Adaptation to hummingbird pollination is associated with reduced diversification in Penstemon

A striking characteristic of the Western North American flora is the repeated evolution of hummingbird pollination from insect-pollinated ancestors. This pattern has received extensive attention as an opportunity to study repeated trait evolution as well as potential constraints on evolutionary reversibility, with little attention focused on the impact of these transitions on species diversification rates. Yet traits conferring adaptation to divergent pollinators potentially impact speciation and extinction rates, since pollinators facilitate plant reproduction and specify mating patterns between flowering plants. Here we examine macroevolutionary processes affecting floral pollination syndrome diversity in the largest North American genus of flowering plants, Penstemon. Within Penstemon, transitions from ancestral bee-adapted flowers to hummingbird-adapted flowers have frequently occurred, although hummingbird-adapted species are rare overall within the genus. We inferred macroevolutio...