One useful way to determine if a disease is caused by faulty human genes is look towards the central principle of evolutionary biology: evolutionary fitness. Evolutionary fitness is defined as the extent to which an organism is adapted to or able to produce offspring in a particular environment. The fitness concept can be applied to the problem of disease causation to distinguish evolutionarily feasible hypotheses of causation from marginally feasible or untenable ones.1) 2)
Generally speaking, diseases have three major causes: genetic, environmental, and infectious. Each disease affects, to some degree, an organism's ability to reproduce, that is, their reproductive fitness. As a general rule, infectious disease confers no reproductive benefit but genetic diseases do, either currently or historically.
Major diseases known to be genetic in origin offer some characteristic which confers a reproductive edge, and almost always that edge involves resistance to infection.3) When a group of people having that trait migrates and that advantage is lost or starts being harmful, the gene is weeded out of the population.
Although cystic fibrosis, thalassemias, and polycystic kidney disease are genetic disorders all known to prevent genetic damage caused by infection, the genetic disorder sickle-cell anemia is a prototypical example. In sickle-cell disease, there is a mutation in the HBB gene which confers resistance to malaria, providing a benefit in some regions of the world but doing harm in others. Groups of people who move from areas of the world infested with malaria to areas which are not, quickly lose the trait for sickle-cell within generations:
Recorded history spans about 5,000 years, and the frequencies of deleterious alleles can change substantially in a small fraction of that time. Consider the sickle-cell allele [one of many competing versions of a gene], which protects heterozygotes from malaria and causes a lethal anemia in homozygotes. If a population with a sickle-cell allele frequency of 20 percent were transferred to an environment without malaria, the negative effects of sickle-cell anemia should cause the frequency of the sickle-cell allele to decrease by a factor of about three within about 10 generations. This estimate accords with geographic differences. In the United States, the sickle-cell allele frequency is about half of what would be expected from African source populations after accounting for admixture [the act of mixing or mingling]. A geographic comparison within the Caribbean between malaria-free and malaria-endemic areas is also consistent: the frequency of the sickle-cell allele is low in Curaçao, where malaria has not been endemic, but not in Surinam, where malaria has been endemic.4)
Gregory Cochran, et al. 5)
If the common inflammatory diseases (including autoimmune diseases) were genetic, the only way they would manage not to be weeded out of the population would be if they conferred some sort of beneficial survival trait not related to the disease.
To date, no such benefits have been identified in any disease, schizophrenia being a good example.6) Schizophrenics have a high suicide rate, few children, and a high rate of abnormality in their children.7) 8) Schizophrenic mothers are more likely than non-schizophrenics to have stillborn babies and children with congenital malformations.9)
Among chronic diseases, as opposed to genetic illnesses, there has certainly been no documented increased resistance to infection. Crohn's disease is typical in that patients who suffer from the disease are known for getting substantially more co-infections.
Some researchers have suggested that chronic inflammatory diseases seem to strike after reproductive age and that autoimmune disease is evolution's way of ensuring that humans breed at earlier ages.
Therefore, I argue that the biological function of humans is basically over with at 20–25 years, and that is what nature is used to as well.
Rolf Zinkernagel 10)
The problem with this argument is that many chronic diseases do strike before reproductive age. Consider, once again, schizophrenia, a disease which is not at all unique in this regard. Schizophrenia drastically reduces reproductive fitness. Epidemiological data clearly show that people with schizophrenia are much more likely die before reproductive age.
Zinkernagel's statement is also problematic when one considers how most historical and modern societies have had grandparents rear and care for their children's children. Surely the more family members alive and healthy enough to look after a child, the better that child's chance for having children of his or her own.
Further evidence against the genetic hypothesis for inflammatory disease is that a number of diseases - diseases like Crohn's, atherosclerosis, autism, breast cancer, schizophrenia and multiple sclerosis - tend to strike around or before reproductive age. These are diseases which have been around for at least thousands of years, allowing for plenty of time for harmful traits to be weeded out of the population. Manifestations of both arteriosclerosis11) 12) and cardiac disease13) can be observed in mummies of ancient Egypt. Ötzi the Neolithic Iceman who lived around 3300 BC was found to have arthritis.14) There is currently no evidence that any of the supposedly genetic chronic diseases haven't been around for at least millenia.
Does evolutionary evidence support the theory that disease is the product of the interplay between human genes and environment? The role would have to be a small one as any kind of strong genetic disposition for acquiring disease would be bred out of the population given that any other trait, which impinges on reproductive fitness, is bred out.
Evolutionary theory is clear that any kind of consistent reproductive disadvantage is consistently and ultimately weeded out of the population. People whose genes confer upon them an elevated risk of disease compared to the general population - whether it be because of susceptibility to environmental factors or because of a variety of genes are interacting - have reduced evolutionary fitness.