If population remain geographically isolated for thousands of generations, what would happen?

They would die, or go insane

Explanation: First off, they need to have a steady food supply to maintain society. If, lets say, the food ran out, then everyone would die as they were isolated and had no other food source. Since the human body needs to have interaction, if a certain group of people were isolated, they would most likely also go insane due to the lack of no other people. Of course, the info you gave was very vague, so I couldn't really give a good answer, so yeah

your answer is going to be physical weathering