Researchers scanning the genomes of African-Americans say they see evidence of natural selection as their ancestors adapted to the harsh conditions of their new environment in America.
The scientists, led by Li Jin of the Chinese Academy of Sciences in Shanghai, report in the journal Genome Research that certain disease-causing variant genes became more common in African-Americans after their ancestors reached American shores — perhaps because they conferred greater, offsetting benefits. Other gene variants have become less common, the researchers say, like the gene for sickle cell hemoglobin, which in its more common single-dose form protects against malaria. The Shanghai team suggests the gene has become less common in African-Americans because malaria is much less of a threat.
The purpose of studying African-American genomes is largely medical. Most searches for variant genes that cause disease take place in people of European ancestry, and physicians want to make sure they have not missed variants that may be more common in African-Americans and helpful for developing treatments or diagnosis. Such searches often reveal events in a population’s history by pinpointing genes that have changed under the pressure of natural selection. The unusually common variants identified by the Shanghai team are associated with higher risk of hypertension, prostate cancer, sclerosis and bladder cancer.
“Most of the genes associated with African-American ethnic diseases,” they write, “may have played an important role in African-Americans’ adaptation to local environment.” But the authors have not yet been able to identify the benefits they believe such genes conferred.