Adaptation is an important process that allows species to utilize new habitats and to avoid extinction, contributing to the biodiversity we find on the planet. Many flowering plants rely on animals for pollination; the floral traits that are adaptive for pollination are those that influence attraction, rewards, or efficiency. Milkweeds (Asclepias) have unusual floral structures that consist of a gynostegium surrounded by five nectar-filled hoods. In many species, a horn develops from the inside base of each hood. Pollen grains are packaged into waxy packets (pollinia) and are positioned in the wall of the gynostegium. For fertilization, pollinia must be removed by pollinators and inserted into stigmatic openings in the wall of the gynostegium between adjacent hoods; pollination is carried out passively by a wide variety of pollinators that are almost entirely all insects. Milkweeds are hermaphroditic, so it is possible for the floral traits to be adaptive for male fitness, female fitness, or both. The floral diversity across Asclepias is astounding. My dissertation investigates if and how the floral structures are adaptive, and if the variation among species is the result of natural selection. I used two complementary approaches. For the first approach, I used contemporary measures of selection and functional studies to focus on the process of adaptation in five species of North American Asclepias. Selection works on intraspecific variation within traits and the effect of that variation on fitness. I also utilized paternity analyses in two species to measure selection through male fitness. For the second approach, I used phylogenetic methods to find signatures of past selection on traits across more than one hundred North American Asclepias species. Phylogenetic comparative methods focus on patterns of interspecific variation. I used tests of correlated evolution between pairs of traits, or traits and pollinators, to investigate functional relationships and possible selective agents. I also tested for convergent evolution, which can demonstrate adaptive evolution in response to a similar selective regime. The six floral traits I studied had an effect on fitness, suggesting they are adaptive. I found that many of the floral traits were under significant selection through only one gender, but that the direction of selection was similar across genders, showing little conflict between male and female function or between male and female fitness. I predicted that the size of the hood and gynostegium would influence pollinator attraction, but they were instead more likely to influence the efficiency of pollination. I also found no significant link between female pollination success and female reproductive success in four of the five species, so traits that increased pollen receipt did not in turn affect female fitness, which is consistent with fruit production not being limited by pollen receipt. Using measures of viable seeds produced (annual female fitness) and viable seeds sired (annual male fitness), I determined that selection estimated using total fruit number is a good estimate of selection through viable seeds produced; however, total pollinia removed per plant, a common estimate of male function and fitness in milkweeds is not a good predictor of viable seeds sired. My studies of the gynostegia, hoods, and horns across the phylogeny showed that horn loss likely followed the closure of hoods, suggesting a possible loss of horn function. There were also three convergent floral phenotypes; each may have evolved in response to similar selective regimes. The convergent species provide an excellent starting point for future investigations of possible selective agents.
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