Simulated pollinator decline strengthens selection on different floral traits in female and hermaphrodite Lobelia siphilitica

Pollinator declines are predicted to strengthen natural selection on floral traits that either attract pollinators or increase the amount of pollen transferred per visit. However, the effect of pollinator declines on selection on floral traits of species with separate sexes may differ between sex morphs that offer different rewards to pollinators. To determine whether the effect of pollinator declines on selection on floral traits differs between sex morphs, I exposed female and hermaphrodite Lobelia siphilitica plants to either ambient pollination or reduced pollination treatments. For plants in each treatment, I estimated directional selection on four floral traits (inflorescence height, average daily floral display, flower size, and stigma exsertion) via seeds per plant. I found that experimentally reducing pollination intensified selection on inflorescence height and flower size, but that this effect was not stronger in females than hermaphrodites. Instead, reducing pollination intensified selection for taller inflorescences only in hermaphrodites, and intensified selection for larger flowers only in females. These results suggest that there is potential for both females and hermaphrodites to evolve floral traits that increase pollinator attraction and/or pollen transfer efficiency, and thus that species with separate sexes have the potential to adapt to pollinator declines.