[rdlecler1]

To quote Dobzhansky, this is all becomes obvious in the light of evolution. A challenging environment removes those cells that accumulate mutations. (i) In challenging or resource constrained environments mutated cells tend to behave more aberrantly and our immune system responds by attacking them (this is why radiation therapy works). (ii) Mutated cells will be more prone to pathogenic infection. In either case, we use the external world as a kind of extended immune system or to police our bodies against cellular mutiny.

[mitteldorf]

There is no "natural selection" within an individual body, within an individual lifetime. Yes, there are some cells that continue to live and others that continue to die. But the cells are not competing, with the fittest winning out. Rather the whole scheme is orchestrated centrally for the good of the genome (which is the same in every cell).

[rdlecler1]

Actually it's called somatic evolution. And there is no central orchestration. When cells start to mutate they no longer share the same genome and it's typically in the organism's interest to remove those cells because they can lead to mutation.