Emery's rule

In 1909, the entomologist Carlo Emery noted that social parasites among insects (e.g., kleptoparasites) tend to be parasites of species or genera to which they are closely related.[1][2] Over time, this pattern has been recognized in many additional cases, and generalized to what is now known as Emery's rule.[1] The pattern is best known for various taxa of Hymenoptera. For example, the social wasp Dolichovespula adulterina parasitizes other members of its genus such as Dolichovespula norwegica and Dolichovespula arenaria.[3][4] Emery's rule is also applicable to members of other kingdoms such as fungi, red algae, and mistletoe. The significance and general relevance of this pattern are still a matter of some debate, as a great many exceptions exist, though a common explanation for the phenomenon when it occurs is that the parasites may have started as facultative parasites within the host species itself (such forms of intraspecific parasitism are well-known, even in some species of bees[5]), but later became reproductively isolated and split off from the ancestral species, a form of sympatric speciation.

When a parasitic species is a sister taxon to its host in a phylogenetic sense, the relationship is considered to be in "strict" adherence to Emery's rule. When the parasite is a close relative of the host but not its sister species, the relationship is in "loose" adherence to the rule.[6]