MASS-ACTION MODELS OF PLANT MATING SYSTEMS - THE EVOLUTIONARY STABILITY OF MIXED MATING SYSTEMS

In self-compatible plants the selfing rate is directly related to the relative amounts of self and outcross pollen deposited on receptive stigmas. As a result, the fraction of ovules self-fertilized by a particular plant is a function not only of the fraction of the pollen it produces that is deposited on its own stigma but of the amount of pollen exported by other plants in the population and the probability that exported pollen successfully reaches its receptive stigmas. Thus, the reproductive success of different genotypes is both frequency- and density-dependent. In dense populations in which the probability of successful pollen export is relatively high, complete outcrossing is evolutionarily stable, even in the absence of inbreeding depression. In sparse populations, genotypes that increase the selfing rate in the population may increase in frequency, but genotypes that cause complete selfing can never become fixed, unless reproduction is pollen-limited. Whenever self-fertilization can evolve, mixed mating systems are evolutionarily stable. This model predicts that selfing rates should be both density-dependent and frequency-dependent. The limited data currently available are consistent with both of these predictions, although there are several complications in attempting to apply the mass-action model directly to biotically pollinated plants. These results suggest that the probability of successful outcrossed reproduction plays at least as important a role in determining the mating system of plants as inbreeding depression.