Effective sizes of macroparasite populations: a conceptual model.

Effective population size (N(e)) is a crucial parameter in evolutionary biology because it controls genetic drift and the response to selection. Thus, N(e) influences evolutionary processes in parasites, such as speciation, host-race formation, local host adaptation and the evolution of drug resistance. However, N(e) is a parameter that is ignored almost completely in parasitology. Our goal is to provide a conceptual framework that facilitates future studies of the N(e) of macroparasites. The key feature of macroparasite populations is that breeders are subdivided into infrapopulations. We use a model of subdivided breeders to show how some basic demographic factors that control N(e) in all species could be estimated for macroparasites. An important conclusion is that several features of parasite life cycles probably function in concert to reduce N(e) below that expected in a single free-living population of equivalent census size.