Speciation and diversi®cation of parasite lineages: an analysis of congeneric parasite species in vertebrates

The evolutionary diversi®cation of living organisms is a central research theme in evolutionary ecology, and yet it remains dicult to infer the action of evolutionary processes from patterns in the distribution of rates of diversi®cation among related taxa. Using data from helminth parasite communities in 76 species of birds and 114 species of mammals, the in ̄uence of four factors that may either be associated with or modulate rates of parasite speciation were examined in a comparative analysis. Two measures of the relative number of congeneric parasite species per host species were used as indices of parasite diversi®cation, and related to host body mass, host density, latitude, and whether the host is aquatic or terrestrial. The occurrence of congeneric parasites was not distributed randomly with respect to these factors. Aquatic bird species tended to harbour more congeneric parasites than terrestrial birds. Large-bodied mammal species, or those living at low latitudes, harboured more congeneric parasites than small-bodied mammals, or than those from higher latitudes. Host density had no apparent association with either measures of parasite diversi®cation. These patterns, however, re ̄ect only the present-day distribution of parasite diversi®cation among host taxa, and not the evolutionary processes responsible for diversi®cation, because the apparent e€ects of the factors investigated disappeared once corrections weremade for host phylogeny. This indicates that features other than host body size, host density, latitude, and whether the habitat is terrestrial or aquatic, have been the key driving forces in the diversi®cation of parasitic helminth lineages.