I read that Neanderthals and Humans interbred in Europe ,so is this another case of two branches fusing together ?although this was due to sexual reproduction and not HGT.
I like what paleanthropologist John Hawks and speciation specialist Jerry Coyne writes on this. They agree that anatomically modern humans, Neanderthals and Denisovans are subspecies, so we are today a hardy hybrid rather than a puerile purebred. [From the recent sequencing of 4 kyrs old african we can now see that there was a backflow from west Asia of agrarians ~ 6 kyrs ago all the way sub-south Sahara, bringing back Neanderthal genes to almost every living african.]That's certainly a possibility. Biologists have observed examples of despeciation—species disappearing due to hybridization with related species. A recent example was observed with Darwin's finches in the Gallapagos, where it seems one species of finch may have gone "extinct" though interbreeding with other species:
"So what about “modern” H. sapiens, Neandertals, and Denisovans? Clearly they hybridized, and some of the hybrids were fertile, for traces of Denisovan and Neandertal genes remain in our genomes. On this basis, anthropologist John Hawks deems Neandertals, modern humans, and Denisovans members of the same species; Gibbons quotes him as saying “They mated with each other. We’ll call them the same species.” (I hope by “mating” he means “mated and produced fertile offspring”.)
But a little bit of gene flow isn’t enough to convince most of us that these groups were conspecific. On that basis, the Darwin’s finches would be deemed conspecific, but nobody does that. The question is whether that gene flow reflected lack of opportunity for mating (in which case they might be the same species), or pervasive hybridization (between, say, modern humans and Neandertals) but only weak viability or fertility of the “hybrids” (in which case they’d be different species). We will probably never know the answer to this.
Does this make the species status of these three groups purely arbitrary? I don’t think so. What we can do is get a “yardstick” by seeing whether other species of primates that were separated for as long as Neandertals, Denisovans, and modern humans—roughly half a million years—have evolved into reproductively isolated groups. I’m not sure what the answer is (it’s probably sitting there somewhere in the literature), but I’d guess that they wouldn’t be separate species, especially because humans have much longer generation times than other primates and so would speciate even more slowly. If it were my call, I’d agree with Hawks (but for somewhat different reasons), calling Neandertals, Denisovans, and modern humans all members of Homo sapiens.
But as for the hobbits, H. floresiensis, I’d stick with calling them a different species. They diverged from modern H. sapiens much further in the past, although they may have been contemporaneous with us."
[ https://whyevolutionistrue.wordpress.com/2011/01/28/how-many-species-of-humans-were-contemporaries/ ; my bold]
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