Abstract
In plants, pollination syndromes (the correlated presence of many features of relevance to pollination mode, for instance pollination by a particular animal clade) are a striking feature of plant biodiversity, providing great floral phenotypic diversity (Fenster et al . ). Adaptation to a particular animal pollinator provides an explanation for why recently diverged plants can have such extreme differentiation in floral form. One might expect such elaborate adaptations to provide a high degree of pollinator specificity and hence reproductive isolation, but there are many cases where substantial gene flow exists between extreme floral morphs (see Table 1), and the resulting hybrids may be highly fertile. This gene flow provides tremendous opportunities to study the genetics and biology of the pollination syndromes by providing intermediate forms and segregating genotypes. If it is true that pollination syndromes result from adaptation under strong selection, we will expect such flowers to be crucibles of natural selection. If strong selection for particular floral phenotypes can be shown, then this, when coupled with hybridization, will give us one of the most valuable of all experimental systems for evolutionary research: gene flow and selection in balance. In this issue of Molecular Ecology, the paper of Milano et al . ( ) delivers this. It shows that in populations of the Ipomopsis aggregata complex, gene flow between pollination morphs is high and selection to stabilize those morphs is also high: a probable case of gene flow–selection balance.