PARENT SESSION
Oral Session #31: Plant Reproductive Ecology, Pollination, and Dispersal. Presiding: J. Karron.
Tuesday, August 7, 2001. 1:00 PM to 5:00 PM. Hall of Ideas L&M.
Ecological causes of selection for self-pollination in Clarkia xantiana: experimental manipulation of biotic interactions.
Moeller, David1, Geber, Monica1, 1
ABSTRACT- In flowering plants, self-pollination has repeatedly evolved from outcrossing. While the barriers to and consequences of the evolution of selfing are well-known, the causes of selection for selfing remain unclear. Ecological factors governing pollinator availability are likely to affect which of the main proposed advantages of selfing is supported. The objective of this study was to evaluate the determinants of pollinator abundance and the causes of selection for self-pollination in Clarkia xantianaby manipulating putative selective agents and phenotypic range using experimental populations. Clarkia xantianais composed of a selfing and outcrossing subspecies, which have distinct geographic ranges and differ in community associations and population size. Previous work suggests that pollinator abundance and reproductive success in outcrossing populations are facilitated by the presence of coexisting congeners. We conducted a field experiment to assess the effect of population size and a congener (C. cylindrica) on bee visitation, seed set, and selection on floral traits affecting selfing rates. Bee visitation was twice as great in populations in the presence of C. cylindrica. Selection by bees favored more "outcrossing" phenotypes in the presence of C. cylindrica. Population size had a significant positive effect on reproductive success and fecundity selection favored more "selfing" phenotypes in small populations. These results suggest that the abundance of conspecifics and congeners influences the direction and magnitude of selection on floral traits and supports the reproductive assurance hypothesis for the evolution of selfing.
KEY WORDS: self-pollination, selection, community interactions , population size