Hafta agree with the "bit of both" trend here.
Mainly, IMC, the "divinely created" races (as they only know themselves as, since there is still a fair amount of unrecorded prihistory) existed in 1 form long ago, and essentially "evolved" into the form they are now. To a certain degree, evolution is responsible for the current "incarnations" (for lack of a better term at hand at the moment) present in the campaign world.
However, magical experimentation & mortal "tampering" has altered the path of this evolution (the divine forces decreeing long ago to abstain from directly interacting in mortal affairs). Creatures such as the owlbear, hippogriff, yuan-ti, and gnoll are obvious products of magical experimentation (intentional or not). Some much more recent creations by such methods include hobgoblins, bugbears, blues (from PsiHB), & ogre mages.
Such tampering by mortals on the natural order has had an effect on the world around them. Though there are multiple types of dragon species (chromatic, gem, metallic, etc.), many of these creatures are being wiped out: either by competition for food by other dragons (both by natural dragons and some of the mage-made newer breeds), or by other races (humanoid dragon-slayers & whatnot). In many cases, some dragon species are starting to interbreed, creating a new species of dragon from 2 or more older ones.
And, of course, never forget the extraplanetary/extradimensional visitors from beyond: aboleths, illithids, & quite a few other abberations. These creatures are quite rare--most likely, encountering & slaying these creatures will wipe them out IMC.