It seems that the definition of a species changes everytime biologists develop new and cooler tools to answer the question of what constitutes a species.
I prefer to think of two animals as being of different species if the two animals can't mate and produce viable and fertile offspring. For instance a horse is a different species from a donkey because the resulting hybrid offspring is a sterile mule. This is of course not the only criteria for saying two animals are of a different species but its a good rule of thumb.
Now you all will have to help me out when applying this logic toward deer species because I certainly don't know the details as well as many members of this forum. But my understanding is that mule and whitetail deer do indeed mate and produce viable offspring (or hybrids). I can't say for sure whether these hybrids are sterile or not but my guess is that they are indeed sterile because so few are seen in the wild (and also the species names are indeed different for mule and whitetail deer).
The reason you will see different characteristics (say size or coloration) among different populations of a single species is because various populations are under different pressures based on their environment (as Bill pointed out). Keep in mind that just because the populations are separate doesn't necessarily mean they have become a different species. So you could easily imagine that the coloration that helps keep a deer in say rural Virginia hidden would not work as well in Arizona. I would also argue that the amazing variety of habitats we have in Arizona causes us to see different charcteristics among separate populations of whitetails within the state. Again the coloration keeping deer hidden on the rim would probably be less effective in say southern Arizona.
To add to that, I imagine that the elk in Arizona are fairly uniform is color, etc. because they were reintroduced to the state less than 100 years ago (that is my understanding anyway). Now 100 years is a relatively short amount of time and the populations have not had a chance to change that much. Elk of course migrate long distances and this will also keep the populations from becoming genetically isolated. Since whitetails are native to Arizona and don't migrate, it is easy to imagine that the populations in northern AZ have been separated from the populations in southern AZ for a long period of time.
As far as a definition of a subspecies goes you're on your own. I have never found one that I like. It seems unlikely though that any whitetail in AZ would be given a subspecies name other than coues even though some populations are rather different from one another.
And l agree a new species of deer is highly unlikely to found in the new world. But I bet if we came back in a couple of million years there would be at least one!
