In the 19th century, the Great Plains were ~500,000 square miles of prairie, steppe, and grasslands that stretched from northern Mexico to southern-central Canada. North of the 50th parallel, the boreal forest still occupies a wide swath of North America comprised of coniferous forest interspersed with vast wetlands, mostly lakes, rivers, bogs, and fens. At that time, the American bison once occurred across the full scope of these vast continental ecosystems. Across all these lands, waters, and places; were all these bison the same species, the same animal? Indeed, when are bison not exactly the same?
Through extensive long-term natural history studies and scientific investigations, even to the genetic and cellular levels, we now understand that there is one species of bison that is comprised of two subspecies in North America and another species of bison in Europe. The American bison that evolved and lived across the vast plains and woodlands are aptly named - the plains bison (genus: Bison, species: bison, subspecies: bison) and wood bison (genus: Bison, species: bison, subspecies: athabascae). So for someone who really pays close attention to formal taxonomy, it is about as simple as it gets. Bison bison bison - for the animals that evolved and lived primarily on the Great Plains and Bison bison athabascae - for the animals that evolved and lived primarily in the boreal forest. "Athabascae" is a formal taxonomic name that recognizes the anglicized version of the Cree native name for vast Lake Athabasca and surrounding watershed in Canada, "athap-ask-a-w," that means "grass or reeds here and there."
We now recognize evidence of sufficient difference between plains bison and woods bison to warrant two different subspecies names. But what are these differences? We expect that this variation arose due to long-term geographic separation. In other words, the plains bison didn't travel to Canada, just like the wood bison didn't go to the Great Plains. While at first glance, it is clear that both sub-species are large with brown hair and black horns; upon more careful observation there are clearly distinct differences in their cranial and skeletal characteristics and variations in horns, head, hump, hair, and pelage coat. Plains bison have massive heads with short noses and have clearly defined shaggy capes that cover the upper portion of their bodies. Woods bison, on the other hand, have large triangular heads and have less defined shoulder capes and head hair, and they have more distinctive and bigger shoulder humps.
This subspecies distinction remains a potential controversy amongst some specialized professional taxonomists, who feel it may be important to argue about such things. Bison conservationists agree on several things: 1) multiple morphological and genetic characteristics distinguish plains bison from wood bison; 2) plains bison and wood bison continue to be morphologically and genetically distinct, despite some historic forced hybridization; and thus 3) wood bison constitute a subspecies of bison, and therefore, should be managed on par with plains bison. An eminent Canadian wildlife biologist once observed that "debating taxonomy does not absolve humans of the responsibility to protect intra-specific diversity as the raw material of evolution."