Course hundreds of times you think of a person and don’t meet him. Like a man walking in his sleep.
Two recent articles in TiGS by Gerald Crabtree float the notion that we, as a species, are gradually declining in average intellect because we are accumulating mutations that deleteriously affect brain development or function. The observations that prompted this view seem to be: (i) intellectual disability can be caused by mutations in any one of a very large number of genes; and (ii) de novo mutations arise at a low but steady rate in every new egg or sperm. He further proposes that (iii) genes involved in brain development or function are especially vulnerable to the effects of such mutations. Considered in isolation, these could reasonably lead to the conclusion that mutations reducing intelligence must be constantly accumulating in the human gene pool. Thankfully, these factors do not act in isolation.
If we, as a species, were simply constantly accumulating new mutations, then one would predict the gradual degradation of every aspect of fitness over time, not just intelligence. Indeed, life could simply not be sustained over evolutionary time in the face of such genetic entropy. Fortunately (for the species, although not for all individual members), natural selection is an attentive minder. […]
Whether causally or as a correlated indicator, intelligence is strongly associated with evolutionary fitness, even in current societies. The threat posed by new mutations to the intellect of the species is therefore kept in check by the constant vigilance of selection. Thus, despite ready counter-examples from nightly newscasts, there is no scientific reason to think that we humans are on an inevitable genetic trajectory towards idiocy.
