POLLINATOR-MEDIATED SELECTION ON FLORAL DISPLAY AND SPUR LENGTH IN THE ORCHID GYMNADENIA CONOPSEA

Floral diversification and specialization are thought to be driven largely by interactions with pollinators, but the extent to which current selection on floral traits is mediated by pollinators has rarely been determined experimentally. We documented selection through female function on floral traits in two populations of the rewarding orchid Gymnadenia conopsea in two years and quantified pollinator-mediated selection (Delta beta(poll)) by subtracting estimates of selection gradients for plants receiving supplemental hand pollination from estimates obtained for open-pollinated control plants. There was directional selection for taller plants, more flowers, larger corollas, and longer spurs in the study populations. Pollinator-mediated selection ranged from weak to moderately strong (Delta beta(poll), range -0.01-0.21, median 0.08). All observed selection on spur length could be attributed to interactions with pollinators, while the proportion of observed selection on plant height (0%-77%), number of flowers (13%-42%), and corolla size (13%-97%) caused by pollinators varied among populations and years. Our results demonstrate that pollinators can mediate selection on both traits likely to be involved in pollinator attraction and traits affecting pollination efficiency. They further show that spatiotemporal variation in the strength of pollinator-mediated selection can contribute substantially to differences in selection between years and populations.