POLLINATORS, HERBIVORES, AND THE MAINTENANCE OF FLOWER COLOR VARIATION: A CASE STUDY WITH LOBELIA SIPHILITICA

Conflicting selection by pollinators and herbivores is thought to be an important mechanism maintaining variation in flower color within plant populations. However, evidence for this mechanism is lacking because selection and the agents of selection on flower color have rarely been estimated. We estimated selection by pollinators and a predispersal seed predator on the three fundamental components of color (brightness, chroma, and hue) of Lobelia siphilitica flowers. We compared phenotypic selection on flowers of supplemental hand-versus open-pollinated plants to infer whether pollinators were an agent of selection on color. We compared attacked and unattacked plants to infer whether the seed predator was an agent of selection on color. Selection on brightness, but not chroma or hue, differed significantly between both pollination treatments and predation categories. Both pollinators and the seed predator exerted selection for less bright flowers, suggesting that they do not cause conflicting selection on flower color. However, we also detected phenotypic selection for brighter flowers that was not caused by pollinators or by the seed predator. Consequently, pollinators and herbivores are not sufficient to generate conflicting selection that could contribute to the maintenance of flower color variation in L. siphilitica.