The mammal fauna of California was significantly shaped by Beringian migrations that occurred episodically from the Eocene through the late Pleistocene. Wide-dispersing mammals show evidence of continental divisions across North America, with western clades retreating variously to the coast or ice-free regions during glaciation periods. For small mammals, deep phylogeographic divergence is common due to vicariant events, and due to their limited dispersal, they often remained isolated. Severe habitat loss and degradation in the past few centuries has decimated many populations that occur in riparian corridors from east to west, further isolating historically contiguous populations and preventing migration. Barriers to dispersal existed in the Monterey Bay region from about 8–2 Ma, because the southern Sierra Nevada drained into the Pacific Ocean from this location and presented a significant obstacle for the dispersal of small mammals. The central Sierra Nevada, southern Sierra Nevada (particularly around the San Joaquin River), the outer Coast Ranges (particularly around the San Francisco Bay south to Monterey), San Bernardino Mountains, Tehachapi Mountains, Peninsular Ranges, and Inyo Valley are identified as focal areas for endemic taxa. A number of studies now verify that divergence for a number of taxa from the warm deserts began in the Miocene east of the Mojave.
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