What major geological event likely coincided with the divergence of whitetails and mule deer?

The uplift of the Rocky Mountains

The significant separation between the white-tailed deer lineage and the mule deer lineage, which is estimated to have occurred two to three million years ago, is strongly associated with major geological restructuring in western North America. Specifically, the rising topography caused by the uplift of the Rocky Mountains created substantial natural barriers. These barriers physically isolated developing populations, preventing gene flow between them. This geographic isolation subjected the diverging groups to different selective pressures—funneling mule deer ancestors toward more arid western regions and allowing whitetail ancestors to dominate eastern woodlands and plains—which ultimately solidified their status as distinct evolutionary lines.