Geographical covariation and local convergence of flower depth in a guild of fly-pollinated plants

Plant adaptations to pollinators are usually studied at the species level, but are expected to occur at the local population level and be reflected in fine-scale patterns of floral variation. Here, we examined whether a guild of c. 20 South African plant species pollinated by the long proboscid fly Prosoeca ganglbaueri (Nemestrinidae) exhibits fine-scale patterns of geographical covariation and convergent evolution at a local scale. Fly proboscis length is highly variable among sites (20-50 mm). Plant adaptation results in floral depths of plants within the guild being closely matched with the proboscis length of their fly pollinator across numerous sites. This results in patterns of divergence among allopatric populations and convergence among species within a site. The most likely evolutionary processes driving these patterns include coevolution between the fly and plants with consistent and abundant rewards, as well as one-sided evolution in rare and nonrewarding species that do not influence the coevolutionary process. Pollinator-mediated selection on spur length was confirmed for a nonrewarding orchid species in the guild by a reciprocal translocation experiment. Thus, rarer and nonrewarding species in the guild are forced to keep pace with the coevolutionary race between common rewarding flowers and flies.