Interesting topic
I'm studying evolutionary biology at the moment, so this post could be a long one... I'm also not a lore buff so will disregard much of it (the horror!) It's all legend and hearsay anyway, and this is a purely scientific explanation of what may have happened.
Firstly, a species is a group of organisms capable of interbreeding to produce fertile young. Lore states that the Bretons arose from the interbreeding of man and mer, which would suggest they are of the same species. This seems unlikely, so perhaps it is possible that an ancestor of the elves and an ancestor of the men, two subspecies of one common ancestor, who have began the process of speciation (evolving into separate species) but not so much as to actually be incapable of breeding together. Are men and mer still capable of reproduction? Are there any cases in the modern world of this?
The Grey Prince. We're told he has both Imperial and Orc parentage. However, this does not necessarily make them the same species. He has no children of his own so it's entirely possible that men and mer are capable of breeding to produce
infertile young, much as a horse and a donkey may produce a mule, which is incapable of having it's own young.
So, we have these nine humanoid races in Tamriel (I'm not including the others because I don't know enough about them) made of up (up to) four species. Men, mer, Argonians, and Khajiits. Argonians seem to be the least closely related to the other species - what exactly they are is hard to say. Reptiles? Amphibians? Even plants, due to their strange relationship with the Hist? I'm not sure. Anyway it seems unlikely they share a common ancestor with the other humanoid races (or at least, not for billions of years back) and so it would seem their relatively similar appearance to the other races (two arms, two legs, bilateral symmetrical etc) is a result of convergent evolution rather than sharing a humanoid ancestor.
The other species are clearly mammals, descended from the Ehlnofey, also likely mammalian. I would guess Khajiits were the first to diverge from the others due to their differing physiology. Men and Elves are clearly very similar. Assuming that Azura's curse on the Dunmer is just a creationist-style myth, the darkened skin of the Dunmer is possibly similar to the darkened skin of the redguards and can be explained by their habitat. Black skin developed in humans as an adaptation to protect against intense UV radiation. Perhaps the presence of the volcanic Red Mountain led to disruptions in Nirns O-zone layer (if it even has one), and the Dunmer developed darkened skin as a necessary defence against it the resulting radiation. Another possible explanation is that it developed as camouflage, helping them blend in with the rock and ash of Morrowind.
Yes, I just scientifically anolysed Tamriel... call me what you like
And I realize much of what I say is probably nullified by lore, or just not relevent to Nirn since it's not Earth. It just happens that these are two subjects I find pretty interesting.
Its a very interesting read, but you have to rememer that Tamriel differs from Earth in a major way.
It is a mythical universe, the kind of universe our ancestors believed in before we had the tools to figure out physics and the like.
Its geocentric, meaning everything revolves around Nirn, even the sun and stars.
Its a small universe, really only consisting of one 'solar system', the stars are holes in the veil unto Aetherius.
Its also very young, roughly 6500 years have passed since the Dawn Era.
Its a universe where gods are manifestly real and meddle in day-to-day affairs of mortals.
You can postulate that the Dunmer adapted to their habitat, but when as close as 200 years ago there were still survivors of the event that changed them from golden-skinned to ashen its hard to argue with the sudden and divine origins of the change.
The Ehlnofey who were the predecessors of everyone but the Hist (and the Argonians) differ from present day mortals in 'size'.
Size of soul, size of mystic power. The dragons you see in Skyrim are likely original Ehlnofey, a species that found a trick or balance that allowed them to keep that size without having to resort to reproduction and mortality just to survive.
Note that Skyrim states dragons are not born, nor do they reproduce or can they die in the normal sense.
Interestingly enough your scientific explanation states that Khajiit probably diverged early, and that is exactly what the Khajiit say.
The Ehlnofey changed and wandered and the Khajiit were shown their mystery by Azura, which is how they got their sugar and became Khajiit.
The mannish races and merish races came from different tribes of Ehlnofey, those that wandered (mostly) became the mannish races and those that stayed the merish.
Those that went to Yokuda became the Redguards, and those that went to Atmora became the Nedes.
When the Nedes returned to Tamriel divergence and interbreeding produced the Nords, Cyrods and Bretons.
(Some of) those that stayed on Tamriel became the Aldmer, from who all elven races come.
Probably interfertility on Tamriel is guided by very different factors than on Earth.
It likely has more to do with sentience and mythical influences than anything strictly biological.
Even when something like evolution can be identified there are still magical influences and even a conscious effort discernible.
I agree it is all very interesting and if this was not a mythical universe but a scientific one I would applaud your article, but on Tamriel you need to wear a different hat than the scientist one in order to make sense of what is going on.
Which is why I find it all so much fun. You dont often get to toy with the mystical and metaphysical in the real world and doing so on Tamriel has the added bonus there is very little chance of getting the Spanish inquisition after you.
Here are some books you might find interesting:
http://www.uesp.net/wiki/Skyrim:Racial_Phylogeny
http://www.uesp.net/wiki/Lore:The_Nirnoot_Missive
http://www.imperial-library.info/content/cosmology