Evolutionary origins and diversification of mutualism

© Oxford University Press 2015. Published 2015 by Oxford University Press. Molecular phylogenies and ecological associations for approximately two hundred closely related plant species and their specialized, obligate pollinators are depicted without taxonomic names so that we might focus our attention instead on general problems of interpretation. A glance at this web in the tree of life suggests the parallel diversification of plant and pollinator clades, as well as instances in which ancient pollinators might have diversified after switching from one host plant lineage to another. For example, a lineage of seven plant species at the upper left is pollinated exclusively by a particular insect lineage, and congruent branching patterns in the two phylogenies suggests diversification in partnership (see arrows in Figure 3.1). However, the plant lineage at the upper left descends from the base of the tree and appears to be more ancient than the associated pollinator lineage. Regardless of whether more recent diversification catalyzed by host shifts or ancient codiversification (parallel cladogenesis) might have dominated the history of this mutualism, the multiplicity of lineages engaged in partnership is impressive. In fact, most evidence for extensive patterns of codiversification involves exceptionally specialized and obligate mutualisms. Might this reflect a coincidence, a bias in sampling, or a biological reality? This chapter examines this question by comparing evidence for origins and diversification across the spectrum of species interactions, from mutualism to antagonism. We regard mutualisms as reciprocal exploitations that nonetheless provide benefits 3.1 Webs in the tree of life