Frequency-dependent pollinator foraging in polymorphic Clarkia xantiana ssp. xantiana populations: implications for flower colour evolution and pollinator interactions

Under many circumstances pollinators are expected to practice positive frequency-dependent foraging in colour-polymorphic plant populations. Theory suggests, however, that competition for floral resources might favor negative frequency-dependent foraging by some pollinator species, possibly contributing to the maintenance of flower colour variation by negative frequency-dependent selection. We addressed this idea with pollination studies of the California annual plant Clarkia xantiana ssp. xantiana (Onagraceae), which is polymorphic for the presence of conspicuous petal spots and is pollinated by several specialist bee species. At the level of entire pollinator assemblages, we did not detect significant fixed flower colour preferences or frequency-dependent foraging. Three species of specialist bee pollinators, however, showed contrasting forms of frequency-dependent foraging. The most widespread species, Hesperapis regularis (Melittidae) exhibited positive frequency dependence. Two other common species, Lasiglossum pullilabre (Halictidae) and Ceratina sequoiae (Apidae), preferred to visit whichever morph (unspotted or spotted) was locally in the minority. All three species were found to be effective at transferring C. xantiana pollen; H. regularis appeared most effective. Our findings suggest that a mixture of positive and negative frequency-dependent selection on flower colour occurs in C. xantiana, with the form and intensity of selection varying in space and time with pollinator assemblages. Negative frequency-dependent selection via pollination dynamics may play a larger role in maintaining genetic variation in flower colour than was previously thought. Our results also suggest an unappreciated form of niche partitioning among specialist pollinators. Genetic polymorphism in flower colour may sometimes facilitate pollinator coexistence.