RATES OF FLORAL EVOLUTION - ADAPTATION TO BUMBLEBEE POLLINATION IN ANALPINE WILDFLOWER, POLEMONIUM-VISCOSUM

Animal pollinators are thought to shape floral evolution, yet the temp o of this process has seldom been measured. I used the prediction equa tion of quantitative genetics, R = h(2)S, to predict the rate at which a change in pollinator abundance may have caused divergence in floral morphology of the alpine skypilot, Polemonium viscosum. A selection e xperiment determined the rate at which such divergence can actually pr oceed. Corolla Bare in this species increases by 12% from populations pollinated by a wide assemblage of insect visitors to those pollinated only by bumblebees. To simulate the evolutionary process giving rise to this change, I used a pollinator selection experiment. Plants with broad flowers set significantly more seeds than plants with narrow flo wers under bumblebee pollination but had equivalent fecundity when vis ited by other insects or hand-pollinated. Bumblebee-mediated selection for broad corolla flare intensified from 0.07 at seed set to 0.17 at progeny establishment. Maternal parent-offspring regression yielded a confidence interval of 0.22-1.00 for trait heritability. Given these p arameter estimates, the prediction equation shows that broadly flared flowers of bumblebee-pollinated P. viscosum could have evolved from na rrower ones in a single generation. This prediction is matched by an o bserved 9% increase in offspring corolla flare after a single bout of bumblebee-mediated selection, relative to offspring of unselected cont rols. Findings show that plant populations can adapt rapidly to abrupt changes in pollinator assemblages.