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Although animal pollination is often proposed as a major driver of floral divergence, questions remain about its importance in plant speciation. One issue is whether pollinator specialization, traditionally thought necessary for floral isolation, is prevalent enough to have played a major role in speciation. Furthermore, the ecological and geographic scenarios under which pollinator transitions occur are poorly understood, and the underlying genetic factors are just beginning to be uncovered for a few systems. Nevertheless, macroevolutionary studies consistently show that transitions to animal pollination are accompanied by an increase in diversification rate. Here we consider several models and diverse empirical data on how pollinators could influence speciation. We conclude that floral isolation is rarely, if ever, sufficient to cause speciation on its own, but that it acts synergistically with other isolating mechanisms. A more comprehensive approach is the key to an improved understanding of the role of pollinators in angiosperm speciation. 637 Annu. Rev. Ecol. Evol. Syst. 2009.40:637-656. Downloaded from www.annualreviews.org Access provided by University of British Columbia on 08/18/17. For personal use only.