A typical animal pollinator forages non-randomly among plants in a community, using floral cues to recognize the available options. The tendency of individual foragers to restrict their visits to a subset of the available flowering species increases the proportion of pollen grains that arrive on appropriate stigmas. Pollinators partition themselves among plants in several ways, with the common result of assortative mating according to floral type. First, I discuss the evolutionary implications of assortative mating, in light of recent models that emphasize its importance for species divergence, then review the ways in which pollinator behavior contributes to assortative mating among floral types. Finally, I consider how the different forms of non-random pollinator behavior might influence floral evolution and plant speciation.
There is a long-standing tradition of thought that visitation by different pollinators drives divergence of floral form and provides reproductive isolation among incipient plant species (reviewed by Waser, this volume). However, pollinators rarely specialize completely on a single floral type (plant species or distinct phenotype within a species), leading some investigators to question the role of pollinators in the radiation of the angio-sperms, and to suggest that floral evolution is largely decoupled from plant speciation (Waser 1998; Chittka et al. 1999). None the less, the remarkable radiation of angiosperms in parallel with pollinators (Grimaldi 1999), and findings that plant families with animal pollination are more speciose than those with abiotic pollination (Dodd et al. 1999), suggest that animal pollination was a key innovation in flowering plant evolution. Exactly how pollinators might contribute to speciation and diversification in plants is currently debated (see Waser, this volume), with discussion centered on the importance of pollinators as agents of reproductive isolation and whether divergence can occur despite ongoing gene flow (Grant 1994; Waser 1998).
Insights from recent models of animal speciation (Dieckmann & Doebeli 1999; Kondrashov & Kondrashov 1999) may help to resolve this debate. Application of the modeling results to plants suggests that even moderate pollinator specialization can be important to the initiation or maintenance of divergence in floral form and the process of plant speciation, because such behavior causes some degree of assortative mating among the plants (Jones 1997). Plants have many different mechanisms to promote assortative mating (Levin 1978), manipulation of pollinator behavior being but one. However, genetically simple changes in floral traits can alter pollinator choices significantly (Schemske & Bradshaw 1999); consequently assortative mating may evolve relatively easily via this mechanism. Without single-handedly conferring complete reproductive isolation, pollinators may nevertheless advance plant diversification in their role as agents of assortative mating, acting in concert with other factors as explained below.
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