Thesis #4 Natural selection results in the evolution of good health only when there is sufficient heritable variation affecting survival and reproduction.
Natural selection accomplishes nothing without an adequate and appropriate supply of genetic variation on which to act. That is why dolphins and whales breathe air in order to get oxygen to their cells; they haven’t had genetic variants that natural selection could have acted on in order to re-evolve gills.
What this means for medicine is that there are a lot of good things that natural selection might have accomplished in our evolution, but didn’t. The most concrete, and indeed deadly, example of this is that humans haven’t evolved the capacity to give birth through their abdominal walls. As a result, for most of the last million years, one of the most dangerous things that happened to each human was being born. That’s because our birth requires that our evolutionarily enlarged neonatal cranium has to pass through the narrow bottom opening of the pelvis. It would have been brilliant if natural selection had solved this “design problem,” but again, like the lack of gills in whales, it didn’t have any genetic variation that it could have used to solve this design problem.
In some cases, this failure of natural selection is remarkably specific at the genetic level, yet medically devastating. The hemoglobin gene that produces sickle-cell anemia when the normal gene is absent kills thousands of people every year. But in this case natural selection actively sustains this allele in sub-Saharan African populations because genomes that have one copy of the sickle-cell gene together with a copy of the normal gene are better able to resist malaria. In an ideal world, there would be a hemoglobin gene that confers resistance to malaria without causing a deadly genetic disease. But mutation has yet to produce such a variant in sufficient numbers for natural selection to make use of it in regions where malaria is common.
Thus there is an entire spectrum of medical problems which arises from this key limitation on what natural selection can accomplish. When we can conceive of a simple morphological or biochemical solution to a medical problem, it is not a falsification of the evolutionary foundations of human health that natural selection did not produce it. Natural selection is severely limited in what it can accomplish, not least because of this problem of missing ideal genes.