Evolutionary consequences of ecological factors: pollinator reliability predicts mating-system traits of a perennial plant

The reproductive-assurance hypothesis predicts that mating-system traits will evolve towards increased autonomous self-pollination in plant populations experiencing unreliable pollinator service. We tested this hypothesis by assessing the relationships among pollinator reliability, outcrossing rates, heterozygosity, and floral traits relevant to pollination processes across populations of the woody vine Dalechampia scandens in Costa Rica. Mean outcrossing rates ranged from 0.16 to 0.49 across four populations, and covaried with the average rates of pollen arrival on stigmas, a measure of pollinator reliability. Across populations, genetically based differences in herkogamy (anther-stigma distance) were associated with variation in stigmatic pollen loads, outcrossing rates, and heterozygosity. These observations are consistent with the hypothesis that, when pollinators are unreliable, floral traits promoting autonomous selfing evolve as a mechanism of reproductive assurance. Extensive covariation between floral traits and mating system among closely related populations further suggests that floral traits influencing mating systems track variation in adaptive optima generated by variation in pollinator reliability.