Pollinator-mediated selection and the evolution of floral traits in orchids

Sammanfattning: In this thesis, I combined manipulations of traits and pollination environment with analysis of phenotypic selection to examine causes of variation in strength and mode of selection on floral traits, and I conducted a reciprocal sowing experiment to test for local adaptation in germination success. I tested the following predictions (1) the opportunity for selection, and the strength of pollinator-mediated and net selection increase with increasing pollen limitation, (2) the effects of traits affecting pollinator attraction and traits affecting pollination efficiency are non-additive and this leads to pollinator-mediated correlational selection, (3) the effects of spur length on pollen removal, pollen receipt, and female fitness differ between populations with short-tongued and populations with long-tongued pollinators, and (4) local adaptation at the stage of germination contributes to the maintenance of ecotypes growing in grasslands and woodlands, respectively.A study including natural populations of 12 orchid species that varied widely in pollen limitation showed that opportunity for selection, pollinator-mediated selection and net selection were all positively related to pollen limitation, whereas non-pollinator-mediated selection was not. In the moth-pollinated orchid Platanthera bifolia, experimental reductions of plant height and spur length decreased pollen removal, pollen receipt and fruit production, but non-additive effects were not detected. Effects of plant height translated into pollinator-mediated selection for taller plants via female fitness, but there was no current pollinator-mediated selection on spur length. An experiment using artificial nectar spurs demonstrated that in P. bifolia pollen receipt saturated at shorter spur length in a population with short-tongued pollinators than in a population with a long-tongued pollinator. Effects of spur length on pollen receipt did not translate into current pollinator-mediated selection indicating that also plants with the shortest spurs for the most part received sufficient pollen for full seed set. Reciprocal sowing of seeds from grassland and woodland populations detected no evidence of local adaptation at the germination stage between ecotypes of P. bifolia.Taken together, the results illustrate how a combination of trait manipulation and analysis of strength and causes of selection can throw light on both the functional and adaptive significance of trait variation within and among natural populations.