Mechanisms of Hummingbird-Mediated Selection for Flower width in Ipomopsis Aggregata

The form of angiosperm flowers is thought to have evolved in part via selection that excludes ineffective pollinators and increases the efficiency of pollen transfer by effective ones. In previous studies with the montane plant Ipomopsis aggregata, we documented pollinator-mediated selection on several aspects of floral form, including flower width. This character varies continuously within natural populations of I. aggregata, and individuals with wider flowers export more pollen per flower to surrounding plants. We previously showed that this component of phenotypic selection is due primarily to the pervisit effectiveness of hummingbirds, the most important pollinator in our study populations. Here we investigate mechanisms of differential visit effectiveness in greater detail. First, counts of pollen in unvisited flowers showed that pollen production itself increases with width. In aviary experiments, hummingbirds also removed a greater proportion of available pollen as width increased. We next videotaped visits to flowers that varied in width either naturally, or by experimental treatment, and found that hummingbirds inserted their bills more deeply into wider flowers. Finally, we directly manipulated how deeply birds could insert their bills, and found that more pollen was removed after deep insertion. Thus, several mechanisms appear to underlie selection on corolla width via visit effectiveness in pollen export. One involves a phenotypic correlation with pollen production; this underscores the value of experiments for untangling indirect from direct selection. Another mechanism involves direct selection due to the depth of insertion of a hummingbird's bill; this is rare evidence for one form of selection involving the "fit" between pollinator and flower.