OUTCROSSING AND SELFING EVOLUTION IN POPULATIONS UNDER DIRECTIONAL SELECTION

Many plant species have a mixed mating system using self-pollination and cross-pollination. Such behaviour has no simple evolutionary explanation. When inbreeding depression is low the theory predicts that self-pollination should be fixed. A residual crossing habit exists in practically all self-pollinated species that show low inbreeding depression. As recombination is useful to generate new genotypes it can be valuable when the environmental conditions are not stable. We used a simulation model to study the dynamics of adaptation with regard to different mixing mating systems. The evolution of a quantitative trait related to fitness has been related to the mating behaviour. In a first approach, all the individuals of a population outcrossed with the same proportion. The comparison between populations differing for their outcrossing rate, their effective size and their initial level of adaptation for new environments, led us to the conclusion that the smaller the population and the lower its adaptation level, the more valuable outcrossing is. An optimum level of outcrossing has been found for each situation studied. In a second approach, selfing rate was controlled by a polymorphic additional locus. Thus, individuals with different outcrossing behaviours competed within each population. We found that a low level of outcrossing can be maintained and randomly fixed in such conditions, in spite of no inbreeding depression. The outcrossing rate that can be fixed when individuals with different mating behaviour compete is much lower than the optimum outcrossing rate found for the whole population.