Behavior is a directed response of an organism to the environment. It has its own parameters of being understood within its domain and we saw that behavior is a directed response of an organism to the environment and these responses are chemical and physical.

For Tooby and Cosmides:

“It is important to realize that our circuits weren't designed to solve just any old kind of problem. They were designed to solve adaptive problems. Adaptive problems have two defining characteristics. First, they are ones that cropped up again and again during the evolutionary history of a species…”

This is the first characteristic of adaptive problems while the second characteristic is that “they are problems whose solution affected the reproduction of individual organisms – however indirect the causal chain may be, and however small the effect on number of offspring produced. This is because differential reproduction (and not survival per se ) is the engine that drives Natural Selection.”

So there has to be a difference among other competing members of the species or other species, owing to which some survive and reproduce while others do not.