I suppose one could say there was "natural selection" after a manner of speaking, not unlike the phenomena evident in our own world, that lighter skinned people that tend towards light hair and blue eyes seem to be naturally selected to best survive in northerly climes, whereas dark skinned people with black hair and brown eyes seem to be naturally selected to best survive in tropical regions. So maybe hobbits were just a natural strain of humankind that flourished in rivervalleys with rich soil for gardening and hillocks readily available to dig a smial in.
