The Most Effective Pollinators Predict Intraspecific Floral Color Diversity Across an Edaphic Matrix

Colonization of new environments can spur floral evolution when plants encounter novel selective regimes such as unique pollinator communities. Although edaphic transitions are frequently associated with floral diversification, the extent to which pollinators mediate such divergence remains poorly understood. A generalist-pollinated granite outcrop endemic, Phacelia dubia, colonized ruderal meadow-like roadsides and lawns within the last ~60 years in the Southeastern United States. We quantified flower color (blue chroma) and size in outcrops and meadows in the field and greenhouse, characterized pollinator communities, and measured pollination efficiencies of common pollinators. We tested whether edaphic transitions restructure generalist pollination systems and contribute to floral divergence consistent with the Grant-Stebbins model of floral evolution. Flowers in natal outcrops were darker on average, and more variable among populations than meadows-outcrop populations were either predominantly purple or white-flowered, while meadows were consistently white-flowered. There was a strong genetic contribution to color divergence among populations. Family-level pollinator composition differed between habitats and correlated with color, especially when weighted by pollinator efficiencies. Pairwise pollinator community distance was positively correlated with flower color distance, and this was strongest for between-habitat comparisons. Spatial variation in the relative abundance of the most effective pollinators (Megachilidae, Apidae) predicted color variation better than more common but less effective pollinators (Andrenidae, Syrphidae). Our study supports that edaphic transitions can expose plant populations to novel pollinator communities and create opportunities for floral evolution. Geographic variation in the most effective pollinators predicted flower color variation, providing support for the Grant-Stebbins "most effective pollinator" principle in a plant with generalized insect pollination.