The most common school of thought is that environment is responsible for the adaptation process. For instance, man at one time, had 5 molars. With changes in the environment came changes in the diet. Now, man has only 3 molars, and those are usually impacted and need to be extracted (wisdom teeth). There are many people currently that do not have even wisdom teeth, as they have moved farther along the philo-genetic scale that Darwin referred to.
