Thesis #5 Natural selection produces good health only when population size is large enough to overcome genetic drift; inbreeding reliably impairs health in outbreeding species.
Many members of the human species have a great propensity to mate with other humans. You may know such a person yourself. This makes eminent evolutionary sense: we can’t reproduce asexually or by self-fertilization. It is this absolute requirement that we had to have sex to reproduce that generated our high levels of genetic variability. We are one of nature’s outbreeding species, a species in which mating is not normally kept within close families, unlike some social insects and fig wasps, or even the naked mole rats of Africa.
This feature of our mating history gives rise to a medical problem which should be more widely appreciated: mating within small ethnic groups gives rise to an increased frequency of genetic diseases caused by recessive deleterious genes. One of the most horrifying examples of this problem is the prevalence of Tay-Sachs disease in the offspring of matings among Ashkenazi Jews. Tay-Sachs is an incurable and fatal disease that causes severe brain degeneration in young children, with accompanying blindness, mental disability, and severe pain. This medical tragedy arises insidiously from the common human inclination to mate with someone from a similar background.
Generally, mating among biological relatives is associated with a wide variety of impairments, from shorter stature to lower IQ. This is an effect that natural selection abhors; as a result the best single example of an apparently inherent human behavioral pattern is an aversion to mating with siblings or parents.
This is the first thesis which leads to a simple piece of medical advice. People should either avoid having children with individuals with whom they share ancestry or, if they decide to go ahead with such a plan, they should actively seek genetic counseling and the best prenatal genetic diagnostics available. Fortunately, we are entering an era of rapid progress in genetic characterization, and we can all hope that ways will be found to circumvent the tragedies that genetic diseases produce.