The evolution of novel traits: Adaptation in milkweed floral traits

Background/Question/Methods Asclepiadoideae (Apocynaceae), the milkweed sub-family, is a family that has an array of unusual traits. It is in one of only two families to transfer pollen in discrete clusters called pollinia. The fused male and female structure that houses these pollinia, combined with the nectar-filled hoods, make flowers in this sub-family unlike any others. We studied Asclepias incarnata (swamp milkweed), A. syriaca (common milkweed), and A. tuberosa (butterfly-weed), which are pollinated by a similar guild of pollinators comprised mostly of bees, butterflies, and wasps. The goal of the study was to identify which floral traits are adaptive by looking for traits under selection. We measured selection gradients on seven floral traits using multiple components of fitness for each species to determine if present-day selection was acting on those traits. We measured selection for two growing seasons (2007 and 2008) to look for consistency across years. Results/Conclusions We found that for most traits, there was not consistent significant selection between years. Asclepias syriaca, however, showed significant directional selection to increase the height of the hoods across years, whereas A. incarnata had conflicting directional selection on another hood trait between years. Most of the selection occurred through the removal of pollinia, a male fitness component, suggesting that many of the floral traits that we measured are adaptations to increase pollen removal.