This is a question for those who are familiar with taxonomy (the study of classifying life forms), and one to which I have gotten a decent answer on another forum but would like some further opinions or a broader consensus. When someone states that two individuals from different populations (especially distant ones) belong to the same species (simple example: the Kodiak Bear of Alaska and the European Brown Bear, both “Ursus arctos” according to current science), will such a statement ever carry the weight of “fact”, or will it always be no more than an educated opinion, subject to perpetual reassessment?
I will illustrate my quandary on examples from real life. As a teenager reading about animals, I was confused by the fact that I would read in a book that a given species was found in both North America and Eurasia (the term for this is “holarctic”), but some other book (typically an older one) would give a different species name. For example, the moose, Alces alces, is a holarctic species. It occurs in Europe, Asia and North America. However, back in the day I read “Alces americana” in the entry on the moose in the Merriam-Webster, implying that North American individuals belonged to a separate species. I soon found other examples. For example, some sources referred to the North American (and East Asian) Elk or Wapiti as being the same species as the Eurasian Red Deer (thus Cervus elaphus), while others referred to it as a separate species, Cervus canadensis, at least one source stating that there was no general agreement as to whether or not the two forms were conspecific. I wanted to have a way of knowing which species occurred on both continents and which did not, and found myself stumped by these disparities. So I started reading up on the matter, got introduced to the study of taxonomy, and ended up going down a rabbit hole from which I came out with no concrete answers. What I found was that there were “splitters” and “lumpers”. The former liked to divide life forms into multiple species (or other taxa, e.g. genera, families), while the latter liked to put together related forms into one species, etc. I found that of all recognized holarctic species, there were apparently few that hadn’t at some point been considered by some writer to be two or more different species on different continents, especially in the early/first half of the 20th century, when “splitters” dominated. I also eventually got the impression that there was not one way of determining what a species was. It used to be common to practice a form of craniometry with scientists measuring various points on many skulls of similar life forms and decide if they were one or more species or subspecies depending on how similar they were. Eventually, analysis of chromosomes, mitochondrial and, most importantly, nuclear DNA became more prevalent. Also, the acceptance of Ernst Mayr’s “biological species concept” defining species as individuals that could actually or potentially interbreed (under natural conditions, though disparity in locality should, for logistical reasons, probably not be considered “unnatural” conditions) and produce fertile offspring, would have contributed to a lot of the “lumping” of formerly separate species. All that said, though, after reading through various scholarly publications, I found to my disappointment that there didn’t seem to be an authority that could “determine”, or a method of definitively proving, that two individuals from different races/populations were of one species or separate species. At most, I found studies expressing opinions for a given species (e.g. University of Toronto researcher C. S. Churcher’s 1959 study in the Journal of Mammalogy where he indicates that Old and New World Red Foxes should be considered one species - Vulpes vulpes), and then other literature would either generally start using or not using the resulting scientific name. You would think that DNA analysis would make this an exact science, but apparently it hasn’t. New opinions still continue to be published. Here are a few examples (am sticking to mammals as above):
Since the 1950s or so, there doesn’t seem to have been any controversy over the notion that North American grey wolves are the same species as Eurasian ones - Canis lupus. However, in recent years, some researchers, based on genetic evidence, have restored a formerly recognized species, the Eastern Wolf (Canis lycaon) for those wolves occurring mainly in Eastern Canada. There are others that think this may be an intermediary species (the result of ancient hybridization?) between the Wolf and the Coyote (the same has long been postulated about another species/subspecies, the threatened Red wolf - Canis (lupus) rufus).
It used to be common for North American red foxes to be classified as Vulpes fulva, a distinct species from Vulpes vulpes, the red fox in general. Following Churcher’s 1959 article, mentioned above, it became common to treat them as one species; further research seemed to confirm this classification, although “V. fulva” would still sometimes come up in the literature. However, in 2014, some DNA research was published claiming that, as the North American and Eurasian forms have supposedly diverged some 400,000 years ago, Vulpes fulva could be considered a distinct species again (how the time of divergence makes the foxes themselves different from each other is unclear to me - it again seems to be based on different definitions or concepts of a species).
In the 1990s, it was common to treat Elk and Red Deer (mentioned above) as one species, Cervus elaphus, based in part on the biological species concept that supposedly applied to them. More recent DNA testing has indicated that they are less closely related than it was thought before, and Cervus canadensis is again commonly recognized a separate species. However, one of the East Asian subspecies (local forms) of Elk (Cervus canadensis sibiricus) is recognized by some as the same subspecies as the North American ones - Cervus canadensis canadensis. In the 1990s, this notion was postulated by Canadian deer expert Valerius Geist based on a supposedly identical morphology between them; one genetic study has confirmed that they could be genetically similar enough to warrant them being considered one race on both sides of the Pacific.
Going away from holarcticity, African elephants used to be considered one species, but now two are recognized: the African Bush Elephant (Loxodonta africana) and the African Forest Elephant (Loxodonta cyclotis). Some would consider that a third species, the West African elephant, could be split as well. Likewise, whereas in the past, giraffes were treated as one species, recently, it has been proposed to split them into anything between 2 and 9 species!
I hope these examples give a clear idea of the rabbit hole I went down with this topic. This brings me to my ultimate question: Is there any way in which we could “objectively” or “unequivocally” demonstrate that any of these or other examples represent the same or different species? Or will the question of whether red foxes from one area and those from another (or insert any other two populations of an animal) are the same or different always be a matter of opinion, a subjective question that is subject to changing criteria and definitions? (Example: as I mentioned above, Elk from North America and Siberia are now considered so similar morphologically and genetically by some as to likely warrant being considered not only the same species but also the same subspecies. Could someone still come along and justifiably claim they are not only different subspecies but in fact separate species, or is this unlikely)?
Depending on what answers I get, I may have one or two subsidiary questions, but will leave those for later.