Parasites tend to be highly specific to their hosts, thus it seems reasonable to expect a positive co-variation between the taxonomic richness of hosts and that of their parasites. Wolfdietrich Eichler (1912–1994), a German authority in zoology and parasitology[1] was the first to point out this relationship in 1942 and it was later dubbed 'Eichler’s rule'. It is one of the first three coevolutionary rules.[2]
Recently, Vas and his co-authors[3] tested Eichler's rule, and concluded that exceptionally strong correlational evidence supports the positive co-variation between the species richness of avian and mammalian families and the generic richness of their parasitic lice.
^Klassen, G. J. (1992). "Coevolution: a history of the macroevolutionary approach to studying host-parasite associations". Journal of Parasitology. 78 (4): 573–587. doi:10.2307/3283532. JSTOR3283532. PMID1635016.