Why a person doesn't evolve in one lifetime

From Nature:

It's not easy making a human. Getting from a fertilized egg to a full-grown adult involves a near-miracle of orchestration, with replicating cells acquiring specialized functions in just the right places at the right times. So you'd think that, having done the job once, our bodies would replace cells when required by the simplest means possible. Oddly, they don't. Our tissues don't renew themselves by mere copying, with old skin cells dividing into new skin cells and so forth. Instead, they keep repeating the laborious process of starting each cell from scratch. Now scientists think they know why: it could be nature's way of making sure that we don't evolve as we grow older¹.

Evolution is usually thought of as something that happens to whole organisms. But there's no fundamental reason why, for multicelled organisms, it shouldn't happen within a single organism too. In a colony of single-celled bacteria, researchers can watch evolution in action. As the cells divide, mutants appear; and under stress, there is a selective pressure that favours some mutants over others, spreading advantageous genetic changes through the population. In principle, precisely the same thing could occur throughout our bodies. Our cells are constantly being replaced in vast numbers: the human body typically contains about a hundred trillion cells, and many billions are shed and replaced every day. If this happened simply by replication of the various specialized cells in each tissue, our tissues would evolve: mutations would arise, and some would spread. In particular, mutant cells that don't do their specialized job so well tend to replicate more quickly than non-mutants, and so gain a competitive advantage, freeloading off the others. In such a case, our wonderfully wrought bodies could grind to a halt.

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