Natural selection by insect predispersal seed herbivores on morphological, chemical, and phenological floral traits of Penstemon digitalis

Background/Question/Methods

Complex ecological characters such as flowers are likely the outcome of diverse, interacting selection pressures from both mutualistic pollinators and antagonistic herbivores. These interactions are expected to be particularly strong when both pollinator and herbivore behaviours are directly linked to floral traits themselves. For example, many insect pre-dispersal seed herbivores are strongly expected to influence natural selection on flowers since these species lay eggs in flowers or buds and are likely to use floral traits as oviposition cues. Preference for particular floral traits, coupled with direct fitness costs of seed consumption suggest that these organisms may either modulate the strength of natural selection by pollinators or drive selection themselves. We tested this hypothesis using an experimental common garden of Penstemon digitalis (Plantaginaceae), consisting of 35 plants from each of three wild populations that differ in their floral and vegetative morphologies as well as the frequency and intensity of seed predation by a microlepidopteran herbivore. This common garden experiment thereby extended the phenotypic range of traits, facilitating the detection of natural selection.  We measured a large number of plant traits that we hypothesized could be under selection by pollinators, herbivores, or both, including flower size, flower colour, floral phenology, floral volatile organic compound emission, display size, plant size, fruit number and seed set, and assessed seed herbivory by the seed predator. Selection was assessed using both conventional selection gradients and path analyses.

Results/Conclusions

We found significant trait variation and covariation both within and among the three source populations, and significant natural selection on many of these phenotypic traits. To our knowledge, this represents one of the first experimental tests of the presence and strength of natural selection on the specific volatile components of floral odour. When we included pre-dispersal seed herbivory in a path analysis, we found that herbivory could explain some, but not all, of the significant natural selection on floral traits. These results support the hypothesis that antagonistic seed herbivores could be directly influencing floral evolution in Penstemon digitalis.