Curtis M. Lively

George Price showed how the effects of natural selection and environmental change could be mathematically partitioned. This partitioning may be especially useful for understanding host–parasite coevolution, where each species represents the environment for the other species. Here, we use coupled Price equations to study this kind of antagonistic coevolution. We made the common assumption that parasites must genetically match their host's genotype to avoid detection by the host's self/nonself recognition system, but we allowed for the possibility that non‐matching parasites have some fitness. Our results show how natural selection on one species results in environmental change for the other species. Numerical iterations of the model show that these environmental changes can periodically exceed the changes in mean fitness due to natural selection, as suggested by R.A. Fisher. Taken together, the results give an algebraic dissection of the eco‐evolutionary feedbacks created during host–parasite coevolution.

Asexual reproduction is expected to have a twofold reproductive advantage over sexual reproduction, owing to the cost of producing males in sexual subpopulations. The persistence of sexual females, thus, requires an advantage to sexual reproduction, at least periodically. Here, we tested the hypothesis that asexual females are more sensitive to limited resources. Under this idea, fluctuations in the availability of resources (per capita) could periodically favour sexual females when resources become limited. We combined sexual and asexual freshwater snails ( Potamopyrgus antipodarum ) together in nylon mesh enclosures at three different densities in an outdoor mesocosm. After 1 month, we counted the brood size of fertile female snails. We found that fecundity declined significantly with increasing density. However, sexual females did not produce more offspring than asexual females at any of the experimental densities. Our results, thus, suggest that the cost of sexual reproduction in P. antipodarum is not ameliorated by periods of intense resource competition.