The conundrum of subspecies: morphological diversity among desert populations of the California vole (Microtus californicus, Cricetidae)

Abstract

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.

On the Taxonomic Identity of Pteronotus davyi incae Smith, 1972 (Chiroptera: Mormoopidae)

“…Our results suggest that gene flow has only been recently disrupted or still occurs between P. d. davyi and P. d. incae; a more detailed investigation using nuclear markers is needed for focusing on this issue. However, the view of subspecies as genealogical networks of populations, without cladistic structure, does not preclude validation of subspecies nomenclature (Patton and Conroy, 2017). The two populations of P. davyi have unique geographical ranges and show diagnosable phenotypic characters, two criteria that fit recognition as distinct subspecies (Braby et al, 2012).…”

Delineating modern variation from extinct morphology in the fossil record using shells of the Eastern Box Turtle (Terrapene carolina)

“…carolina stands in contrast to the identification of intraspecific units in the fossil record of marmots and shrews [ 1 , 20 ]. It is possible that intraspecific entities could be recognized in additional vertebrate taxa, but those possibilities can only be explored if variation is documented in the appropriate morphological system for each taxon [ 1 , 20 , 176 ]. That work is an important step in creating comparable datasets that can be used to test hypotheses of long-term evolution derived from the relatively short-term records contained within the modern biota.…”

Misinterpretation of genomic data matters for endangered species listing: the subspecific status of the Peñasco least chipmunk (Neotamias minimus atristriatus)

“…Second, even if the recovered phylogenetic pattern was consistently well-supported paraphyly or polyphyly among subspecies within the Southern group (i.e., evolutionary non-independence that suggests either that interbreeding is still occurring or that not enough time has passed for populations to exhibit fixed genetic differences), it would still not be appropriate to invalidate subspecies status. Subspecies are well-established as potentially interbreeding units of analysis and represent taxa on the continuum of the formation of species (Wilson and Brown 1953;Padial et al 2010;Patton and Conroy 2017). Lack of strong support for a relationship does not signal strong support for the alternative (unless the alternative is strongly supported).…”

“…By extension, reciprocal monophyly is explicitly not an acceptable criterion for defining subspecies (Patten 2015). Subspecies are characterized by heritable diagnostic traits including morphological or molecular differences that vary in frequency between geographically discrete but potentially interbreeding units of analysis (Hennig 1966;Patten 2010;Patton and Conroy 2017). Thus, gene flow is expected among subspecies, which would result in a lack of reciprocal monophyly (Patten 2010, Patten andRemsen 2017).…”

“…In our view, the question of taxonomic validity of N. m. atristriatus therefore becomes: Have extended isolation, unique population ecology, and an associated independent evolutionary trajectory resulted in diagnosability of this subspecies? There exists an extended literature on the various definitions of subspecies (for thorough reviews see Haig et al 2006;Remsen 2010;Patton and Conroy 2017;and citations therein). Puckett et al (2021) used a definition of subspecies devised specifically for cetaceans by Taylor et al (2017:174): "…a population, or collection of populations, that appears to be a separately evolving lineage with discontinuities resulting from geography, ecological specialization, or other forces that restrict gene flow to the point that the population or collection of populations is diagnosably distinct."…”