Muleys are probably the youngest of all the large mammal species in North America, most likely evolving into a species in the last 10,000 years. As Geist pointed out Muleys are mostly whitetail genetics with a big dose of blacktail genetics thrown in.
About 10,000 years ago the glacier in the central part of North America temporarily expanded rapidly south, and partly isolated the precursor to muleys from white tails, and is believed to have allowed muleys to “solidify” themselves into a separate species.
Like most more recently evolved species muleys are more of a specialist, whereas whitetails are more of a generalist. In most cases generalists do better in the face of human altered habitat. Because of this, whitetails have expand westward, probably increasing the frequency of hybrids over the past century.
