Genome study reveals evidence of natural selection occurring during slavery
Scientists probing the genetic makeup of African-Americans say they have found evidence that their early ancestors experienced natural selection as they adapted to the harsh conditions of life in America.
Li Jin of the Chinese Academy of Sciences in Shanghai led the team of researchers who conducted a recent study that illuminated this unexplored area. Since most research studies on the variant genes that cause diseases are conducted on people of European ancestry, the Shanghai team studied the genomes of the earliest African-Americans to ensure that variants specific to this group could be discovered.
The study may contribute to the development of treatments and enhanced disease diagnosis, according to the Shanghai team.
The researchers used a method for studying admixture, the genetic phenomenon that occurs when two populations or races mix. By using gene chips (a technique that allows scientists to parse DNA sequences in large amounts), the Shanghai researchers were able to deconstruct the composition of the chromosomes of the first Africans to arrive in America and their immediate ancestors. In addition, modern African populations, such as the Yoruba, were studied to compare differences between blacks of today and the likely genetic makeup of their antecedents.
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.
Some experts believe the study misses the fact that the natural selection Shanghai scientists are accounting for may have occurred in other environments, such as the ships slaves traveled on — before reaching American shores. For example, University of Southern California Professor and anthropologist Craig Stanford, who has studied human evolution for more than twenty years, believes the study misses this crucial point.
“It’s long been thought that the slave ships bringing Africans to the new world were places of intense natural selection, since 90 percent of the slaves may have died on a trip,” Stanford told theGrio. “Those who survived may have had genetic traits — the ability to withstand starvation, lack of water, etc. — that served them well on the ship but left their descendants with genetic problems that may have been associated with the genes that conferred protection on the ships.”
It is acknowledged that researchers can analyze and trace the genomes of contemporary mixed populations, but some experts claim that it is difficult to prove that natural selection can take place within a period as short as 300 years through the means used 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.
“This is another example of over-reaching by people in the field of medical genetics who seem to have no understanding of population genetics, or of the historical events they seek to explain,” he said. “Africa is the most genetically diverse place on earth for humans — so the very idea that you can easily separate out ‘European’ genes from ‘African’ ones is not founded in good science, or good history.”
These conclusions suggest that the methods used by the team at the Chinese Academy of Sciences were not thorough. By contrast, because evidence of natural selection is subtle, the Shanghai team asserts that scientists in the future will need to analyze many thousands of genomes to make their exploratory research more conclusive.