Doctor of Philosophy (PhD)


Biological Sciences

Document Type



Most biogeographic studies on the Mexican biota have assumed that the dramatic climate cycles of the Pleistocene epoch and the prominence of the Trans-Mexico Volcanic Belt have played major roles in the origin and diversification of species. Here the pylogenetics and biogeography of four codistributed rodent species were studied. In each case, a phylogenetic hypothesis for the taxon and allied species using two mitochondrial (Cytochrome-b and 12S), and two nuclear genes (GHR and IRBP) was generated, appropriate taxonomic changes were recommended, and a temporal framework was generated to identify events that may have produced the phylogenetic pattern. Nelson’s woodrat Neotoma nelsoni and the Perote ground squirrel Xerospermophilus perotensis, were confirmed as having their closest relatives in the Mexican Plateau. The findings also confirmed that N. nelsoni and X. perotensis are genetically well-differentiated from their sister taxa. Genetic distances in combination with low levels of morphological differentiation suggest that they should be recognized only at the subspecific level as N. leucodon nelsoni and X. spilosoma perotensis. Molecular estimates of divergence times suggested that N. l. nelsoni and X. s. perotensis diverged from their sister taxa to the north during early Pleistocene times. The rock mouse Peromyscus difficilis was divided into two well-supported clades, a northern clade including the subspecies P. d. difficilis and P. d. petricola, and a southern clade containing the subspecies amplus, felipensis, and saxicola. Molecular-based estimates of divergence times suggested that separation of these clades occurred in the Pleistocene. The study of the Phillips’ kangaroo rat, Dipodomys phillipsii, revealed a biogeographic pattern different from that seen for other taxa. D. phillipsii was divided into two well-supported clades: one distributed on the Mexican Plateau, and a southern clade in the TMVB. Several lines of evidence supported the decision to return the Mexican Plateau clade of D. phillipsii to full species status as D. ornatus. The study showed that D. phillipsii, D. ornatus, D. elator, and D. merriami form a well-supported clade of kangaroo rats, however, the dataset was unable to resolve relationships among these four species. Molecular-based analyses of divergence times suggests that D. phillipsii, D. ornatus, D. elator, and D. merriami diverged in mid-Pliocene times, probably in or near the Mexican Plateau. Unlike the Pleistocene divergence dates reported in previous chapters this Pliocence divergence suggests that the morphotectonic processes that gave rise to the Trans-Mexico Volcanic Belt may have influenced early diversification in Mexican species of Dipodomys.



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Committee Chair

Hafner, Mark S.