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What's funny to me is that all the "splitters" magically become "lumpers" when we're talking about hominids
@The_Nybbler replied to you above:
I'm kind of replying to you both. West Africans (and other Sub-Saharan Africans) have an estimated 2%-19% of their genome derived from an archaic hominin ghost population, a population less related to modern humans than Neanderthals or Denisovans (both of whom have introgressed into modern humans, especially Eurasians). That is, this ghost population split from the modern human line prior to Neanderthals and Denisovans.
As this ghost population gets better characterized genetically (or maybe even fossily, but fossils are tough to get in Sub-Saharan Africa) and the admixture percentage in Sub-Saharan Africans is better ascertained, I suspect that there will be an increased push to consider this ghost population as a sapiens subspecies or population (and thus Neanderthals and Denisovans would get lumped in, as well), especially if the admixture percentage from this ghost population in Sub-Saharan Africans is in the mid-single digits or higher.
Can't have Sub-Saharan Africans with the most archaic non-sapiens admixture, especially from an even more distantly related member of Homo.
I mean, clearly all three of these species were capable of producing fertile offspring with H sapiens, that's how the genetics got into the populations. Calling everything one species is therefore following the actual definition of a species.
That's a definition of species (or at least a variant of one), albeit arguably the most popular definition. I'd likely recite a similar definition if I got suddenly cold-called by God. However, see the Wikipedia article on Species I also linked to you elsewhere in the thread for more definitions. There are many cross-species hybrids that can produce fertile descendants, and sometimes even cross-genera hybrids as well.
Several feline taxa are capable of hybridization. The Chausie is fully fertile by the fourth generation, but the jungle cat and the domestic cat remain separate species.
The serval and the domestic cat are in different genera, but can hybridize to make the Savannah cat. Female hybrids are fertile right off the bat, and male hybrids can be fertile by the fifth generation.
Beefalo are fertile. Most Bison herds are actually partially descended from cattle. Yet not only do bison and cattle remain in separate species, they remain in different genera.
Grizzly bears (Ursus arctos horribilis) and polar bears (Ursus maritimus) can produce fertile female offspring; it has yet to be seen if they can produce fertile male offspring. Brown bears in general are also partially descended from polar bears.
Although hybrids can likely be had via IVF, it's doubtful if unadmixed Great Danes and Chihuahuas can mate due to the massive size and anatomical differences between them. Some on the internet claim that any photos of Great Dane-Chihuahua offspring are all—or at least mostly—hoaxes. Nonetheless, Great Danes and Chihuahuas are not only considered firmly within the same species, they're considered firmly within the same subspecies.
Speaking of canines, wolves produce fertile hybrids with species such as the coyote and golden jackal. Additionally:
Yet, grey wolves, golden jackals, and African wolves are all considered separate species.
And these were just examples using some more familiar animals. So producing fertile offspring does not appear to be a sufficient condition for being considered the same species (and perhaps for genus too); it may not even be a necessary one. To circle-back to the original Homo example, Neanderthals are still "generally regarded as a distinct species", thus likewise for Denisovans. Hence, if race can be dismissed as merely a social construct (except when justifying racial preferences and income/wealth transfers to benefit fashionable minorities at the expense of everyone else)—then so can species—where the social construction aspect is subject to fads, politics, convention, group-think, and outright invention (as in the case discussed by OP).
As a side note—it's funny how, at least within mammals, male hybrids look to have a much rougher go at reproducing than female hybrids. The male burden of performance is not unique to modern humans.
I know there are multigenational ligers. Just keep breeding the mixed females with pure tigers or lions. I don't know if someone is trying to make a stable fully fertile hybrid population.
I was thinking about including ligers/tigons, but I already had multiple examples so I de-prioritized ligers/tigons, and ended up not getting around to them for the reason you mentioned: males have been established to be sterile with a fair degree of certainty.
I went with the grizzly/polar bear example, since at least two female hybrids have been shown to be fertile (in the wild, no less!), it's merely unknown if male hybrids are fertile or not, and brown bears are partially descended from polar bears.
It also made me chuckle that you mentioned ligers but not tigons. Ligers (lion father, tiger mother [not that kind of tiger mother]) are more famous than tigons (lion mother, tiger father), likely due to the large body size of ligers (larger than both lions and tigers, whereas tigons aren't any larger [and may be smaller] than lions and tigers).
Life must be rough for male tigons. Small, infertile, and forgotten, while female tigons, ligers, lions, and tigers put in their Panthera dating profiles: "Don't bother if you're under 10' or 1,000 lbs."
However, it's noteworthy that lions and tigers are able to produce fertile female offspring as it is. They diverged about 4 million years ago. Lions are actually more closely related to leopard and jaguars than they are to tigers and snow leopards; tigers are more closely related to snow leopards than they are to lions + leopards + jaguars. Humans and chimpanzees split about 5.5 million years ago.
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Not an expert, but I’d imagine this has to do with the male only having one copy of certain genes due to XY while the female has two.
It's indeed a thing called Haldane's Rule:
Most mammals abide by the XX-XY system, where males with the XY are the heterogametic sex.
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Really? Could you provide me with some further reading?
IIRC, Denisovan admixture is only significant among Austronesians, though East Asians have a tiny amount.
Interbreeding between archaic and modern humans, summarizes Neanderthal, Denisovan, and African ghost population admixture.
Current state of affairs suggests Melanesians and some other populations in Australasia have about 4%-6% Denisovan DNA on top of the 1%-4% Neanderthal DNA of Eurasians in general.
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