Metazoan parasites and resource heterogeneity: constraints and benefits.

The usually narrow specificity of parasites still represents a puzzling question. We suggest that specialisation provokes aggregation of individuals at three levels (host species, hosts individuals, host microhabitats) and that one benefit for metazoan parasites lies in the frequency of genetic exchanges, in agreement with Rohde's hypothesis. We discuss two mechanisms that may maintain specificity after host speciation or host switching: alloxenic speciation by habitat preference and alloxenic speciation by assortative survival. We suggest that specialisation provokes aggregation, that aggregation increases genetic diversity, and that genetic diversity favours specialisation which is thus indefinitely restored.