As a species, Homo sapiens are only around 200,000 years old, so it wouldn't be surprising if we were still evolving rapidly. However, some arguments have been made to suggest that our physical evolution stopped around 40,000 years ago, or with the invention of agriculture, to be replaced by social and technological evolution. A new paper in the Proceedings of the National Academy of Sciences refutes this theory with evidence evolution continues.
Evolution can run at different rates, but normally keeps on ticking, even if very slowly. However, according to one argument, humans are no longer subject to the forces of natural selection. Diseases that once proved fatal are now no impediment to people not only living long lives, but having children as well. Where this has occurred there is no longer Darwinian pressure to remove susceptibility to those conditions from the gene pool.
Against this, certain traits are becoming more common in the gene pool. Perhaps the most famous is the capacity to digest lactose as an adult. Once exceptionally rare, this spread through the European population from 4,000 years ago thanks to the advantage conferred by the extra source of nutrition.
Harvard University's Dr Jonathan Beauchamp, the paper's author, quotes the spread of resistance to malaria and the capacity to function in the low oxygen conditions at high altitude as further examples of recent forms of human evolution.
Beauchamp has brought the evidence to the late twentieth century by exploring the relationship between certain genes and numbers of children, using data from 20,000 people who took part in the Health and Retirement Study around the time they retired.
Among the genes Beauchamp chose to focus on are those known to influence cholesterol concentrations and fasting glucose levels. Others affect factors such as the age of puberty for girls, that might change the number of children people have.
There was no clear evidence of natural selection acting on five of the seven measures Beauchamp explored. Although larger samples might reveal something statistically significant in some cases, any effects are likely to be small and slow.
Beauchamp did find evidence he describes as “weakly suggestive that genetic variants associated with [older age of puberty onset] may have been selected for.” The finding conflicts with occasional panics about falling ages of puberty, and an alleged association with teenage pregnancies.
The one strong association Beauchamp found was between low numbers of children and a set of gene variations linked to high education attainment. The fact that people with university degrees have fewer children has sparked alarm in some quarters – the government of Singapore even established programs to try to get graduates to breed – but Beauchamp is the first to show that certain genetic traits are becoming rarer as a result. Nevertheless, he notes that the effect is slow, writing: “Although natural selection is still operating, the environment appears to have achieved an 'evolutionary override.'” After all, cholesterol levels are rising, ages of puberty are falling and more people are still getting degrees.
3