Hello! I got into an email conversation with my mammalogy professor on mammal subspecies that started from my hesitation on accepting subspecies of Virginia opossums. I thought some of you may be interested in seeing what I had to say since a lot of it stems from my time as an iNaturalist curator.
In general, I believe that the number of subspecies are exaggerated for most mammal species. Many of them were described before advancements in phylogenetics and are instead based on minute variations in skull morphology or pelage. While these can be indicative of a unique population of a mammal, they were mostly described from pretty small sample sizes; didn’t take individual variation into account; and contradict conditions of the environment that would suggest subspeciation had not actually occurred. For example, before being subjected to genetic analysis at one point there were ~90 subspecies of brown bears described, but now it’s understood that most of them are untenable and can be divided into just 5 clades (Miller et al. 2006; Hirata et al. 2013).
I believe there are “good” subspecies, and that these subspecies can be supported (and identified) genetically. But even when a subspecies (or even a species) is described on genetic grounds, sometimes researchers compare the individuals from the farthest extremes of a species’ range and don’t consider that the genetic differences between them could be clinal. At the suspected “boarder” between subspecies A and B, there could be no genetic differences between them. It is on these grounds that the IUCN has recently accepted Alces alces and Alces americanus as one species (Hundertmark, 2016) and that there are no valid subspecies of jaguars (Kitchener et al. 2017). Alternatively, sometimes a subspecies is described from a living population that has been greatly reduced due to extirpation. Consequently the genetic cohesion of the population is a product of inbreeding depression, not natural occurrences of isolation. An example would be Florida panthers (previously Puma concolor coryi), which was more genetically diverse prior to 19th century and samples taken from museum specimens indicate they are untenable from Puma concolor couguar (Culver et al. 2000).
I do not think the subspecies described are “good” or at least need to be legitimized, especially Florida opossums (Didelphis virginiana pigra) and Eastern opossums (Didelphis virginiana virginiana). I do not think they have been subject to any phylogenetic analysis of Virginia opossum subspecies, or I could at least not find one. I am personally skeptical of their distinctiveness because:  phylogenetic analysis of other widely-dispersed US mammals (cougars, bobcats, black bears, raccoons) with southeastern subspecies have found that they are untenable (Culver et al. 2000; Kitchener et al. 2017; Puckett et al. 2015; Cullingham et al. 2008);  there aren’t any boarders or other means of isolation that would prevent D. v. pigra and D. v. virginiana from interbreeding, potentially supporting a huge “subhybrid zone” that would make recognizing them as distinct meaningless;  the adoption of these two subspecies are largely derived from Gardner (1973), who did not cite any phylogenetic or morphologic justification for the subspecies; and  the one study I could find that critically compared D. v. pigra and D. v. virginiana - a morphological comparison between their pelvises - found “no significant difference in males or females for any absolute dimension of the pelvis or nonpelvic structures (Wilcox-on-Mann-Whitney test)” (Tague, 2003).
I think too many people accept subspecies (and species) at face-value, and may not understand underlying issues that would make them untenable. Taxonomic units aren’t discrete, absolute truths, and treating them as such sometimes have real world consequences for the animals involved. Recognizing Florida panthers as a separate subspecies from other cougars in the US burdens the population by encouraging inbreeding (by adopting the false implication that they are a unique subspecies) or pushing genetic revival of museum specimens for genetic diversity (which would be unnecessary, invasive, expensive). The best thing for Florida panthers would be to introduce cougars from Texas, Mexico, or California to the Florida Everglades to bring some much needed genetic diversity, but that will only happen if the concept of Puma concolor coryi is abandoned. This isn’t an issue for the successful Virginia opossum, but it’s still something I keep in mind.