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).
I thought species were defined as being broadly unable to interbreed? (Yes, mules and ligers exist but they’re exceptions to the rule produced by human intervention.)
In biology they would call that a "ring species". Sometimes two nearby populations can interbreed, which means they are the same species. But distant populations are so different they can't interbreed, and so must be different species. But figuring out exactly where to draw the line is problematic.
I don't think the concept of a species is well defined. Tiger and lions can interbreed, as can donkeys and horses. And then there are ring species. Biology does whatever it does and then humans try to fit words around it.
Some mules (a very few - something like 60 in the past 500 years) are actually fertile. Does that change your mind about whether or not horses and donkeys are distinct species?
The reality is that there is no good definition of species, and cannot be. Yours is a common one, but has a lot of problems - declaring wolves and pomeranians the same species is a tough bridge to cross for most people: you've made the definition so broad it means little. Biology is messy and in the real world there are no clear-cut, convenient dividing lines.
Another problem with that definition is that there is no transitivity in real biology. Animal A and Animal B may be able to produce fertile offspring. Animal B and Animal C may be able to produce fertile offspring. But Animal A and Animal D [a sibling of Animal C, thanks KMag for that point!] may not be able to.
Of course, we also would find it strange to declare that two humans incapable of producing fertile (or any) offspring different species.
In the past the rules for 'species' was not whether or not they will interbreed, but whether or not their offspring would be fertile.
A horse and a donkey can breed just fine, but a mule cannot.
This makes sense because we know that species eventually differentiate and whether or not they can recombine is a good indication of how far apart that separation is.
But nowadays all that stuff is thrown out of the window. I don't really understand why it is no longer considered a useful distinction.
There are even not-uncommon situations where Population A can breed with Population B, and Population B can breed with Population C, but Population A and Population C can't interbreed.
Using "can breed" as the metric for "same species" creates the situation where A and B are the same species, and B and C are the same species, but A and C are not. That's clearly contrary to what most people have in mind when they use the word "species".
Species really should be a continuum rather than a binary.
In practice, that is often not the case. Most species haven't been tested to see whether they can produce viable offspring with each other(it's usually impractical), and there are plenty of different species known to breed successfully with one another. Mules, horse-donkey hybrids, while the vast majority of them are infertile, are considered different species. Camels and Llamas can produce viable offspring even though they evolved on two different continents.
Isn’t the definition/line drawn for the delineation of a species (compared to a sub-species/variety/variant) if offspring will be produced that can also reproduce.
Horse vs Donkey == mule (which can’t reproduce) being one of the few examples where anything is produced, and why they are considered different species.
One challenge here of course is while we have examples of Neanderthal/human hybrids, we don’t have enough of them to say it’s anything but a crazy fluke. But that could just be due to not having enough data points, not due to it being hard.
If it wasn’t a crazy fluke, then yes they shouldn’t be a separate species.
We could make that the definition, but we'd doing a lot of redefinition: coyotes and wolves would become the same species, as would lions and jaguars. Fertility issues tend to increase with genetic distance but aren't guaranteed; for example, mules are usually but not always sterile.
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?
There are many species that are separated by 100K/1M years that are still able to interbreed. Horses, Donkeys, Zebras are a good example. These animals can produce viable offspring but still have significant anatomical differences (those differences tend to define what we call species)
Ligers are not infertile and species does have a definition in biology. Where the argument comes about is if two different populations are different species or not.
You can't use such a literal-minded interpretation in the real world. The real world is complex and fuzzy.
For instance, in species with sexual differentiation, two males or two females cannot mate, but are considered the same species.
For a more precise definition, you would have to, say, look at the probability that a pair of organisms of the appropriate sex from two different populations would be able to mate, and produce viable offspring (that is, offspring that could themselves mate and produce more offspring).
Asking about a single organism that happens to be infertile, or a single pair that happens not to be able to mate or produce viable offspring, is not particularly interesting, but at a population level you could use a statistical test to get a probability distribution, rather than a yes or no answer.
Note also that it can be the case that two different species can mate, and produce offspring, but the offspring itself is not viable (cannot mate and produce offspring). This happens in the case of mating two closely related species, such as when mating a horse and donkey to get a mule (https://en.wikipedia.org/wiki/Mule), or a lion and a tiger to get a liger (https://en.wikipedia.org/wiki/Liger).
If we interbred with them, how are they actually species? It seems more to me you're talking more about racial or breed differences akin to breeds of dogs if two species actually successfuly had viable offspring in the wild.
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).
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