Selection Process
Nature is an experimenter. Random mutations produce changes that either promote survival or impede it. The ones that promote, survive, the ones that don't, disappear. It is necessarily a slow process since the results of the random changes may not be immediately apparent.
It is a wondrous process, unless, of course, your line is selected for discontinuance. I've played with the concept of the survival of our species because of the increasing girth of Americans. It is interesting to attempt to outline the consequences of the growing obesity rate as we spread our "nutrition" choices to other countries. It may be that those prone to obesity will be "selected" out and 500 years from now, an obese person would be an anomalistic throw-back.