Divergence in inflorescence height: an evolutionary response to pollinator fidelity

The above-ground heights of inflorescences of 8 species of wild-flowers in a subalpine meadow in the Colorado Rocky Mountains were measured in two successive years. An index of overlap (of height-distributions) was computed for pairwise comparisons of all species. The species were assorted into 4 groups based on their usual pollinators: long-tongued bumblebees (3 species were pollinated by long-tongued bumblebees), short-tongued bumblebees (3 species), hummingbirds (1 species), and solitary bees (1 species). The values of the sample of overlap indices for plants pollinated by the same animals was significantly smaller than the values for plants pollinated by different animals; plants which share pollinators are less alike in height than those that don't share pollinators. It is suggested that this is a result of selection for enhancement of pollinator fidelity. The selective mechanisms, based on the 'horizontal' flight pattern of pollinators and the consequences to the plant of interspecific flights, are discussed.