Flowering plants that self-fertilize despite high costs (i.e., inbreeding depression) have classically been posed as a conundrum. Much theory and empirical work have highlighted how ecological context can resolve this inconsistency, emphasizing the role of pollinator failure and selection for reproductive assurance. Ignored but equally paradoxical is the presence of species that have low inbreeding depression, yet primarily outcross. Genetic models of mating system evolution predict that such species should be completely selfing. Here, we ask the question, if pollinator failure can explain the evolution of selfing under high inbreeding depression, can pollinator persistence help explain the maintenance of outcrossing when inbreeding depression is low? Using a modeling approach, we evaluate the extent to which pollinators’ reliance on plants might actually thwart the evolution of selfing unless selfing is coupled with traits needed to “escape” persistent pollinators. We present consumer resource models that explicitly consider plant and pollinator densities, selfing rate, inbreeding depression, plant attractiveness to pollinators and its associated costs. First, we ask, under what conditions can a pure selfing mutant replace a resident population of partial selfers? Next, we consider selfing rate as a function of attractiveness and explore how the relative difference in attractiveness of the invader compared to the resident can influence these dynamics.
Results/Conclusions
Whether a pure selfer can invade a population of partial selfers depends on the balance between the costs of attractiveness, inbreeding depression, and their influences on plant density. When residents are relatively unattractive, pure selfers can outcompete them when inbreeding depression <0.5. In such a scenario, residents have a relatively high selfing rate yet still pay some cost of attractiveness, while pure selfers pay no floral costs. When residents are highly attractive, conditions for the pure selfer to win become more restrictive, requiring lower levels of inbreeding depression. This occurs because, despite the cost of attraction, residents can support larger pollinator populations and are nearly completely outcrossed, whereas pure selfers pay the cost of inbreeding depression on all progeny. We find similarly restrictive conditions when considering invasion criteria for a mutant that is a fraction of the resident’s attractiveness. Because low inbreeding depression fosters high equilibrium resident densities, high density-dependent mortality combines with any floral cost still paid by a partially attractive mutant to prevent invasion. Our work casts new light on how ecological context influences the evolution of selfing, identifying conditions that may leave some populations “stuck” as outcrossers.