We examined geographic trends in 4 morphological data sets (both craniodental and colorimetric measurements and scores of cranial foramina and qualitative craniodental variables) within and among 5 subspecies of the California vole, Microtus californicus, that occur within or adjacent to the Mojave Desert in eastern California. These analyses are corollary to those previously published on the same samples using both mitochondrial and nuclear gene sequences and microsatellite loci. The morphological and molecular data sets are generally concordant in supporting the existing infraspecific taxonomy, although important conflicts are evident. With the combination of data sets, we are able to assign previously unknown samples to these infraspecific taxa. We define and provide an emended diagnosis of each subspecies using the criteria established by Willi Hennig in his 1966 assessment of the qualities and characteristics of infraspecific taxa, which he understood to be fundamentally different than the hierarchical phylogenetic system he established as the conceptual framework for taxa at the species level and above. Hennig's views are contrary to how the subspecies concept has been applied in much of recent mammalian taxonomy.

To examine the processes that maintain genetic diversity among closely related taxa, we investigated the dynamics of introgression across a contact zone between two lineages of California voles (Microtus californicus). We tested the prediction that introgression of nuclear loci would be greater than that for mitochondrial loci, assuming ongoing gene flow across the contact zone. We also predicted that genomic markers would show a mosaic pattern of differentiation across this zone, consistent with genomes that are semi‐permeable. Using mitochondrial cytochrome b sequences and genome‐wide loci developed via ddRAD‐seq, we analyzed genetic variation for 10 vole populations distributed along the central California coast; this transect included populations from within the distributions of both parental lineages as well as the putative contact zone. Our analyses revealed that (1) the two lineages examined are relatively young, having diverged ca. 8.5–54 kya, (2) voles from the contact zone in Santa Barbara County did not include F1 or early generation backcrossed individuals, and (3) there appeared to be little to no recurrent gene flow across the contact zone. Introgression patterns for mitochondrial and nuclear markers were not concordant; only mitochondrial markers revealed evidence of introgression, putatively due to historical hybridization. These differences in genetic signatures are intriguing given that the contact zone occurs in a region of continuous vole habitat, with no evidence of past or present physical barriers. Future studies that examine specific isolating mechanisms, such as microhabitat use and mate choice, will facilitate our understanding of how genetic boundaries are maintained in this system.