Depends on the setting, really-- its cosmology and history. Dark Sun, for instance, is thoroughly steeped in themes of evolution, and there's numerous examples of mutation. There is even a magical artifact (in structural form) that causes large-scale mutations in anyone injured in its vicinity.
Obviously, the post-apocalyptic influence of Dark Sun imposes certain evolutionary concepts.
Most fantasy settings, however, do not feature evolution-- in subraces or otherwise. The races were created as they are by their gods, and any change hence comes from the evolution of sociology, not physiology. Since this social evolution is heavily influenced by the gods, through their priests, it could be considered the long-term strategy of the gods in their struggle for supremacy.
As a rule of thumb, unless the setting is based on societies and cultures that can be compared to post-Darwinian Earth, life was created as-is by the gods (or other powerful entities). Artificial evolution, such as imposed by the Illithids, is a common feature of aberrations and cannot be linked to any kind of physical law. (I'm seriously pondering whether or not the Xixchil qualify as Aberrations, as envisioned by Beyond the Moons.)
What I'm having problems with is when there is an intersection between settings which hold contradictory views-- there's a project on Planewalker called Urban Planescape, that involves a modernized form of the Planescape setting. Some interpretations simply involve a technologically aware Multiverse, while others involve the reintroduction of our real-life universe to the Planes. I'm personally working on a crossover between the Star*Drive and Planescape settings.
If one world (or worse, universe) contains a fossil record that clearly indicates the existence of proto-humans (Int 3-4 primates bridging the gap between animal and humanoid) and another provides clear evidence that the humanoid races were created as-is by the gods, how do you reconcile this?
For example, in my crossover, I have decided to conflate the races of Kobolds and the T'sa. The T'sa, it turns out, are the long-lost metallic tribes, deprived of their magic (and a good deal of their coloration) by the loss of most magical energy to our galaxy. I've decided that the Oortlings (another victim of Illithid bioengineering) were originally derived from the Fraal/Thaal. I'm still working out the details of the I'krl/Illithid relationship, but I've decided they are not one and the same.
I'm just trying to figure out how to reconcile these-- and worse, the exsistence of identical species in alternate universes in which humanity was created, as-is, before they even evolved in our universe. (Thankfully, most Primes seem to set the creation of humanity within the last 10k years.)