Floral trait evolution: Evidence of genetic differences among natural populations of wild radish and the role of pollinators in mediating natural selection on floral traits

One of the most well-known evolutionary relationships is that between flowering plants and their pollinators. Abundant circumstantial but little direct evidence supports the central role of pollinators in directing the evolution of floral morphology. Using wild radish (Raphanus sativus, L: Brassicaceae), a species widely studied in ecological and evolutionary investigations, four progressively specific topics related to the role of pollinators in floral evolution were examined. First, to determine whether among population variation occurs in traits likely to be affected by natural selection in wild plant populations, 32 separate natural populations were studied. Field surveys found that significant phenotypic differences in five floral traits, and nine whole-plant traits exist among populations. Common garden experiments found that these populations have diverged genetically with respect to floral traits. Second, the association between pollinator species and floral phenotypes among populations within a plant species is not well-documented. Significant statistical relationships were found between pollinator classes and floral phenotypes among populations. These associations are indirect evidence of the influence of pollinators on short-term floral evolution. Third, an experiment to test the direct effect of different assemblages of pollinators on selection was conducted. The strength of natural selection on floral traits depended significantly on the species of insect pollinator to which plants were exposed. The experiment demonstrates the potential of pollinator distribution and abundance to affect the strength of natural selection of floral traits. Finally, to establish whether natural selection results in evolutionary change, theory requires that a measurable response to selection be detected between generations. For a pollinator environment with few large pollinators relative to ambient natural pollinator densities, the equation, R = {dollar}hsp2{dollar} S, predicted a 17% to 27% increase in mean petal area in a population in one generation. This magnitude of change suggests that the extremes of petal area phenotypes measured in wild radish populations in the field could have arisen in only two to three generations. The evidence from this study suggests that pollinators play an important role in directing the evolutionary trajectory of floral traits of natural populations of wild radish, and that some plant species can quickly adapt to different pollinator species.