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We synthesize observations from 1979 to 2016 of a contact zone involving two subspecies of pocket gophers (Thomomys bottae connectens and T. b. opulentus) and their respective chewing lice (Geomydoecus aurei and G. centralis) along the Río Grande Valley in New Mexico, U.S.A., to test predictions about the dynamics of the zone. Historically, the natural flood cycle of the Rio Grande prevented contact between the two subspecies of pocket gophers. Flood control measures completed in the 1930s permitted contact, thus establishing the hybrid zone between the pocket gophers and the contact zone between their lice (without hybridization). Since that time, the pocket gopher hybrid zone has stabilized, whereas the northern chewing louse species has replaced the southern louse species at a consistent rate of ~150 m/year. The 0.2–0.8 width of the replacement zone has remained constant, reflecting the constant rate of chewing louse species turnover on a single gopher and within a local pocket gopher population. In contrast, the full width of the replacement zone (northernmost G. centralis to southernmost G. aurei) has increased annually. By employing a variety of metrics of the species replacement zone, we are better able to understand the dynamics of interactions between and among the chewing lice and their pocket gopher hosts. This research provides an opportunity to observe active species replacement and resulting distributional shifts in a parasitic organism in its natural setting. [ABSTRACT FROM AUTHOR]

The hispid pocket mouse (Chaetodipus hispidus) is one of the most genetically and morphologically divergent species within the heteromyid genus Chaetodipus. Four subspecies of C. hispidus currently are recognized, C. h. hispidus, C. h. paradoxus, C. h. spilotus, and C. h. zacatecae, ranging from Noith Dakota south through the Great Plains and Texas to central Mexico. We investigated the phylogeographic structure within C. hispidus by examining mitochondrial DNA from both freshly collected and museum specimens from localities distributed throughout the range of the species. We also examined 11 cranial characters in 303 specimens to assess morphological variation within the species. Although morphometric analyses were unable to differentiate the subspecies, phylogenetic analyses of molecular data indicated that the 4 currently recognized subspecies of C. hispidus are not genetically distinct. Instead, our results indicate that there are 4 distinct mitochondrial clades of C. hispidus that do not correspond to the currently recognized subspecies, but whose geographic limits instead coincide with major geographical features in the southern United States and northern Mexico. The Southern Coahuila filter-barrier (Durango and Coahuila), the Deming Plains (New Mexico), and the Balcones Escarpment (Texas) likely have acted as intermittent physical barriers to gene flow among the distinct mitochondrial clades, which we recognize as subspecies within C. hispidus. [ABSTRACT FROM AUTHOR]