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.