MUTATIONAL DECAY
Every human, by the time he on she reproduces, has accumulated an average of about 100 mutations. Some proportion—the proportion is unknown but scientifically important—of the 100 are deleterious. In nature, offspring with too many deleterious mutations die, and the number of those mutations is prevented from increasing in the population. But wealthy populations are featherbedded from natural selection, and mutations multiply. We know, from experiments in flies, that populations deteriorate quite rapidly—viability declines by about half a percent per generation in the flies—if natural selection is relaxed. This is almost certainly going on in human populations now.
