Eichler's rule

Eichler's rule is a coevolutionary principle in parasitology stating that because parasites tend to be highly specific to their hosts, the taxonomic richness of a host group should positively correlate with the taxonomic richness of its parasites, so that host groups with more species tend to carry proportionally richer communities of parasites. The German parasitologist Wolfdietrich Eichler, working at Leipzig University, formulated the rule in 1942, making it one of the earliest coevolutionary rules on record; later research, particularly on parasitic lice of birds and mammals, has supported the relationship. This description is adapted from Wikipedia contributors under CC BY-SA 4.0; changes were made. https://creativecommons.org/licenses/by-sa/4.0/