. The observation that some species possess mixed mating systems does not necessarily indicate that they have reached an evolutionary equilibrium. The increasing efforts to understand the evolution of plant mating systems are a clear indication of how much there is yet to learn. We encourage further empirical and theoretical work in this area.