Evolutionary dynamics of an Ipomopsis hybrid zone: confronting models with lifetime fitness data.

Interspecific hybridization is a recurring aspect of the evolution of many plant and animal groups. The temporal dynamics of hybrid zones and the evolutionary consequences of hybridization should depend on fitness of parental and hybrid individuals expressed in different environments. We measured lifetime fitness, including survival and reproduction, of plants of Ipomopsis aggregata, Ipomopsis tenuituba, and their F1 hybrids, in experimental plantings in a natural hybrid zone. Fitness, measured as the finite rate of increase ( lambda ), depended strongly on environment. Each parental species performed well in its home locale and poorly in the locale of the other species. Hybrids performed as well as parents overall but enjoyed their highest fitness in the hybrid site. Furthermore, F1 hybrids with I. tenuituba as maternal parent survived well only at the hybrid site, suggesting a cytonuclearxenvironment interaction. These results support an "environmental cline" model of hybrid zone dynamics, with complexities in the fitness of hybrids consistent also with an "evolutionary novelty" model. Combined with those of earlier studies of pollination, our results suggest that both vegetative adaptation to physical environment and floral adaptation to pollinators contribute to observed patterns of phenotypic expression in this hybrid zone and to persistence of the hybrid zone.