Thesis #6 Natural selection produces good health only when new deleterious mutations are rare or small in magnitude; very few novel mutations will have large and generally beneficial effects, in an environment to which a population is well-adapted.
Mutation is the source of the genetic variation that natural selection acts on. But very few new mutations are beneficial. Instead, the vast majority of mutations have no selectable benefit, or are actually deleterious. It is only in comic books or bad science fiction that mutations suddenly produce a much-improved human.
This has significant implications for human health. Circumstances that elevate mutation rates will generally impair health. The best example of a situation where this arises is exposure to radiation. At extreme levels, radiation directly degrades biological tissues, causing immediate death in the worst cases.
But more subtly, radiation also readily increases mutations rates in exposed tissues. In somatic tissues, such mutation can lead to combinations of mutations which allow unlimited cell proliferation. When somatic selection among such mutant cells operates over time, selection within the body can favor the most proliferative of these mutant somatic cells, leading to the development of malignant cancer.
In the cells that will eventually produce sperm or eggs, mutation will lead to mutant genes that can be transmitted from generation to generation, potentially causing a trail of havoc in the descendants of the individuals whose testes or ovaries were irradiated.
The obvious medical significance of these basic points it that we should avoid exposure to radiation. There are five contexts in which we are routinely exposed to elevated levels of radiation: (i) sun exposure; (ii) diagnostic medical or dental radiology, (iii) high-elevation flight, (iv) radon emissions from basements, and (v) proximity to radioactive wastes, such as those produced by nuclear power plants, mining, and medical radiology. This is a spectrum of exposure risks from the unavoidable to the relatively rare and avoidable.
In addition to radiation, there are a number of synthetic and naturally-occurring substances that cause mutations as a result of chemistry alone. Most of the worst of these are products of scientific or industrial chemical synthesis. On the other hand, Bruce Ames and others have found weaker carcinogens in many of the food products that we routinely ingest, from caffeine to burnt meat.