Evolution only says what "could have" happened?

I've run across the following idea: while Newton explained some phenomenon as a necessary consequence of physical laws, evolution only sets forth an explanation of biological diversity as it could have happened. Newton's discoveries say what must happen, but evolution merely says that an observed adaptation could have arose by natural selection.

This seems to limit the claim of evolution in a way that goes beyond the usual critique of "just-so-stories" that one hears. Mixed in with this is the question of what is a prediction and what is an explanation.

Any insight is appreciated.

via International Skeptics Forum http://ift.tt/1RrykjY