Thesis #2 – Natural selection reliably produces high levels of biological fitness, and thus good health, only under the environmental conditions in which it has been acting for many generations.
A common caricature of evolution by natural selection is that it produces some type of universal progress that is sustained and predictable over millions of years. Not only is this NOT how present-day evolutionary biologists think of evolution, it isn’t even how Charles Darwin himself thought of the effects of evolution by natural selection. This is, unfortunately, how many naïve and perhaps temperamentally optimistic intellectuals have thought of evolution by natural selection. Perhaps evolutionary biologists have little right to complain about this; the perversions and misappropriations of Einstein’s relativistic mechanics in twentieth-century popular and intellectual culture were even more ludicrous. But this misinterpretation of evolution by natural selection has certainly impeded the use of this central biological theory by medicine, because it strips evolutionary biology of much of its salience as a foundation for medicine.
The key point is that natural selection predictably acts to increase fitness only in the environments in which it has long acted. This is why, for example, colonizing other parts of our solar system is such an inherently difficult proposition. Few, or no, species on this planet have adaptations that would enable them to thrive in the environments supplied elsewhere in our solar system. At a less extreme level, most insects have extreme difficulty surviving and reproducing during circumpolar winters, as do most plants. Environmental conditions define the context for natural selection. And such environments are defined not only by physical conditions; they also involve the biological communities of species found in particular habitats, whether those species are competitors, prey, predators, or pathogens.
This constraint on natural selection is not only a matter of habitat. It is also a matter of time. Natural selection takes many generations to produce high levels of fitness in a particular environment. Since particular environments require the evolution of corresponding particular biological functions, generally called “adaptations,” in order to achieve high levels of fitness, the longstanding environmental history of any population determines which specific adaptations that population will possess.