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Is taxonomic conspecificity ever "factual", or only based on "opinions"??

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There's an issue that's been bugging me since my early teens and which now, at 39, I would like to put to rest. I think I have long figured out the answer to my question, but would like to get confirmation of my conclusion from the experts. My dilemma concerns the "accuracy" of taxonomic species classifications as to conspecificity of different forms. As you likely know, across the geographic range of similar life forms, not all scientists are in agreement over which ones belong to the same species or not, and may "split" or "lump" them into the same or different species based on different criteria. For example, the African elephant has traditionally been treated as one species, Loxodonta africana. However, they have recently been split into two species: the larger African bush elephant (Loxodonta africana) and the smaller African forest elephant (Loxodonta cyclotis). My ultimate question for you is: is it ever possible to state unequivocally, or beyond reasonable doubt at any rate, that two subspecies, geographic forms, etc.  are not separate species, but are "definitely" conspecific, and if so, what criteria need to be fulfilled to do so? (adhere to X or Y definition of a species? Do a very precise DNA analysis?) Or is attempting to do so a futile exercise, and is the "splitting" and "lumping" of species always going to be a matter of opinion (albeit hopefully an educated opinion) of a particular author and his/her adherents or detractors?

In order to give a practical background to my question, or at any rate to illustrate why it has been so interesting to me, here is my history with attempting to answer it. I have always loved nature and wildlife. When reading about different animals back in my school days, I noticed after a while that different authors referred to different geographical forms of a given animal as either being of the same species, or else of different species. After a while I noticed a trend: especially older books would refer to North American species that had similar counterparts in Eurasia as being separate species, while newer books would refer to them as belonging to the same species. For instance, the Grizzly and Alaskan Brown Bear being treated as either the same species as the Eurasian Brown Bear (Ursus arctos), as is still common today, or as several distinct species (the number of which varied from two - Ursus horriblis and Ursus middendorffi  - to at the extreme, the 87 species and subspecies proposed by C. Hart Merriam in 1918). As another, less extreme example, the Red Fox in North America has often been treated as at least one separate species (Vulpes fulva) from the Old World Red fox (Vulpes vulpes), though since C. S. Churcher's 1959 revision, it has become customary to treat them as one species. An even bigger dilemma seemed to concern the question of whether the North American (and, in fact, East Asian) Elk was a distinct species (Cervus canadensis) or a subspecies or set of subspecies of the Eurasian Red Deer (Cervus elaphus).  I really wanted to know which of the species I read about "did" or "did not" occur on both sides of the Pond (partly due no doubt to simply wanting a factual classification, and partly for sentimental reasons as I am Canadian but with very recent European heritage, so was interested in finding similarities between the two climes). So I started going through all the books I could, reading the descriptions they gave of the different relationships between the (sub)species, and trying to interpret the authors' justifications for their stance, apply my own logic to it, and draw conclusions about it. Long story short, I found that I couldn't get any "exact" answer as to whether any given species was indeed holarctic or whether the ones in North America were actually a different but related species to the ones in Eurasia. It always seemed to boil down to the opinion of one or more given authors, and whether individual other authors adopted his or her nomenclature or not.  After a few years, I gave up seriously trying to find "factual" proof of various life forms' conspecificity.

I  realize that the question is complex in part because there are different definitions of what a species is (for example, some people have criticized Mayr's popular "Biological Species Concept", which is kind toward "lumping"). Also, the methods used for determining similarities and differences have changed over time. Older researchers had a method where they applied a kind of craniometry, measuring many different skulls of a given life form and coming to conclusions on their speciation based on how much, or little, they varied over a geographic range. This seemed to assume that the structure of the skull was static, and did not depend on e.g. varieties in the gene pool or the diet of the individual. More recently, comparing genes and DNA has been used, and I assumed at first that this was a more exact science. However, I have found that it is not really. Different scientists have come to different conclusions about a given species based on their own studies of the DNA as well. It's also interesting to note that whereas in the past, the splitters seem to have treated to separate species by geographic location (of all species of animals considered to be native holarctic, I think there are very few that have never been split into separate North American vs. Eurasian species by someone at some time), today you will often find that "splitters" will erect a species somewhere at the edge of a life form's geographic range. For example, the Gray Wolf (Canis lupus) seems to have been consistently regarded as a holarctic species since the 1950s or so, but recently one of the former species, Canis lycaon has been recognized again for those wolves that live roughly in Eastern Canada (this is controversial, though, and some think they are, for example, the result of a hybrid wolf-coyote population). And the criteria used for determining conspecificity seem to vary from researcher to researcher. For example, I hope that this is not too much of an oversimplification, but I think one recent study claims that, because the North American and European Red Fox, referenced above, have been genetically isolated for a very long time, the older classification of them as separate species may be correct. I have even read that some think that the Giraffe, long considered a single species, may well be multiple species; a 2001 study applying the "phylogenetic species concept" recognizes a complex of 8 species of Giraffe! On the other hand, the aforementioned North American Elk, while genetically now considered to be a distinct species from the Old World Red Deer, are supposedly still so similar morphologically and genetically to some Elk in Eastern Asia, that it might be desirable to consider them not two subspecies (Cervus canadensis canadensis and Cervus canadensis sibiricus), but one (C. c. canadensis), though they are geographically isolated and thus cannot exchange genes. (Could the fact that they are have been found to be so similar as to warrant potential con-subspecificity be used as "proof", at least until evidence is found to the contrary, that they are in fact the same species? This is an argument I used for myself in the past, but have long wondered if it's a valid argument).

All this considered,  I get the impression that there is no way of telling for certain whether two geographical forms (e.g. European and Alaskan Old World Swallowtail butterflies, or North American Caribou and European Reindeer) are in actual fact one species or not, but that it is always ultimately a matter of opinion. Is my conclusion correct, or might there be nevertheless be some way of finding the conspecificity of two geographical forms to be a factual truth, at least under certain definitions of a species? And if so, what might that method be? I hope I didn't bore you to death with these musings. This is an old question on which I would really like to have some sort of closure after all these years.

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From my viewpoint, which is heavily biased due to the fact that I started work with microbes, I would say that in itself it is impossible to make clear and universal delineations. That being said, it is not the same as to say that existing categories are not factual. There is certainly divergence that we can measure using different tools (which may yield different results), but since everything is on a continuum, we won't find an universal measure to be applicable for every species with the same level of usefulness. I.e. while we could arbitrarily say define a certain amount of genetic distance as the threshold, it will most likely lead to categorizations that are not terribly useful in certain cases. E.g. using thresholds developed for bacterial species would probably lump almost all primates into one group. Doing the reverse would fragment bacterial phylogeny to an incredibly useless degree.

So while certain groupings can be debated, they are not merely based opinion, at least no in an arbitrary sense. But the justification of one categorization vs another may depend on viewpoint, or more specifically, the objective of a given study leading to certain categorization. 



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  • 2 weeks later...

Isn't it the case that classifying of species is just a way of organising knowledge? It is convenient to group organisms together in order to study their common features and behaviour, which in turn may contribute to a new (improved?) classification. Classification is artificial and is a simplification of nature. As long as this aspect it recognised the simplifcation is a strength, not a weakness. If it is agreed that classifications are for convenience, then as CharonY suggests the appropriate rules for defining species depend on viewpoint and will be the ones most convenient for the intended purpose.

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On 3/7/2019 at 4:31 PM, erracht said:

is it ever possible to state unequivocally, or beyond reasonable doubt at any rate, that two subspecies, geographic forms, etc.  are not separate species, but are "definitely" conspecific, and if so, what criteria need to be fulfilled to do so? (adhere to X or Y definition of a species? Do a very precise DNA analysis?) Or is attempting to do so a futile exercise, and is the "splitting" and "lumping" of species always going to be a matter of opinion (albeit hopefully an educated opinion) of a particular author and his/her adherents or detractors?

I do not think so.  It all gets very murky when identifying a different subspecies or species.  As 2 separated populations of a single species evolve towards different species there is no digital point at which we can say the 2 populations are separate species, it is instead a smooth transition and so it is very difficult to say with any clarity at which exact point along this smooth transition that the dividing line between species occurs.

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