Explaining evolutionary shifts between bee and hummingbird pollination: Convergence, divergence, and directionality

In certain angiosperm genera, closely related species have diverged from one another to converge on different pollination syndromes, whereas species with intermediate phenotypes are rare or absent. Convergent conformity to syndromes implies the existence of evolutionary attractors'' toward which phenotypes are drawn; divergent breaks from conformity show that populations can escape one attractor and be drawn to another. We discuss how these two opposed processes can be reconciled for the special case of evolutionary transitions between bee pollination and hummingbird pollination. In this case, a third phenomenon, the directional bias in favor of transitions from melittophily to ornithophily, also needs explanation. Older treatments chiefly ascribed convergence to cognitive and morphological properties of pollinators and ascribed transitions to geographical differences in pollinator availability. Those treatments did not specifically address what factors would overcome and disrupt the stabilizing selection that would be expected to preserve the pollination syndrome of a plant species. Here, we focus on possible contributors to destabilization, especially considering the possible roles of ( 1) differences among pollinators in pollen-transfer efficiency, ( 2) mutations with large effects on floral phenotypes, and ( 3) losses of function in the biochemical pathways that produce floral pigments. We conclude that all of these can influence the evolution of pollinator transitions but that the process usually needs to be initiated by external ecological factors that change the visitation rates of pollinators. We discuss the roles of particular floral characters in several plant genera that have undergone transitions. We expect that transitions reach completion through a centripetal'' process of selection that incrementally recruits changes in multiple characters.