Systematics and taxonomy of the Peromyscus difficilis species group (Rodentia: Cricetidae), with descriptions of 2 new species

Abstract The Peromyscus difficilis species group inhabits mountainous regions in western North America from Colorado, United States, to Oaxaca, Mexico. Previous work generated a robust phylogeny in which 8 monophyletic lineages representing putative species were identified. This phylogeny coincided only partially with previous systematic arrangements. Based on phylogenetic, geographic, environmental, morphometric, and external traits, and karyotypic attributes, we reassessed the taxonomy and nomenclature of the group. Because cranial and external characteristics were strongly correlated with local environmental attributes not necessarily related to phylogeny, considerable within- and among-lineage variation was detected. Yet, the definition of taxonomic units was possible using the genetic species concept and the complete suite of attributes. Six species were recognized, 2 new to science: P. nasutus, distributed on mountainous areas from Colorado, New Mexico, eastern Arizona, and western Oklahoma to the Rio Grande; Peromyscus sp. nov. 1. from central Sierra Madre Occidental (SMOc); P. sp. nov. 2 from eastern Durango and Coahuila; P. difficilis from the southern portion of the SMOc, Mexican Plateau, and northern Sierra Madre Oriental (SMOr); P. amplus from northwestern Oaxaca, southern SMOr, and central Mexico; and P. felipensis with disjunct populations, 1 from Oaxaca, and 1 from Morelos and southern Mexico City valley. A taxonomic hypothesis for the group based on mitochondrial genetic information, and further testing of this hypothesis using additional nuclear genetic markers, will allow for further exploration of the genetic or epigenetic processes that result in the observed, environmentally mediated phenotypic patterns.