A wolfdog, also called a wolf-dog hybrid or wolf hybrid, is a canid hybrid resulting from the hybridization of a domestic dog (Canis lupus familiaris) to one of four other Canis sub-species, the gray (Canis lupus), eastern timber (Canis lupus lycaon), red (Canis rufus), and Ethiopian wolves (Canis simensis). Intra-hybridization between dogs and other subspecies of gray wolves are the most common wolfdogs since dogs and gray wolves are considered the same species, are genetically very close, and have shared vast portions of their ranges for millennia. Such hybridization in the wild has been detected in many populations scattered throughout Europe and North America, usually occurring in areas where wolf populations have declined from human impacts and persecutions. At the same time, hybrids are also often bred in captivity for various purposes. Inter-hybridization of dogs and two other North American wolf species have also occurred historically in the wild, although it is often difficult for biologists to discriminate the dog genes in the eastern timber and red wolves from the gray wolf genes also present in these wolf species due to their historical overlaps with North American gray wolves as well as with coyotes, both of which have introgressed into the eastern timber and red wolf gene pools. At the same time, because many isolated populations of the three wolf species in North America have also mixed with coyotes in the wild, it has been speculated by some biologists that some of the coywolf hybrids in the northeastern third of the continent may also have both coydogs and wolfdogs in their gene pool. Hybrids between dogs and Ethiopian wolves discovered in the Ethiopian Highlands likely originated from past interactions between free-roaming feral dogs and Ethiopian wolves living in isolated areas.