Emery's rule

In 1909, the entomologist Carlo Emery noted that social parasites among insects (e.g., cleptoparasites) tend to be parasites of species or genera to which they are closely related. Over the years, this pattern has been recognized in many additional cases, and generalized to what is now known as Emery's Rule. The pattern is best known for various taxa of Hymenoptera, but also in members of other kingdoms such as fungi, red algae, and mistletoe. The significance and general relevance of this pattern is 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), but later became reproductively isolated and split off from the ancestral species, a form of sympatric speciation.