Conclusions

Floral traits generally may be under selection both to increase pollinator service and to improve targeting of pollen to compatible mates; this is not a new idea (Raven 1972; Heinrich 1975). However, pollen targeting becomes a more important issue if it plays a key role in the process of spe-ciation, as well as serving to reinforce existing barriers to hybridization and reduce gamete wastage. Floral traits that are used as recognition cues by pollinators, such as color and scent, may evolve primarily to improve pollen targeting rather than to increase pollinator visitation.

Floral differences sufficient to induce some degree of pollinator constancy and thus assortative mating can be much more subtle than those that change the taxonomic identity of pollinators, particularly to different "syndromes." For purposes of assortative mating, the divergent groups do not need to adapt to different pollinator taxa, and no selection directly on floral traits is required. Instead, the species or incipient species need to accomplish the relatively easy task of diverging sufficiently for individual pollinators to use their variable trait(s) as a basis for constancy or labile preference, thereby improving pollen targeting. The pool of pollinators is thus partitioned on a much finer scale than by taxa.

Assortative mating due to flower constancy by pollinators may be a critical step toward reproductive isolation between ecologically divergent incipient species. In light of recent simulations indicating expanded conditions under which assortative mating may induce reproductive isolation, it is reasonable to envision scenarios in which plant divergence is propelled by assortative pollen flow arising from modified floral signals, often without a change in pollinator identity. I have presented several scenarios for pollinator-mediated divergence in sympatry and reinforcement of differences evolved in allopatry, not because I think they are ubiquitous, but rather to prompt tests of these often overlooked possibilities as mechanisms of diversification and maintenance of species boundaries in angiosperms.

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