Going off on a tangent here, I'm investigating the possibility of a fantasy world based on Darwin's principles of evolution and natural selection. What if all races evolved naturally? Would "humanoid" be a species (implying that all or most kinds of humanoids could interbreed) or a class of species (implying that no races could interbreed)? What would this mean for the history and make-up of a world?
I'm envisioning a world with a number of separate continents that were connected in the past (either because of lower sea level during an ice age or because of continental drift). Apes started evolving into men and spread across the world. BUt the continents were separated and humanoid-kind split up into different races, one or a few for each continent. In the end we have a world with each continent dominated by a single race (a dwarf-continent, an elf-continent, etc.), most likely focusing on an age where they start interacting.
Hot or not?
Túrin
An interesting idea. In the past, I generally went with different races not being able to reproduce together, due to chromosome count and so forth (if they could reproduce, they wouldn't fit the definition of different species).
But your idea would literally make them different races of the same species (humanoid). While interesting, my first question would be why? It seems more like a feature of a setting than something you could really base a setting around.
And unless the races are advanced enough to understand genetics, it will likely not matter to them.
Unless the end result is something unusual, I don't see the interest. If evolution is just a nifty way to explain and justify the status quo, it strikes me as a somewhat flimsy premise for a setting.
However, if you went for the "weird" angle in terms of harsh environments and the strange natural selection that goes with them, things might get more interesting. I think it'd be an interesting thought experiment to see what kind of creatures might evolve in a world dominated by ash-spewing volcanoes, or completely submerged in acidic water, et cetera.
For clarity: I do not believe this idea is enough to hang a campaign setting on, and even if I would I probably wouldn't plan to actually make such a setting. But what if this was one of the premises for a world that was being created, what effects would that have?
Túrin
It would probably help to purify a "realistic-styled" setting from wingèd elves, energy beings, and other excesses of fantasy (depending, of course, on the type of conditions that led to selection and evolution in the game world.)
As for the process of evolution conceived of by the setting's author, I suppose that would be of significant interest to devout biology geeks, and almost no one else.
There is an issue of Ends v. Means here. Evolutionary species design is a means to the end of a realistic-seeming world. But I think that if I just wanted a realistic-seeming world, I'd just write one that way, without bothering to put evolution in place. There are enough concerns to be dealt with that I consider more vital than evolution (a prime example being the geographical origin and dispersal of cultures through time.)