Did slavery cause rapid natural selection among African Americans?

Ashley Michelle Williams has a fascinating article over at theGrio about a recent study that offers evidence that the harsh conditions of slavery subjected the African American population to strong evolutionary pressures. This could help explain what appear to be recent mutations in the genomes of African Americans. Williams writes:

Researchers found that of the African-American genomes in their sample, only 22 percent of the DNA analyzed came from Europeans. The remaining DNA was found to come from purely African ancestors, a finding in alignment with previous discoveries.

The main result of the study was that certain disease-causing variant genes were found to have become more common in African-Americans after their ancestors reached American shores — possibly because they presented greater benefits, according to an article published by the team in Genome Research.

Additionally, the researchers found that other gene variants have actually become less common over time in African-Americans. For example, the gene for sickle cell hemoglobin — which protects against malaria when a carrier has two copies of the gene — has become less common in African-Americans than native Africans, possible because malaria was less of a threat on America's shores.

Interestingly, the new gene variants that African-Americans acquired in America are connected with higher risks for hypertension, prostate cancer, sclerosis, and bladder cancer. The researchers assert in their paper that these disease-causing genes may have played an adaptive role for blacks in their new home country. "Most of the genes associated with African-American ethnic diseases may have played an important role in African-Americans' adaptation to local environment," they stated in their paper.

However, the authors have not been able to identify the specific benefits these genes may have conferred to the ancestors of modern blacks.

Williams also notes that not all researchers agree with the conclusions that scientists proposed in the study:

Richard Wilk, for example, a cultural anthropolgist at Indiana University, told theGrio that biological evolution does not work as fast as the study is claiming.

"People do not adapt physically in a couple of generations," he said. "A couple of thousand years is the minimum... And not all of Africa is hot and tropical either."

Wilk emphasized that the study also may not be plausible since it is based on the assumption that the people exported from Africa as slaves were all from the Yoruba people — a West African ethnic group. The study ignores the vast genetic and environmental diversity of Africa.

Given that people from Africa exhibit the greatest genetic diversity of any human group on Earth, it does seem extremely unlikely that the ancestors of African Americans would have similar genetic makeups. Still, it's hard to believe that no selection pressure was exerted on this group during the harsh passage to America from Africa, along with the brutal conditions of slavery. Read the rest of Williams' article on theGrio.