Two extant and six extinct species are recognised. Of the six extinct species, five went extinct in the Quaternary extinction event. Bison palaeosinensis evolved in the Early Pleistocene in South Asia, and was the evolutionary ancestor of B. priscus (steppe bison), which was the ancestor of all other Bison species. From 2 MYA to 6,000 BC, steppe bison ranged across the mammoth steppe, inhabiting Europe and northern Asia with B. schoetensacki (woodland bison), and North America with B. antiquus, B. latifrons, and B. occidentalis. The last species to go extinct, B. occidentalis, was succeeded at 3,000 BC by B. bison.
Of the two surviving species, the American bison, B. bison, found only in North America, is the more numerous. Although commonly known as a buffalo in the United States and Canada, it is only distantly related to the true buffalo. The North American species is composed of two subspecies, the Plains bison, B. b. bison, and the Wood bison, B. b. athabascae, which is the namesake of Wood Buffalo National Park in Canada. A third subspecies, the Eastern Bison (B. b. pennsylvanicus) is no longer considered a valid taxon, being a junior synonym of B. b. bison. References to "Woods Bison" or "Wood Bison" from the eastern United States confusingly refer to this subspecies, not B. b. athabascae, which was not found in the region. The European bison, B. bonasus, or wisent, is found in Europe and the Caucasus, reintroduced after being extinct in the wild.
The American bison and the European bison (Wisent) are the largest surviving terrestrial animals in North America and Europe. Bison are good swimmers and can cross rivers over half a mile (800 meters) wide. They are nomadic grazers and travel in herds. The bulls leave the herds of females at two or three years of age, and join a male herd, which are generally smaller than female herds. Mature bulls rarely travel alone. Towards the end of the summer, for the reproductive season, the sexes necessarily commingle.
American bison are known for living in the Great Plains, but formerly had a much larger range including much of the eastern United States and parts of Mexico. Both species were hunted close to extinction during the 19th and 20th centuries, but have since rebounded; the Wisent owing its survival, in part, to the Chernobyl Disaster, ironically, as the Chernobyl Exclusion Zone has become a kind of wildlife preserve for Wisent and other rare megafauna such as the Przewalski's Horse, though poaching has become a threat in recent years. The American Plains bison is no longer listed as endangered, but this does not mean the species is secure. Genetically pure B. b. bison currently number only ~20,000, separated into fragmented herds—all of which require active conservation measures. The Wood bison is on the endangered species list in Canada and is listed as threatened in the United States, though there have been numerous attempts by beefalo ranchers to have it entirely removed from the Endangered Species List.