From DD's link:
This relatively close relationship between whitetail and mule deer and their genetic differences from blacktails has spawned a new theory that the mule deer itself is actually a hybrid form; the result of a mixing of genes from coastal blacktail bucks mating with whitetails does when the glaciers of the last Ice Age receded, bringing the two species together. The resulting offspring then changed drastically over the last 10,000 years in response to environmental conditions. This would explain the similarities between whitetail and mule deer mtDNA. Analysis of the rest of the DNA (nuclear DNA) with Polymerase Chain Reaction-based microsatellites or any of the other rapidly developing techniques will add much to our knowledge of the origin of deer.
The majority of the website is crap, but the author eventually mentions the above (which is based in science, not conjecture).
Size, and many other morphological characteristics, provides poor criteria to make species identification. In a scientific sense there is always a chance (one trillion to one is still "a chance") that a species may occur far outside of its known range, but it is not likely that a blacktail deer will show up in Alberta unless BC sinks into the sea and they swim here.
