Eichler's rule

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When comparing host sister clades we tend to find taxonomically richer parasite fauna on the taxonomically richer group of hosts.

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 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.

Recently, Vas and his co-authors tested this '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.