(August 23, 2018 at 1:49 pm)Khemikal Wrote: Species can be difficult to pin down (not as difficult as some might imply) - but this isn't a particularly good example of that.
Because the mule example has been brought up...I'll elaborate by reference. A certain percentage of our dna is neanderthal in origin. If this relationship were similar to mules..then we would expect that modern horses had a similar percentage of donkey dna. They don't. Their shared genetics comes at the level of genus.
The reason that they don't is obvious, and it's related to why horses and donkeys aren't the same species (or subspecies of the same species). Their offspring are overwhelmingly sterile. They are not breeds of horses or donkeys but an entirely different animal. Chromosomal incompatibility forces this.
We couldn't have a noticeable amount of neanderthal dna, today, if the outcome of our mating with them was similar to the outcome of a horse and a donkey mating. Not only do we have neanderthal dna in modern human populations, we have denisovan dna in modern human populations. Combine this with a denisovan/neanderthal breed....and this strongly suggests that the relationship between the three is one of concurrent subspecies..rather than distinct species with both freak interbreeding instances..and even more freak -fertile- offspring and subsequent interbreeding between all three of those freak fertile interbreeding instances...all amounting to an unrepresentative showing of dna in modern populations.
Could it happen? Maybe....the odds are beyond long, and there's a much simpler and much more probable explanation which fits available data in every way that the long odd hypothesis doesn't...and all of this is explained by the genus, species, subsp, breed model, rather than being muddied up by it, just as modern variations within subspecies are.
The confusion, such as it is, goes back to a quick generalization we all learned in school as a shortcut for a complex picture. That if two animals -can't- breed with each other, they aren't the same species. This is true. However, it doesn't actually follow from that that there are no two species which can, but our mind makes this connection and creatards latch onto it. Combine this with the fact that popularizing articles often say "two different species" rather than "two different subspecies" and the knot is well and truly tied.
If the genetic contribution of Neanderthals and denisovans to HSS were highly advantageous to HSS ancestors, there need not be any substantial number of successful interbreeding between HSS and other populations for the genetic contribution from those other populations to be highly selected for, and thus prevalent amongst modern HSS gene pool.
The fact that all humans outside Africa today carry a significant Neanderthal genetic contribution only says successful interbreeding occurred at least once. It diesn’t Say interbreeding was frequent or trouble free. So it diesn’t Say Neanderthal and HSS are necessarily close enough to have no barriers to interbreeding that would have made them difference species.
