Can taxonomy "prove" that two separate populations belong to the same species?

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.

The “species” concept is largely based on the ability or lack thereof for two groups to boink and produce viable, fertile boinklings. But the species concept is in real world use kind of a fuzzy boundry, not a sharp dividing line. Sure, you can absolutely say that a squirrel and a tarantula are two different species (for the love of all that is holy, don’t put them together and play Barry White to test it) but if two animals are kinda different but close enough to produce fertile offspring (for example, Neaderthals and Cro-Magnons) you can go either way.

If you’ve not heard of ring species, try reading about them. There is a species of gull in England that is considered the same species as a gull in eastern North America. It continues west and into Alaska where it jumps the Bering strait and continues through Asia, Europe, and crosses the channel into England, where it is a different species from the original. At no point can you identify that the species has changed, but it has. I once read about a ring species of snail around a lake in the west, maybe Tahoe. The concept of species is fuzzy and always will be.

As a Zoology major in college, I watched professors debate this issue and never come to any true consensus. Based on that I decided that species distinction is mostly a matter of opinion and not based on anything real. A tiger (Panthera tigris) and a lion (Panthera Leo) can successfully mate but I don’t think most people would consider them the same species. Perhaps using DNA to compare different groups you can draw some conclusions, but I’m not too sure about that.

It doesn’t really matter, and it’s fluid over time with some species splitting while others merging. It’s useful to scientifically classify animals into groups down to the species level, but arguing about whether two seagulls are the same or slightly different species seems like a waste of time. YMMV.

Under the Biological Species Concept? By definition, this would be determined by whether they can produce fertile offspring.

Are you asking whether it is possible to predict whether two physically separated populations could interbreed in the absence of direct evidence?

So the subject area you need to understand here is theories on the genetic basis for hybrid incompatibility. It’s complex and technical.

There are cases with model animals such as flies that are amenable to experimentation and extensive genetic analysis where much careful research has uncovered the genetic loci involved. But for a novel genome where we don’t already have an extensive understanding of the organism’s genetics and don’t already understand the specific genetic basis for potential hybrid incompatibility in that organism, there is no way that we could just look at a marginal case (two similar genomes) and predict whether the offspring would be viable. So the bottom line is - no, we cannot use genetic analysis to reliably predict (in a marginal case) whether two similar genomes could produce fertile offspring.

The other thing to bear in mind, of course, is that if two populations do not interbreed in practice, i.e. there is no gene flow, they will drift apart and eventually inevitably form two species. In cases where we are unsure if two populations are interbreeding in practice, genetic analysis can reliably determine and quantify the amount of interbreeding (gene flow) that has been taking place. But I don’t think that’s quite your question.

Two words: Saint Bewawa.

A high level view is necessary. The concept of taxonomy and species categories is an artificial set of rules that humans have created because we find it useful in describing the natural world. There is no law of nature that says species must be separated by clear and distinct boundaries.

Because these are human invented categories, we can change the rules for them as we want. So depending on how we want to define “species” sometimes certain animals may be grouped with other similar animals, and with other rules they may not be. We also can learn more information, so the rules don’t change, but what we know about the animals does.

You win the internet.

Agreed. I got a good chuckle out of that one.

Coincidentally, somebody almost coined another one a couple days ago:

Docklings. As in Ugly …

The latter, because…

Echoreply has it exactly correct. Now that doesn’t mean species are exactly fictional - they are defined by something ( and Ernst Mayer’s BSC is not the only widely used definition of species, though it is the best known). But they are artificial creations for the sake of categorization and categorized for the sake of systematic study. Taxonomy/systematics is always in flux and always will be. Not just because there are fringe cases that are problematic under a variety of definitions (hybrid parthenogens for example), but also because beyond that there is great deal of dispute over definitions.

This doesn’t imply total chaos. Experts parse and argue, but they do so based on at least some factual support. Nobody is going to accept you creating a new random species Brownus bobicus just because you like the name. It just means things change - I learned the Pacific Treefrog as a single critter, Hyla regilla. Now it is loosely (but not completely) accepted as three critters in a different genus. That’s just life in the world of systematic biology :slight_smile:.

Thank you, this is essentially the view that was expressed on the other forum. In this thread I’m detecting a consensus on this matter - that conspecificity of different related forms cannot be unequivocally determined as fact. I have suspected as much since at least my late teens. OK, then. I would like to ask a subsidiary question, which aims at getting closure on this question that has bothered me for years. If the thesis that regional form X and regional form Y are one species can never be absolutely proven, then what, in people’s opinions, is a healthy way of thinking about these related animals that exist across continents as to their potential conspecificity? Would it be to simply go by how they are classified in most current literature until the tendency to question the generally accepted classification arises? To think of them as clearly closely related transcontinental forms, while acknowledging that where exactly one species ends and another begins is not a quesiton with an absolute answer? Other suggestions?

This is very interesting; I vaguely recall either reading about this or postulating that something like this could exist (given that many species occurring over a broad area show “clinial” or gradual variation across that area).

It’s not the crux of my question, but the information you provide is pertinent, interesting and useful to know.

This is four years old and doesn’t give any answers, but it does describe some of the outcomes and questions raised from the increased use of DNA to define/redefine species: What Does It Mean to Be a Species? Genetics Is Changing the Answer | Science | Smithsonian Magazine

Modern humans all demonstrably belong to the same species, even though scattered over a vast range [say Cape Town to Ushaia overland by foot]. With modern travel and Tinder our genetic unity can be proven and maintained, but if we’d not developed mechanised transport then one or two glacials down the track it is quite possible that there would have been sufficient drift at the extremes to create distinct species.

This isn’t just a philosophical debate either. There was a legal case a few years back(it might have been decades) involving stopping logging on federal land on due to it being the habitat for an endangered species. One of the arguments the pro logging side used was there wasn’t enough differentiation to establish that the animals were a distinct species from the close relatives. I wish I could remember the animals involved to provide a link to the story. Does anyone else remember this?

The latter.

Your choice of the word “healthy” is disturbing. To me it implies that you have some psychological need for ideas about the real world to be perfectly bright-lined and have no overlaps, gaps, ambiguities, or uncertainty.

If so, that kind of thinking really doesn’t fit the natural world. Purely human inventions of pure thought can be that kind of crisp and simple. The real world is not, and cannot be made so. The best we can do is say “our model of X generally resembles most of the reality of X as long as we don’t look too, too close at the details.”

Which came first, the chicken or the egg?

More seriously, at what point in hominid evolution did the population that had a moved to Europe and Asia a couple hundred thousand years ago become a distinct species (if it ever did) from the one that stayed in Africa and only moved north 30,000 years ago? The two could and did interbreed and there are descendants today (about 8 billion of them).

I understand. Yes - for a long time, I had an unhealthy obsession with this question. I’m a person who likes factual knowledge and have spent a lot of time in my life looking for clear answers to various questions. I have come to realize that much in life does not have a convenient straight answer and is dependent on artificial human catergories. I’d also pretty much figured out long ago what I asked this thread to confirm about species (and other taxonomic categories) being essentially human constructions subject to change rather than unequivocal factual truths. I just wanted confirmation of my conclusions by those more knowledgeable.

Am not familiar with this exact case, but I know the principle very well. Having more “split” taxonomic classifications can provide conservation authorities with leverage to lobby for protection of a habitat (and/or of a life form found on that location) by pointing out that said habitat contains a specific taxon of that life form. As I don’t have a real-life example in my mind at present, I will illustrate my point with a hypothetical example. I referred to the Elk (Wapiti) above. Currently, it is classified as Cervus canadensis, a species that occurs in North America and Eastern Asia. Based on morphological differences, North American individuals have traditionally been split into six subspecies: Cervus canadensis canadensis (extinct), C. c. merriami (extinct), C. c. nelsoni, C. c. manitobensis, C. c. roosevelti, and C. c. nannodes. However, as already stated, there are those who believe that the differences among these forms do not warrant separation into subpecies (these differences supposedly arising from environmental factors and not genetics) and would prefer all North American Elk treated as “Cervus canadensis canadensis”, perhaps even lumping the Siberian C. c. sibiricus into C. c. canadensis" (though some might recognize that roosevelti (Pacific Northwest form) and nanodes (Tule Elk from California) are different enough to warrant subspecific status. Now, say there’s a natural area in California where Tule Elk reside. Someone wants to develop that land to the point of destroying their habitat. It would be a more powerful argument for the conservationist to say that a population of the distinct local Californian subspecies “Cervus canadensis nanodes” was under threat than to treat those individuals as just one population of the general type “Cervus canadensis canadensis”, as treating it as a distinct subspecies would highlight the loss of genetic material (which would doubtless happen anytime a local variant or population would die out, whether or not an actual subspecies was lost).

The spotted owl. The Northern Spotted Owl was threatened by logging. The California Spotted Owl was thriving. Are they different enough that the Northern needs protecting? Or is the Northern just a slight variation of the California?

An experiment sanctioned by the USFWS investigated the suitability of protecting the spotted owl by controlling its competitor, the barred owl. Work began in 2009 by removing populations of barred owls in designated areas. The studies found that in the absence of barred owls, the population of spotted owls did not decline, and were able to bounce back.

That’s the one. I remembered it was an owl but I didn’t remember the specific name.

Turns out to have not really been the case though. Probably more accurate to say they were declining more slowly. The Northern and Mexican Spotted Owls were just ahead of the curve.

And in fact just recently it is highly likely that last year’s severe high-intensity fires on the west slopes of the Sierras were very damaging to local populations.