This is the best explanation for how natural selection works.

"Chance alone cannot explain the marvelous fit between individuals and their environment. And it doesn't. True, the raw materials for evolution--the variations between individuals--are indeed produced by chance mutations. These mutations occur willy-nilly, regardless of whether they are good or bad for the individual. But it is the filtering of that variation by natural selection that produces adaptations, and natural selection is manifestly not random. It is a powerful molding force, accumulating genes that have a greater chance of being passed on to others, and in so doing making individuals even better able to cope with their environment. It is, then, the unique combination of mutation and selection--chance and lawfulness--that tells us how organisms become adapted."
-- Jerry Coyne, University of Chicago biologist, author of Why Evolution is True

If you are interested in this subject, this blog has 84 posts about natural selection at Natural Selection.