Cellular natural selection works like natural selection on the savanna, but smaller. Harsh habitats weed out weak animals and weak cells; this has implications for cancer and health.
Imagine a savanna with herds of slow-moving antelope, lots of fresh water, and benevolent temperatures. Lions would abound. Weak lions, fat lions, mentally-deficient lions, and three-legged lions would all thrive. A portion of the lion population would remain fit and strong, but most would begin to drift in aberrant directions.