Santa Rosalia revisited:: or why are there so many kinds of parasites in 'the garden of earthly delights'?

As is the case for free-living species, a very large number of parasitic species are not described adequately by the biological species concept. Furthermore, Thierry de Meeus, Yannis Michalakis and Francois Renaud argue that because hosts represent a highly heterogeneous and changing environment as well as a breeding site, favouring the association of host-adaptation and host-choice genes, sympatric speciation may occur frequently in parasitic organisms. Therefore, parasites appear to be ideal biological models for the study of ecological specialization and speciation. Beyond the relevance of such considerations in fundamental science, the study of the origin and evolution of parasite diversity has important implications for more applied fields such as epidemiology and diagnosis.