Thesis #7 Natural selection will sustain nucleotide sequences that foster biological fitness however numerous and however indirect their benefits, making the genetic foundations for the evolution of health genome-wide and complex.
The kinds of experiments that most biologists favor involve studying large-effect mutations and other ways of mutilating or disrupting experimental animals. It is an irony of much molecular genetic research that it often studies the physiology of large-effect mutations produced by radiation and other means. Experimental animals with such mutations are unlikely to be a useful guide to the physiological variation that arises from the kind of high-fitness genes that are more likely to be common in large human populations. Yet molecular biologists are fond of reasoning from (a) the large experimental signals that such mutants produce to (b) the functional underpinnings of normal physiological function.
Sometimes this works. If a biologist carefully destroys the reproductive organs of a group of animals using irradiation or genetic engineering, but does not inflict severe adverse side-effects, these animals will usually live longer compared to intact controls. This reveals a cost of reproduction that is one of the more durable, if not quite universal, findings of research on aging. An interesting exception is women, who do not appear to live longer if they undergo hysterectomy and ovariectomy. But men, at least in American institutions that used to castrate incarcerated “mental defectives,” who are castrated do live longer.
On the other hand, producing inbred laboratory stocks and then mutating them has been shown to give misleading results in a significant number of cases. This doesn’t mean that such experiments can never be used to reveal how animals work. But it does mean that such experiments can’t be generally trusted.
As an alternative approach, evolutionary biologists, quantitative geneticists, and other types of more Darwinian biologists prefer to work with naturally occurring genetic and environmental variation. This reflects the common assumption among such biologists that natural selection will often favor genetic variants and physiological responses to environmental variation that are limited in their effects, rather than large in their effects. Furthermore, this kind of biologist generally supposes that genetic variation at many sites throughout the genome will underlie the evolution of functional characters, like those which sustain both Darwinian fitness and medical health.
One way of looking at this difference between these two types of biologists is that the first type, let us call them molecular biologists or reductionists, like to suppose that organisms are assembled from well-defined, machine-like, large-effect pathways. The second type of biologist allows that a few cases like that may evolve, but that much of the underpinnings of normal function is genetically and physiological complex. Let us call these Darwinian or evolutionary biologists, as this is fairly close to Darwin’s own thinking on this topic. Darwin and subsequent generations of evolutionary biologists have emphasized that natural selection can act on many inherited variations, even relatively subtle heritable genetic differences. Thus a Darwinian view of health is naturally inclined to emphasize the idea of many contributing genetic and biochemical factors underlying healthy function, not a small number.