Mountain colonization precedes shifts away from bee pollination in Melastomataceae

Shifts among different groups of pollinators are central in the evolution of flowering plants, yet mechanisms underlying pollinator shifts remain unclear. Environment-induced reduction in pollinator availability and hence efficiency may destabilize ancestral plant-pollinator interactions and trigger shifts to new, more efficient pollinators, but formal tests remain scarce. We used a series of phylogenetic comparative methods on 333 species of the pantropical family Melastomataceae to test whether elevation, latitude and climatic variables explain pollinator shifts and the distribution of floral traits governing pollen release. We find that shifts away from bee pollination to generalist insect and vertebrate pollination associate with occurrence in cooler and wetter mountain environments. Also, we show that mountain colonization repeatedly preceded and, hence, likely triggered shifts away from bee pollination. Furthermore, our results suggest that the evolution of floral traits (larger petals and pore sizes) facilitating pollen transfer by bees may have been critical for the initial colonization of mountains by bee-pollinated species. By identifying environments conducive to pollinator shifts, our results do not only provide a much-needed hypothesis for mechanisms underlying the evolution of different pollination systems but also confirm their validity through empirical testing. Whether environment-induced evolutionary pollinator shifts are the norm across angiosperms remains to be explored.