It is very rare to observe adaptive genetic differences between human populations (as was demonstrated in this paper). Why might this be the case? Brainstorm as many reasons as you can think of.

Genetic human adaptations are specific according to the environments which make it rare.

Explanation:

The variations in humans that increases the selection process in a particular environment is considered as Adaptations. There are various biological adaptations that depend on the time length. The type of adaptation depends on the presence of 'stressor' in environment. In genetic adaptation, the stressor remains constant and can last for generations.

The example of such adaptation is the - sickle allele cell in the population of human. It might be kept in mind that genetic adaptation is specific environment dependent. In this case a specific gene can be advantageous and detrimental too in different environments. This make its variations very rare.