Adaptation is NOT part of the basic theory of evolution. For example, the first bipeds didn't just suddenly develop at the end of the Miocene period because the forest habitat was being replaced by grasslands and bipedality became beneficial. There was ALREADY biped hominids thriving in the forest habitat. Fact is, these hominids just 'happened' to be bipeds and bipedalism was a beneficial feature when the grasslands surpassed the forest as the major source of food and shelter.
Get it? The trait comes BEFORE the environmental changes. That is the basis of the theory of evolution...not adaptation.