Abstract
Distributions of fossil and Recent montane mammals in the American Southwest support a non-equilibrium model. This vicariant explanation, based on vegetational disjunctions during Pleistocene interglacials, is extended in an attempt to predict patterns of evolutionary divergence within a monophyletic lineage. A reconstruction of faunal movements during the Pleistocene based on this explanation suggests at least three complementary hypotheses for evolutionary divergence: 1) endemic forms should arise on the largest mountain ranges; 2) montane satellites of these should share derived forms; and 3) small mountain ranges isolated in both space and time should have been colonized during successive glacial periods by migrating boreal forms. Patterns of bacular morphology within the Eutamias quadrivittatus species complex are used to test the hypotheses. Results of the analysis corroborate the hypotheses, suggesting the need for changes in current thought regarding the arrangement of taxa within this species complex and the origin of altitudinal zonations of species on mountains in the American Southwest. Because the patterns are based on vicariance, patterns of divergence among the chipmunks should characterize divergence in a variety of other montane taxa.
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