Factors influencing the composition of mixed populations of a hemiclonal hybrid and its sexual host

Hemiclonal/hybridogenetic hybrids combine demographic superiority of asexua ls and genetic diversity of sexuals, but their need for backcrossing with a parental species tightly couples them with this sexual host. Mow can syste ms like this persist in ecological and evolutionary time? Two discrete-time mathematical models describing the complex life cycle and mating system of hybridogenetic waterfrogs (Rana esculenta) identified four factors and the ir interactions as important. Although female mating preferences, in combin ation with differences in fecundity, determine species coexistence, differe nces in larval competitiveness seem to be more important for the hybrid's a ctual frequency. However, coexistence is possible even when host and hybrid are equally fecund and competitive. Dispersal and competition interact in their influence on species composition, but ecological and reproductive dis persal has opposing effects. In ecological terms our results explain the re markable stability of observed species ratios over lime within natural hybr idogenetic populations, and indicate why the species composition can vary s o widely between localities. In evolutionary terms they explain the old age of these and other hybridogenetic systems. They also suggest interesting c onsequences for other tightly coupled systems.