[1][2] Over time, this pattern has been recognized in many additional cases, and generalized to what is now known as Emery's rule.
[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.