The evolution of novel traits: Measurements of selection on floral traits in two species of milkweed

Background/Question/Methods Asclepiadaceae, the milkweed family, is one of only two families to transfer pollen in packets (pollinia). Most other features of the milkweed flower are likewise radically different from the basic Angiosperm floral plan. To improve our limited understanding of the functional significance of these novel traits, we are studying Asclepias incarnata (swamp milkweed) and A. syriaca (common milkweed), which are pollinated by a similar guild of pollinators comprised of bees, butterflies, and wasps. We estimated selection gradients for seven floral traits in each species to determine what trait values are adaptive and whether the directions of selection match the present day differences seen between these species. Results/Conclusions In Asclepias syriaca, we found significant positive directional selection on the height of the hood, and positive and negative directional selection for different aspects of the horn, which is a claw-like structure that protrudes from the hood. We also found positive directional selection for hood width in A. incarnata. All three significant selection gradients in A. syriaca mirror the floral size differences between the two species, but the selection for increased hood width in A. incarnata is opposite to the predicted direction.