[Keeper of Dol Guldur]

I think it was just plain and simple evolution. Due to the environments of the different clans, certain traits were more beneficial to life there. And since depending on where they lived, they came into contact with elves, dwarves and pretty early on the ancestors of the Rohirrim, they learned from them. You can't tell me if you lived your whole life in an elven sort of way, you wouldn't look good, and so on. Though any thing (the fairy wife) is possible in between.