Analyzing pollinator-mediated selection in a plant hybrid zone: Hummingbird visitation patterns on three spatial scales

Clines across hybrid zones can be produced by several forms of natural selection. We illustrate an approach to studying pollinator-mediated selection in plant hybrid zones, using two species of Ipomopsis (Polemoniaceae) as a model system. We measured visitation to flowers in natural and experimental populations by two major types of pollinators, hummingbirds and hawkmoths, at up to three different spatial scales. Using measures of pollinator visitation, we calculated phenotypic selection gradients and characterized the form of selection in the hybrid zone. Hummingbirds overvisited Ipomopsis aggregata compared with Ipomopsis tenuituba and morphological hybrids at all spatial scales, especially the largest scale of kilometers. These responses may depend in part on the presence of other hummingbird-visited plants in the community. Hummingbird behavior produced directional selection favoring wide corolla tubes and intense red coloration. Hawkmoths, in contrast, overvisited plants with narrow corolla tubes. When both types of pollinators were present, corolla width experienced disruptive selection, consistent with a model of hybrid disadvantage. In the natural hybrid zone, however, hawkmoths are rare participants. In most years plants experience visitation from hummingbirds alone. Thus, selection by pollinators usually fits an advancing wave model in which traits characteristic of I. aggregata are favored everywhere. Modeling the evolution of dines in response to such pollinator-mediated selection will require further theoretical development that allows for selection intensity to vary with spatial scale and with the abundance of unrelated plants visited by the same pollinators.