Nicholas Wade - A Troublesome Inheritance: Genes, Race and Human History

New analyses of the human genome establish that human evolution has been recent, copious and regional. Biologists scanning the genome for evidence of natural selection have detected signals of many genes that have been favored by natural selection in the recent evolutionary past. No less than 14% of the human genome, according to one estimate, has changed under this recent evolutionary pressure. Most of these signals of natural selection date from 30,000 to 5,000 years ago, just an eyeblink in evolutions 3 billion year timescale.

Natural selection has continued to mold the human genome, doubtless up until the present day, although the signals of evolution within the past few hundred or thousand years are harder to pick up unless the force of selection has been extremely strong. One of the

Several instances have now come to light of natural selection shaping human traits within just the past few hundred years. Under the pressure of selection, for example, the age of first reproduction among women born between 1799 and 1940 on LIsle-aux-Coudres, an island in the Saint Lawrence River near Quebec, fell from 26 to 22 years, according to researchers who were able to study an unusually complete record of marriages, births and deaths in the islands parish records.

The researchers argue that other possible effects, like better nutrition, can be ruled out as explanations, and note that the tendency to give birth at a younger age appeared to be heritable, confirming that a genetic change had taken place. Our study supports the idea that humans are still evolving, they write. It also demonstrates that microevolution is detectable over just a few generations in a long-lived species.

Another source of evidence for very recent human evolution is that of the multigenerational surveys conducted for medical reasons, like the Framingham Heart Study. Borrowing statistical methods developed by evolutionary biologists for measuring natural selection, physicians have recently been able to tease out certain bodily changes that are under evolutionary pressure in these large patient populations. The traits include age at first reproduction, which is decreasing in modern societies, and age at menopause, which is increasing. The traits are of no particular importance in themselves and have been measured just because the relevant data were collected by the physicians who designed the studies. But the statistics suggest that the traits are inherited, and if so, they are evidence of evolution at work in present-day populations. The evidence strongly suggests that we

Human evolution has not only been recent and extensive; it has also been regional. The period of 30,000 to 5,000 years ago, from which signals of recent natural selection can be detected, occurred after the splitting of the three major races, and so represents selection that has occurred largely independently within each race. The three principal races are Africans (those who live south of the Sahara), East Asians (Chinese, Japanese and Koreans) and Caucasians (Europeans and the peoples of the Near East and the Indian subcontinent). In each of these races, a different set of genes has been changed by natural selection, as is described further in chapter 5. This is just what would be expected for populations that had to adapt to different challenges on each continent. The genes specially affected by natural selection control not only expected traits like skin color and nutritional metabolism but also some aspects of brain function, although in ways that are not yet understood.

Analysis of genomes from around the world establishes that there is indeed a biological reality to race, despite the official statements to the contrary of leading social science organizations. A longer discussion of this issue is offered in chapter 5, but an illustration of the point is the fact that with mixed-race populations, such as African Americans, geneticists can now track along an individuals genome and assign each segment to an African or European ancestor, an exercise that would be impossible if race did not have some basis in biological reality.

The fact that human evolution has been recent, copious and regional is not widely recognized, even though it has now been reported by many articles in the literature of genetics. The reason is in part that the knowledge is so new and in part because it raises awkward challenges to deeply held conventional wisdom.