Geitonogamy has the unique distinction of having the ecological properties of outcrossing, but the genetic consequences of selfing. We investigate the potential of geitonogamy as a selective force in the evolution of inflorescence design in Asclepias speciosa. We conducted 1,092 hand outcross pollinations (6 per individual) on two treatments, one covered in pollinator-excluding bridal veil before and after pollinations and the other left open to experience the consequences of natural rates of self-pollinations. The bagged treatment had significantly higher female fitness than the open treatment, providing strong evidence that natural rates of self-pollination significantly reduce fruit set in A. speciosa. Results were consistent in both undisturbed and fragmented populations, suggesting geitonogamy may be independent of population size. Fruit set data provides evidence that geitonogamy, and not resource limitation, may limit female success. We will also look at the effect of inflorescence-unit size on geitonogamous pollinations in natural populations. We will manipulate umbels in the field to create five treatments ranging in size (5, 15, 25, 35, 45 flowers per umbel), and then measure effects of size on pollen export and selfing rate with AFLP analysis of inserted pollinia. This will be the most direct study of pollen export and geitonogamy in a natural population conducted to date.

Key words: Asclepias speciosa, geitonogamy, inflorescence design, pollination