Support for the predictions of the pollinator-mediated stabilizing selection hypothesis

Abstract

Aims: Floral traits are frequently used in traditional plant systematics becauseof their assumed constancy. One potential reason for the apparent constancy of flower size is that effective pollen transfer between flowers depends on the accuracy of the physical fit betweenthe flower and pollinator. Therefore, flowers are likely to be under stronger stabilizing selection for uniform size than vegetative plant parts. Moreover, as predicted by the pollinator-mediated stabilizing selection (PMSS) hypothesis, an accurate fit between flowers andtheir pollinators is likely to be more important for specialized pollinationsystems as found in many species with bilaterally symmetric (zygomorphic) flowers than for species with radially symmetric (actinomorphic)flowers. Methods: In a comparative study of 15 zygomorphic and 13 actinomorphicspecies in Switzerland, we tested whether variation in flower size, among and within individuals, is smaller than variation in leaf size and whether variation in flower size is smaller in zygomorphic compared to actinomorphic species. Important findings: Indeed, variation in leaf length was significantly larger than variationin flower length and width. Within-individual variation in flower andleaf sizes did not differ significantly between zygomorphic and actinomorphicspecies. In line with the predictions of the PMSS, among-individual variation in flower length and flower width wassignificantly smaller for zygomorphic species than for actinomorphicspecies, while the two groups did not differ in leaf length variation.This suggests that plants with zygomorphic flowers have undergonestronger selection for uniform flowers than plants with actinomorphicflowers. This supports that the relative uniformity of flowers compared to vegetative structures within species, as already observed in traditional plant systematics, is, at least in part, a consequence of the requirement for effective pollination.