Evaluating the Roles of Pollinators in Shaping the Floral Color Variation of Phacelia dubia

Floral color variation within a species is a common occurrence and its maintenance has been a long-standing question in evolutionary biology. Differences in flower color between populations can be shaped by pollinator-mediated selection. Pollinators can exhibit innate and learned preferences for color that may influence floral color variation. These preferences may be molded by the visual environment in which pollinators forage. Most current understanding of how pollinators shape floral color variation involves dramatic differences in dominant pollinators between populations (e.g., hawkmoth vs. bee) and most behavioral work utilized model pollinators like honeybee and bumble bee. However, less is known about wild pollinator communities and their roles in shaping flower color variation and preferences for color for generalist-pollinated plants. Phacelia dubia is a small annual plant that exhibits floral color variation from purple to white and is found in granite rock outcrops and meadows. Here I test whether Phacelia dubia exhibits color differences between habitat types across its range in the Southeastern US. I found that white flowers dominated all meadows and western outcrop populations, while purple flowers are only present in outcrops, especially those in the east. To understand whether pollinator-mediated selection could shape color variation I sampled pollinators in 17 populations to characterize community composition. I found evidence that outcrop populations, where flowers are purple, were characterized by a unique pollinator community that differs from meadows. I then conducted a pollinator preference experiment in natural populations using arrays of purple and white potted P. dubia plants. Pollinators in outcrops exhibited preference for purple morphs while those in meadows showed no preference. Together, my work has implications for how pollinators may be influencing the floral color variation for taxa that inhabit distinctly different habitats and those that are effectively pollinated by a wide range of generalist pollinators.