Just made me think of something... Are "half"breeds, like animal crossbreeds, sterile? It would make some sense from a genetics viewpoint, and would also give an interesting facet to relations between half- and pure breeds.
It would also give something to giggle about when some twitty fanfic writer tries to make their Mary Sue self-portrayal even more interesting by writing themselves as a half-elf.
But if they are capable of crossbreeding, that must mean that the various races indeed have some sort of common ancestor, so as to have enough genetic similarities. This raises a question regarding the lizardman and kobold races, seeing as they are reptilian rather than mammalian (at least the lizardmen are. And I believe the current Forgotten Realms opinion of kobolds is that they are reptilian. I much preferred them as the yapping Chihuahua-folk). The differences between a human and an elf, or even a gnome and an orc, are understandable to an extent within the boundaries of genetic understanding (after all, look at the differences between Germans, Chinese and chimpanzees), but the difference between a warm-blooded and a cold-blooded creature seem to be something of a stretch.
This could of course be explained away by one of the ancestral subraces being cursed by some deity or whatnot, and being turned into a more "monstrous" creature as a form of punishment. This could result in certain extreme mutations (cold-bloodedness, fangs, claws, scales) while still retaining enough genetic similarity to maintain crossbreeding relations. The reptilian races are really the biggest wrench in the gears, but I wasn't aware of them being a problem until just now (didn't know about kobold crossbreeding). I had assumed that they were simply another strain of intelligent life, and thus were not required to share genetic similarities with the other races.
Or, we could just agree that the Forgotten Realms are built upon ass-backwards high fantasy concepts that were pulled out of a hat randomly and have absolutely no forethought behind them.