It is really laughable to see what different ideas are prominent in various naturalists' minds, when they speak of "species". It all comes, I believe, from trying to define the undefinable.
—Charles Darwin, 1856
The June issue of Scientific American (yes, I'm behind on my reading) features an article on taxonomy entitled "What Is A Species?" It discusses various classification schemes, from the original (creationist) Linnean system to Ernst Mayr's biological species concept (based on reproductive compatibility or incompatibility) to the phylogenetic species concept (based on ancestry), and it opens with an enduring example of taxonomic controversy: classifying North America's wolves.
Most wolves in North America belong to a single species, the grey wolf or Canis lupus. (This is the same wolf found in Eurasia, the "big bad wolf" of folk stories.) Several subspecies exist, including the white wolf of the Arctic regions (C. l. arctos) and the small "lobo" or Mexican wolf found in the montane and desert Southwest (C. l. baileyi). In between are other populations, currently recognized as subspecies occidentalis (Northwestern wolf) and nubilis (the Great Plains wolf, a.k.a. buffalo wolf or timber wolf). The latter can be quite variable in appearance, but usually grey or black: the "standard issue" wolves of southern Canada, Yellowstone, and the North Woods of Minnesota, Wisconsin, and Michigan's Upper Peninsula. That's about as far as the consensus goes, however.
[Top to bottom: Arctic wolf, typical Great Plains wolf, and Mexican wolf: Photos by yours truly.]
Formerly resident throughout the Southeast, and now restricted to the area around Alligator NWR in northeastern North Carolina, the red wolf (Canis rufus) has been a taxonomic puzzle for some time. Some researchers question the validity of the species, thinking it may have originated as a hybrid of the grey wolf (C. lupus) and the "brush wolf" or coyote (C. latrans). Others point to the fact that the red wolf's geographic range was mostly separate from that of coyotes (until recently a strictly western species) and grey wolves (typically a northern species, although this should be considered more of a guideline than a rule).
Bigger questions arise when we consider the eastern timber wolf, the first to be encountered by Europeans in America. Some early naturalists believed it was not conspecific with the Old World wolf, Canis lupus, and so named it C. lycaon. Eventually, the eastern timber wolf was reclassified or "demoted" as a subspecies of the grey wolf, C. lupus lycaon. More recently, Canadian researchers have suggested that C. lycaon is in fact a valid species, and have offered the common name Algonquin wolf (after the population's epicenter in Algonquin Provincial Park). They have also suggested that the red wolf, C. rufus, might represent an isolated southern population of the Algonquin wolf. Skeptics think the Algonquin wolf, like the red wolf, might be a coyote-grey wolf hybrid, as coyotes have definitely made incursions into the Northeast.
Confused yet? This is supposed to be easy: The wolf, a large, reasonably well-known mammal close to home. Imagine taxonomy on, for example, rarely-seen look-alike birds or bats or lizards or fish or insects in remote tropical areas, or worse yet disjunct populations on tropical islands. (That line is for you, Andy; I know you've done some work in that line.) But North America's canids are not reproductively isolated, and so Mayr's BSC is of limited utility. Result: No one agrees on what constitutes a valid species. Arguably, we may have...
I personally incline to the four-species model—and then hasten to emphasize that I have no particular qualifications in taxonomy. I've been fortunate enough to have seen and heard many coyotes, howled with grey wolves in Minnesota, and sighted a red wolf during that species' all-too-brief reintroduction to Great Smoky Mountains National Park. But those experiences, valuable as they are to me personally, have no bearing on the question of classification.
I left out another classification scheme: One species. "We like to call it Canis soup," says a researcher in the Scientific American article. Tongue-in-cheek it may be, but I think Darwin—who after all demolished the concept of completely fixed, created species in favor of a messier but more elegant and realistic view of the natural world—would have approved.