It seems probable that most variation in health, beauty, and IQ is due to genetic load, large numbers of rare genetic variants. About fifteen percent of the human genome is under negative selection, meaning that in about fifteen percent of the human genome, any random change is likely to be harmful, most variants get eliminated by natural selection in a reasonable time.
The mutation rate in humans is one or two mutations per hundred million bases per generation, probably around one mutation per hundred million bases, for a total of around thirty to sixty mutations per generation, implying around five to nine harmful mutations per generation.
Since we must have been near equilibrium in the ancestral environment, this implies five to nine harmful mutations were eliminated every generation.
This amounts to natural selection working fiercely – that just to stay in the same place we were running mighty fast, that survival of the fittest was pretty harsh. Since we have not been experiencing severe natural selection since the Industrial Revolution, the number of harmful mutations in the typical individual must have increased by about forty or so. We must have devolved quite significantly. (more…)