Natural selection in Ipomopsis hybrid zones: implications for ecological speciation

The fitness of hybrids relative to parental species plays an important role in models of speciation. In ecological speciation, reproductive isolation evolves owing to divergent natural selection, which implies reduced fitness of hybrid phenotypes. The source of the divergent selection in flowering plants may be animal pollinators, or environmental features of habitats that lead to physiological adaptations. Reciprocal transplants, combined with examination of pollination and other fitness components, allow exploration of these mechanisms of speciation. Much of this information is available from hybrid zones between Ipomopsis aggregata and Ipomopsis tenuituba. Pollination studies reveal some disruptive selection on corolla width, but also geographical variation in pollinator preference, and hybrids do not generally suffer lower pollination. Survival of hybrids depends on both genotype and environment. One genotypic class of hybrids is as fit or more fit than the parents, while another type suffers reduced fitness in parental environments. The dynamics of this hybrid zone involve a complex mixture of selection mediated by pollinators and other sources, and this combination of selection may have contributed to the original speciation. (C) New Phytologist (2003).