There are costs and benefits to both sides. A low threshold gives a high aerobic fitness and a low risk of disease, but at the cost of a high rate of infertility and poor adaptability. A high threshold gives a low aerobic capacity and higher risk of disease but with the benefits of greater fertility and better adaptability. These are words to conjure with. Fertility. Adaptability. Aerobic fitness. Disease. We can’t cut much closer to the grain of natural selection than that. I reiterate: all these trade-offs emerge inexorably from the requirement for two genomes.
