Dispersal theory suggests that mule deer likely evolved from black-tailed deer populations that expanded eastward following the receding glaciers. As mule deer broke away and moved into better habitats their phenotypic traits were allowed to be expressed from an increase in nutrition and a lack of predators. They increased in body size, altered the colors, structures on their face including ears, and grew larger antlers.

Heffelfinger, J. and Krausman, P.R. (2023) Ecology and management of black-tailed and mule deer of North America. Boca Raton, FL: CRC Press.

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