[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.