Isn't there already a separate word—subspecies—for "isolated reproductive groups, with different phenotypes, which could still interbreed if the opportunity arose"? My understanding was that every phenotypically-distinct isolated reproductive group was considered a subspecies until its genetics diverged enough to have speciated, at which point it was now a species.
It seems to me that if e.g. American black bears and Asian black bears can interbreed, then we could call them all one species—black bears—and put all their subspecies together in into that taxonomic category. Maybe with some optional taxonomic level between "species" and "subspecies" for describing their phenotypic groupings.
But I see, looking at various sources, that those two types of bears are indeed considered separate species. Why do we do that? What's better/more useful about drawing the species boundary there?
This puzzled me as well. Do we not have a word for this? Is _subspecies_ really the word? In linguistics mutually intelligible but distinct `languages' are called dialects. Similarly is there a term for mutually interbreedable but distinct `species'?
Similarly is there a term for mutually interbreedable but distinct `species'?
There's not really any objective standard to draw the line between such populations. A clear line exists between populations that cannot reproduce, and there is probably something similar for populations that can reproduce but largely make sterile offspring (Ligers, Mules).
"Species" is an organizational convenience for biologists. Nature doesn't have such a boundary. It just has varying degrees of reproductive compatibility, inclination, and opportunity.
"Subspecies" is a concession to what lumpers call splitters.
"Species" is very clearly defined by "speciation": the point where something becomes too different to interbreed. Usually when two things can interbreed, but have many differences, they are referred to as separate subspecies of the same species.
I'm sure a proper taxonomist would be able to speak more authoritatively, but I rather doubt it. Subspecies tend not to interbreed for whatever reason, whether geographic isolation, sexual selection, or other factors, whereas humans rather enthusiastically behave in the opposite fashion. I don't think one could convincingly argue that humans are geographically isolated, either. You might be able to make an argument that human races are akin to pet breeds, but my understanding is that zoologists don't really consider pet breeds as subspecies.
The first, but speciation via sexual reproduction is much easier as non sexual reproduction blurs the lines of what counts as a species.
For sexual reproduction simply isolate populations and they drift apart genetically. This is why you can get ring species and populations that can’t get in touch naturally are considered separate species even if when artificially reintroduced they could interbreed. Thus many invasive species technically count if they could naturally get in touch the species would already be in that location.
Anyway that’s based on the current definition used by zoologists though some people find this definition unsatisfying but in both directions. Grizzlies and polar bears can interbreed and therefore count as the same species in that definition even though they are developing different traits not just hair color but body weight is diverging.
In the end biology doesn’t give a fuck about our definitions so things are messy.
I'm not a biologist, but my understanding is that animals that can interbreed but usually wouldn't are (if at all) divided into subspecies within the same species.
"A taxonomist decides whether to recognize a subspecies or not. A common way to decide is that organisms belonging to different subspecies of the same species are capable of interbreeding and producing fertile offspring, but they do not interbreed in nature due to geographic isolation or other factors."
Consider brown bears and polar bears. We consider them 2 species yet they can successfully interbreed. The main reason they don't is that their ranges haven't historically overlapped. Geography, not biology, was the separator. You could view them as one species with a bivariate distribution, in the late stages of diverging into two.
Canids likewise viewed as one species with significant ecological clustering, but divided this time by behavior.
That's actually not the definition used in biology. Simple example: The entire genus Amphiprion (aka anemonefish) are speciated due to appearance and regional locality. They can and do interbreed when introduced into each others populations, and have fertile offspring that are mixed species. They are declared species due to their appearance, and their natural range's prevention of interbreeding. That said, there are areas where some ranges overlap and it's not uncommon that one species mates with another and has hybrid offspring.
Being genetically capable of interbreeding makes sense as a definition of speciation, but that is not how species are currently identified. Natural range, aesthetics, and more than that, whether or not they DO interbreed in the wild (vs physically/genetically capable) play a more significant role in determining species.
There are many 'hybrid' species in nature, which is what I feel warrants some revision, perhaps using genetic data to restructure many classifications as subspecies rather than independent species.
edit: Sorry, I think I may have misinterpreted what you said to some extent and think we generally agree. I will maintain that according to the classification methods described, people such as the Sentinelese, may well be considered a separate species to an unbiased observer using the same guidelines we apply to other animals due to slightly different physical characteristics and social/geographical reproductive isolation prevents the possibility of breeding with anyone outside of their group.
There is certainly a conservative definition for speciation, though: the point where something has zero reproductive compatibility—where there is no known example of viable offspring. At that point, inclination and opportunity cease to matter.
Why not just define “species” by that clear formal boundary, and then call everything that doesn’t manage to reach that line “subspecies”?
Well there are plenty of other cases where we are completely comfortable calling them distinct species, even though we know they can interbreed. The term "species" is certainly fuzzier than we were taught in school in the 70's, but it's not as if biologists have stopped using the concept.
The species distinction is often made when two populations of otherwise compatible creatures do not reproduce due to time, location, or physiological factors. In this case, geography has historically kept Grizzlies and Polar Bears separate, so their physical features have diverged a great deal despite remaining sexually compatible.
In other cases, different mating seasons or physiological incompatibility might prevent breeding, even though the gametes are compatible. Coyotes and wolves and dogs can breed, for instance, but they usually don't and so they're considered separate species.
The mathematician in me hates this abuse of what should be an absolute and well-defined term, and the cynic wonders about eugenicists making a species distinction between the various human lineages.
It extends far further than that. There are cases of interspecies breeding in mammals (A + B = C). What species are the offspring (C + C = C?)? What if such offspring continue to breed? Sometimes, the offspring are not sterile! Do they become a new species. What if, over a few generations, you get a healthy mix of (CCBA + ACBC = ????)?
The idea of categorizing things is a convenience that our minds and our society employ to keep things simple. It works in a large number of circumstances. However, it is important to realize that these categories are not 100% rigid and that situations arise that are not simple to categorize. The result is sometimes broadening the definition of a particular category, creating a new category, or ignoring the uncomfortable situation. In reality, it is important to acknowledge that reality does not answer to our whims and needs to categorize things and just accept that clear lines can't always be drawn.
Polar bears and brown bears can breed and produce fertile offspring, but we reckon them to be not just subspecies, but separate species altogether. Whether to lump or split is rather arbitrary.
Thus, it's observably true that the position that there are no human subspecies isn't a scientific one, but rather a moral one. Personally I'm comfortable with that, because being Catholic I account all human beings to be created in the image of God and equal in worth in His eyes. Whether or not there are genetically determined differences in mean behavior between different ancestry groups in Homo Sapiens doesn't change that one bit. Neither for that matter does how scientists choose to classify them.
MustardTiger I sympathize with the points you are trying to get across (and the undeserved downvotes and careless replies you are getting). Just one thing though
> We have greater genetic difference between races than ANY example of subspecies in existence.
You went too far with that comment. I think I know what you're trying to say: that if intrahuman genetic divergence and superficial morphology differences were observed in any other organism it would be classified as subspecies, but as a matter of fact there remain many examples of strongly genetically divergent populations in nature that have not been elevated from subspecies to species, and some which are unlikely to be due to known introgression etc.
EDIT: yes I know that introgression doesn't contradict species status for many species definitions, but taxonomy literature is determined by practices local to the community studying the taxon in question and for many taxa introgression is still certainly an argument against classifying as species.
Riddle me this, you have 2 populations containing 2 sub-populations each. Lets call them populations Aa and Bb. 'a' can mate with 'b' and produce c. c can produce viable offspring with A or B but A and B can not produce viable offspring. What do we call Aa and Bb if not distinct species? What do we call c?
What if some members A can produce viable offspring with B some of the time but not always? What then? My point is we make taxonomical distinctions for our convenience, they don't really have any hold on the natural world.
Incidentally, the species debate is fairly simply summarized / resolved, at least for typical sexual organisms: There is nothing whatsoever in first-principles evolutionary biology that says that these can always be partitioned into discrete categories that one might call "species". What evolutionary biology says is
1. When two populations overlap in space and time and have the opportunity to interbreed, they might do so.
2. In the absence of interbreeding, populations evolve, divergently w.r.t. most markers other than the occasional selection-mediated convergence.
So, divergence might be the result of lack of opportunity to interbreed (this allows genetic and phenotypic diversity to accrue, but does not itself represent a biologically real division), or it might be the result of inability to interbreed (biologically meaningful, but impossible to discern if opportunity to interbreed is absent). And that's it, for sexual organisms that is the gist. In general there's nothing to say that our systems of related populations spread through space and time can always be divided into "species". On the other hand any cursory look around a single locality shows that many discrete biological entities do exist; there's nothing wrong with giving them the name "species" as long as we acknowledge that for closely related allopatric populations the question of whether they are the same "species" is not defined.
There are various problems. One is that some humans are attached to the idea that entities exist for which the word "species" is apt; or put another way they are convinced that "species" must mean _something_ and they ask what (e.g. because they like their neatly divided up bird lists).
Another is that if scientists go around saying "animals can't always be divided into things which it makes sense to call 'species'" then it significantly weakens certain arguments for the protection of populations in conservation biology / legal contexts, since these invariably name the species-level taxa they are intended to protect. Since the modern trend is towards splitting subspecies (i.e. geographical races) into species (as opposed to lumping), many conservation initiatives are based on protecting "species" for which the species designation is subjective. That doesn't mean they are flawed of course -- they are nearly always protecting valuable ecosystems. It's just that as we split geographical races, we (by definition) create rarer and more vulnerable taxa, and so conservation initiatives arise around these.
It seems to me that if e.g. American black bears and Asian black bears can interbreed, then we could call them all one species—black bears—and put all their subspecies together in into that taxonomic category. Maybe with some optional taxonomic level between "species" and "subspecies" for describing their phenotypic groupings.
But I see, looking at various sources, that those two types of bears are indeed considered separate species. Why do we do that? What's better/more useful about drawing the species boundary there?
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