FREQUENCY-DEPENDENT SELECTION AND ADAPTIVE SURFACES FOR FLORAL CHARACTER COMBINATIONS - THE POLLINATION OF POLEMONIUM-VISCOSUM

By experiment, we show that the pollination success obtained from a bumblebee visit by an individual of Polemonium viscosum depends strongly on the size of nectar rewards offered and weakly on flower size. However, probably because the size of nectar rewards correlates with flower size, bumblebees discriminate among individual plants on the basis of flower size. If the bumblebees' preferences are learned, then it is likely that the nature of the preferences will depend on the bees' experience with reward/advertisement combinations and that this will be affected by the frequencies of the combinations in the population. Thus, pollinator preferences can be a source of frequency-dependent selection on floral traits in insect-pollinated plants. In general, when the pollimation performance of plants is determined in part by pollinator visitation rate, then the adaptive surfaces that Armbruster proposed for the study of relationships between form and function in flowers may alter under frequency-dependent selection. The shape of the adaptive surface may depend on population composition under the following conditions: intraspecific competition for pollinators, a correlation between nectar production rate and standing crop of nectar, and a correlation of standing crop of nectar with a component of a floral advertisement that can be learned by the pollinator.