I like all this stuff about evolution... I've often considered the subject.
The campaign world I'm currently running assumes that the primary races (humans, elves, dwarves, orcs, goblins) are all closely related, probably divergent no more than 300,000 years ago, so they can interbreed with little or no problem. I don't think they're the same species, but they're close enough and have had enough interbreeding that they remain genetically close. The current human evolution theory (in our world) states that humans evolved all over the planet from an earlier species, and only interbreeding through trade and such kept, say, Kenyans roughly the same as Japanese; in this world, the difference just happens to be a little more pronounced.
(Oh, and some evidence in cryptozoology suggests that neandertal crossbreeds exist in human populations, hence I allow crossbreeds.)
Drifting away from core races, we find kobolds, which I treat as unintelligent (rhesus monkeys, I think someone said; with you all the way there; my kobolds are essentially temperate monkeys), and then gnolls. Gnolls are interesting. They are at the same level of development as core races, but are clearly unrelated. They're of canine stock; I imagine they were created by dragon sorcerers. (Dragons have been around far longer than any core race... and I'm talking about individual dragons.)
Finally, there are the far-out races such as lizardfolk, yuan-ti and illithids. Lizardfolk are evolved from lesser lizards in tropical jungles (there are obviously a few more reptiles in my world). Yuan-ti are the magical equivalent of germline genetic engineering; they're descendants of sorcerers who are no longer human. There's a genetic trail leading from the more humanoid pureblood yuan-ti to the halfbloods to the abominations; a human couldn't breed with an abomination, but they could breed with a pureblood. Illithids? They're a freaky alien type thing whose origins I won't go into. They have no Earthly equivalent, thankfully.
Then there are species you can't really call races, such as troglodytes and trolls. Trogs I treat as the reptilian equivalents of gorillas that live in caves. They're closely related to lizardfolk, as you'd expect. Trolls are another oddity; that regen ability isn't reflected in any higher animal in nature. I guess they're the product of magical experimentation by the dragons too.
You'll notice that the dragons take the blame for a lot of the races in this list. Well, of course they do. They're giant reptiles that live to incredible ages and amass incredible mystic power. Frankly, the history of the various dragon cultures (I hesitate to call them species or races) covers tens of millions of years. They're probably pretty close to the dinosaurs. But where did they become hexapods? That's a magical alteration all on its own... even the dragons aren't an evolved species.
Whew, that's more than I meant to write. And reveal. Oh well. Incidentally, this all relates to my campaign-based webcomic linked below. I'm told it's pretty good.