Species Selection Maintains Self-Incompatibility

Incompatible Self-Compatibility Macroevolutionary processes driving species differences in diversification rates are important in explaining the variation we see in nature, but the extent of this process and how much the traits within a single species can drive changes in the diversification rate are unknown. Goldberg et al. (p. 493; see the Perspective by Wright and Barrett) analyzed the phylogenetics of the plant family Solanaceae and found that rates of extinction are greater for self-pollinating species than outbreeding species. Species-level selection against the deleterious effects of inbreeding may explain why self-fertilization, despite its short-term evolutionary advantages, has not spread to become more common in the flowering plants. Self-incompatibility in a family of plants is associated with appreciably higher rates of speciation. Identifying traits that affect rates of speciation and extinction and, hence, explain differences in species diversity among clades is a major goal of evolutionary biology. Detecting such traits is especially difficult when they undergo frequent transitions between states. Self-incompatibility, the ability of hermaphrodites to enforce outcrossing, is frequently lost in flowering plants, enabling self-fertilization. We show, however, that in the nightshade plant family (Solanaceae), species with functional self-incompatibility diversify at a significantly higher rate than those without it. The apparent short-term advantages of potentially self-fertilizing individuals are therefore offset by strong species selection, which favors obligate outcrossing.