Wednesday, 18 January 2012

The human genetic evolution rate controversy

After taking cultural evolution into account, human genetic evolution (i.e. the evolution of human DNA) has accelerated in recent times - as a result of a number of factors:

  • The population has exploded, resulting in vastly more opportunities for positive mutations to happen;
  • Genes are poorly matched for the environment - resulting in directional selection pressures;
  • Shielding protects mutations from selection - allowing them to persist in the population;
So: it is rather strange to see the opposite case being made. Here's Steven Jones, claiming that the evolution of human DNA is effectively over.

Steve claims that selection by death between the ages of 1 month and 21 years has much reduced - which is true in many areas of the world. He claims that selection by differential reproductive success has reduced somewhat - which also seems to be true, to some extent.

However, he then says that the effects of genetic drift are reduced - as a result of larger population sizes and less isolation. This is true - but reasoning from there to reduced levels of evolutionary change gets things rather backwards. Small population sizes results in stochastic sampling effects - which eliminate genetic variation. Larger population sizes result in less of this elimination of variation, resulting in more of the variation introduced by mutations persisting. So, while it is true that larger population sizes prevent a trype of evolution due to sampling errors, larger populations also allow mutations to persist better than small populations do.

These days, we don't have to rely on hand-waving arguments to settle this point. We have data on evolutionary rates from molecular evidence which allow us to see both the current rate of change and the amount of change since humans diverged from our common ancestors with chimpanzees.

This has been done. The conclusions have been reported in the article Culture Speeds Up Human Evolution. To quote from it:

the rate of human evolution over the past few thousand years is far greater than it has been over the past few million years.
The article also says:

Comparing the amount of genetic differentiation between humans and our closest relatives, chimpanzees, suggests that the pace of change has accelerated to 10 to 100 times the average long-term rate
John Hawks has a more detailed refutation here: Human evolution stopping? Wrong, wrong, wrong.

This controversy is surely now dead. We can now see that culture speeds up human evolution.

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