The American west encompassing the Colorado Plateau, Basin and Range, the High Plains, and the Rocky and Sierra Mountains began to uplift rapidly by 20–15 Ma . A middle Miocene flora from Carson Pass in the central Sierra Nevada suggests uplift of about 2300 m since that time . The uplift is a key factor in creating an increasingly drier climate in the North American interior around that time [49, 51]. Paleobotanical evidence suggests that by the middle Miocene the arid interior has become an effective barrier to biotic interchange between eastern and western North America [52, 53]. After the MMCO, an increasingly drier climate, as well as global cooling (Fig. 3) , might thus have resulted in a vicariance event responsible for the divergence of Coptis and Xanthorhiza (node 1; Fig. 3).