Zebras are classified in the genus "Equus" (known as equines) along with horses and asses. These three groups are the only living members of the family Equidae. The plains zebra and mountain zebra were traditionally placed in the subgenus "Hippotigris" (C. H. Smith, 1841) in contrast to the Grévy's zebra which was considered the sole species of subgenus "Dolichohippus" (Heller, 1912). Groves and Bell (2004) placed all three species in the subgenus "Hippotigris". A 2013 phylogenetic study found that the plains zebra is more closely related to Grévy's zebras than mountain zebras. The extinct quagga was originally classified as a distinct species. Later genetic studies have placed it as the same species as the plains zebra, either a subspecies or just the southernmost population. Molecular evidence supports zebras as a monophyletic lineage.