The present Great Basin Floristic Province had achieved roughly its present topographic conformation by some time in the Miocene epoch and had a climate not too different from the present one, though probably a little warmer, moister, and less continental. Both the flora and the fauna took on a fairly modern aspect during the Miocene, as a result of worldwide evolutionary changes and more specific adaptation to the conditions of the region. Changes in the biota since that time mainly reflect evolution and migration at the level of species and, to a lesser extent, genera, in response to regional conditions and the repeated fluctuations in climate. The climatic reversals of the Pleistocene cause repeated inverse migrations of more northern, mesophytic elements in the flora, on the one hand, and more southern, xerophytic elements on the other. These expansions and contractions of range favored hybridization and genetic mixing among related plant species. The fauna of the region, dependent eventually on the flora, must have been subjected to basically the same set of repeated changes in range and local distribution during the Pleistocene. About 10,000 years ago many of the large mammals in the Intermountain Region, as elsewhere in North America, rapidly became extinct, perhaps largely through overkill by primitive man.
"The biota of the Intermountain Region in geohistorical context,"
Great Basin Naturalist Memoirs: Vol. 2
, Article 2.
Available at: http://scholarsarchive.byu.edu/gbnm/vol2/iss1/2