PHYLOGEOGRAPHY AND MOLECULAR SYSTEMATICS OF THE PEROMYSCUS-AZTECUS SPECIES GROUP (RODENTIA, MURIDAE) INFERRED USING PARSIMONY AND LIKELIHOOD

Mice of the Peromyscus aztecus species group occur at mid to high elev ations in several mountain ranges in the highlands of Middle America ( Mexico and Central America), a region of high endemicity. We examined the biogeography of this group by conducting phylogenetic analyses of 668 bp of the mitochondial cytochrome b (cyt b) gene. Phylogenetic ana lyses under both parsimony and likelihood frameworks produced the same topologies, but estimates of nodal support were artificially high in weighted parsimony analyses. This difference is attributed to the inab ility of parsimony to optimize branch lengths when evaluating topologi es. These data indicate that the P. aztecus-like populations from sout h and east of the Isthmus of Tehuantepec currently assigned to P. a. o axacensis represent a distinct species, with genetic distances as high as 0.091. In addition, I! hylocetes is strongly divergent from Mexica n populations of P. aztecus (genetic distances of 0.044-0.069), suppor ting the recognition of this taxon as a distinct species. The history of divergence in this group can be explained by a series of apparently early to middle Pleistocene vicariance events associated with glacial cycles. The Sierra Madre Occidental and Cordillera Transvolcanica eac h appear to be faunistically isolated, the Isthmus of Tehuantepec appe ars to have been a strong Pleistocene barrier, and the Sierra Madre Or iental has affinities with the Sierra Madre del Sur and the highlands of central Oaxaca.