I spent much of December and January reading about these topics. I chatted with "PD" a bit about the topic till he'd had enough and quit talking to me. Without a PhD in genetics, the studies and scholarly articles on these topics will numb your brain pretty quickly. The most recent published studies from 2017 regarding the genetic relationships between deer of the Americas are summed up in the paper below. I'm pasting some of the specific verbiage from it so as not to misstate their words. The gist seems to be:
1. MtDNA studies indicate that North American deer (Odocoileus) should likely be classified under the genus Pandora (rather than Odocoileus), but since the majority of important studies performed on deer in the Americas were done on O. Virginianus and can be cross-referenced by that name, it would create scientific confusion to change the genus name of WT deer.
2. BT deer seem to be more closely related to the species Mazama Pandora, a small deer found in the Yucatan Peninsula, than to Mule Deer. In fact Mule Deer are more closely related to WT deer than they are to BT deer. (Hard to imagine considering the similarity of morphology, adjacent geographic ranges, and ongoing hybridization between MD and BD!)
3. Other discussions on this thread regarding WT hybridiztion: They do hybridize with BT deer. Genetic studies of Columbian WT's show evidence of BT's in their genome (no reference cited below) Google: Cowan Blacktail - WT hybridization. Also, I provided a study examining the hybridization of WTs and Mule Deer in Texas. Y'all can read that if you're interested.
4. (edit) Genetic study on the speciation of deer in North America link at the bottom. This mitochondrial DNA study indicates that BT deer separated from the other Odocoileus species around 2 million years ago, and represent the oldest deer lineage of that Genus in N. America.
"The mule deer mtDNA lineage diverged from the black.-tailed deer, and the white-tailed deer mtDNA lineage subsequently diverged from the mule deer lineage." None of this is set in stone, but the science clearly indicates that the entire subject needs more study to gain a better picture of the evolution of the deer of the Americas.
https://www.ncbi.nlm.nih.gov/pmc/articles/PMC5673856/The traditional classification of species of Odocoileus is incongruent with the phylogenetic information currently available for them. Our results suggest (1) that the columbianus and sitkensis lineages, currently treated as subspecies of Od. hemionus, form a clade that is more closely related to Od. pandora than to Od. hemionus; and that (2) Od. hemionus appears more closely related to Od. virginianus (even to Od. virginianus from South America!) than to its putative subspecies columbianus or sitkensis. In agreement with this possibility, the level of uncorrected genetic divergence, calculated with CYTB sequence data, between the hemionus and the columbianus groups (6.2%) greatly exceeds mean levels of divergences within species (and species-like lineages) of Odocoileini and Rangiferini (all below 3.6%, Table Table4).4). Surprisingly in view of their importance to North American hunters, no phylogenetic study using nuclear sequence data from mule deer, white-tailed deer, and black-tailed deer have been conducted to date. If further analyses based on sequence data obtained from independently inherited loci confirm the topology obtained from mtDNA, then reconciling taxonomy with phylogenetics would require elevating columbianus and sitkensis to species rank (see Future Directions). However, such further analyses based on multiple loci are likely to produce an alternative topology, for example by recovering all lineages of mule deer, white-tailed deer, and black-tailed deer as a monophyletic group and with pandora sister to it. Under this plausible scenario, and for the sake of binomial stability, which has important implications for scientific communication (see discussion on this topic by Gutiérrez and Marinho-Filho 2017), we transfer pandora to the genus Odocoileus, in congruence with the close relationship and overall similarity it shares with other members of Odocoileus. Regardless of which of these alternative topologies will be favored by additional analyses, dense geographic sampling is necessary to produce a suitable taxonomic classification with respect to lineages currently treated as members of Od. hemionus and Od. virginianus. This is particularly important due to the tremendous morphological variation documented among (even geographically close) populations of Neotropical white-tailed deer and the possibility that they might not be conspecific (as proposed by Molina and Molinari 1999, and Molinari 2007).
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From earlier in that article:
The sister relationship between pandora and the columbianus group also suggests that the biogeographic history of these deer is complex, but this topic requires robust phylogenetic inference, enabling ancestral area reconstructions and proper molecular dating. However, discussing the nomenclatural implications of the close relationship between pandora and the genus Odocoileus is necessary, especially after Escobedo-Morales et al. (2016) advocated allocating species of Odocoileus into the genus Mazama. Such an action, which has been contemplated by a few modern authors (Haltenorth 1963, Grubb 2000, Groves and Grubb 2011), would increase congruence between available phylogenetic information and the taxonomic nomenclature of Odocoileini but diminish efficiency in communication of scientific information. Allocating species currently treated as Odocoileus within Mazama would unnecessarily (see below) disrupt the association between the name Odocoileus and at least two—and perhaps more (Molina and Molinari 1999, Molinari 2007)—species epithets and the names of numerous subspecies (between 48 and 71) (Baker 1984, Brokx 1984, Méndez 1984, Smith 1991). This action would pose difficulties for retrieval of data and bibliography from repositories, such as GenBank and the Global Biodiversity Information Facility, and search engines, such as Google Scholar and the Web of Science, respectively. This is not a trivial matter because, given the importance of Odocoileus in aspects raging from public health to landscape ecology, massive amounts of data are associated with the name Odocoileus, whose North American members are among the most studied ungulates worldwide.
Hybridization of WT and MD in Texas:
https://vetmed.tamu.edu/files/vetmed/faculty/derr/publications/Derr1991Deer.pdfOn the Speciation of North American Deer:
Paleogeography of N. American Deer:
https://research.library.mun.ca/1190/1/Greenslade_AnnetteD.pdfThe earliest estimated time since divergence between a mull: deer genotype and a
white-tailed deer genotype is considerably more recent; on the basis ofBrown et aI.'s
figure, the 2.74% sequence divergence between KIMS and ORA (fable 6) corresponds
to a divergence time ofca. 1.1 x 1()6 years ago. Mule deer and black-tailed deer
genotypes diverged from each o1bera maximum ofca. 2.7 x: 1()6 years ago (maximum
sequence divergence of6.73% between AKB and eRDOS; Table 6). Based on these
estimates, the black-tailed deer genotypes represent the oldest Odocoileus mtDNA
lineage in North America. The mule deer mtDNA lineage diverged from the black.-tailed
deer, and the white-tailed deer mtDNA lineage subsequently diverged from the mule deer
lineage. This general onter is indicated by the branching pattern shown in the
pbylogenetic networks (Figures 13 and 14).
Someday I'll hit you with some good stuff on MD/BTD hybridization that might shock you a bit.
George out.