Variation in pollination performance gradients in a Dudleya species complex: Can generalization promote floral divergence?

Abstract

1. Adaptive floral divergence may evolve in plants that are generalized for pollination when geographic variation in pollinator assemblages produces a mosaic of phenotypic optima. Within a Dudleya species complex, I estimated the relationship between pollination success and floral phenotype at four sites, and asked whether this relationship depended on the composition of the pollinator assemblage. 2. Flowers were visited by hummingbirds and insects (primarily bees), and the frequency of visitors varied among sites and between years. In 2000, pollination success did not depend on floral phenotype, but in 2001 visitation by hummingbirds increased, and flowers that were longer, produced more nectar, and had more inserted stigmas received more pollen. 3. An independent experiment with hummingbirds confirmed a positive relationship between pollination success and these floral traits, which was due in part to a humming-bird visitation preference for longer, more rewarding flowers. 4. The absence of a trade-off in obtaining service from insects vs birds suggests that variation in pollination environment will not promote floral divergence in Dudleya. However, if longer, more rewarding flowers incur metabolic costs, then the difference between insects and birds as agents of selection on floral function may be sufficient to generate trade-offs in total fitness. © 2005 British Ecological Society.