How are the host spectra of hematophagous parasites shaped over evolutionary time? Random choice vs selection of a phylogenetic lineage

Among generalist parasites, some species exploit only hosts from one particular phylogenetic lineage, whereas others can use a broader phylogenetic range of hosts, often seemingly using a random subset of the locally available host species. The latter type of generalist parasites should have greater opportunities to expand their geographical range and should not be restricted to stable and predictable host species because they are less prone to extinction than generalist parasites limited to a phylogenetically narrower host spectrum. We analyzed the diversity skewness of the host spectrum of 21 flea species from South Africa and 39 flea species from northern North America. Diversity skewness measures the balance in the shape of the phylogenetic tree of a set of species: the greater the skew, the more one lineage is overrepresented in an assemblage. When compared to a null expectation, i.e., random selections from the locally available pool of host species, the host spectra of most fleas was not more or less skewed than that expected by chance, though there were a few exceptions. Across South African fleas, the diversity skewness of the host spectrum was strongly negatively correlated with the size of a flea's geographic range; this relationship was not seen among North American flea species. There was no evidence among either set of fleas that average host body mass (a surrogate measure of host life span) correlated with the diversity skewness of the host spectrum. These findings are discussed with respect to the evolution of host specificity, its measurement, and historical differences between the two geographic areas considered.