Ecological change may be a common initiator of evolutionary pollinator shifts

Summary Although evolutionary pollinator shifts are accepted as major sources of floral and species diversity in angiosperms, the mechanisms triggering pollinator shifts remain largely unexplored. In 1970, Stebbins proposed that ecological change may be a common initiator of evolutionary pollinator shifts, but empirical tests of Stebbins' hypothesis remain scarce. The lack of studies exploring the drivers of pollinator shifts likely stem from the lack of a clear definition of ecological change, and the inherent challenges of studying past evolutionary events empirically. I here propose a definition of ecological change as any abiotic or biotic macro- or microecological, extrinsic or intrinsic change that significantly alters the effectiveness of a taxon's ancestral pollinator(s). Because such ecological change may happen at different scales, I argue that taking an integrative macro?micro-evolutionary approach will allow us to test where and when which type of ecological change is a common driver of pollinator shifts. I propose macroevolutionary tests of abiotic macroecological change as a driver of pollinator shifts as the easiest and most effective method, allowing for the comparison of a large number of independent shifts to establish whether there are broad generalities in evolutionary pollinator shifts, and determine when and where there are not.