Natural Selection Purged Many Weakly Deleterious Neanderthal Gene Variants from Modern Human Genome

Neanderthal genetic material is found in only small amounts in the genomes of modern humans because, after interbreeding, natural selection removed large numbers of weakly deleterious Neanderthal gene variants, according to a study by Ivan Juric, Ph.D., and colleagues at the University of California, Davis, published online on November 8, 2016 in PLOS Genetics. The open-access article is titled “The Strength of Selection Against Neanderthal Introgression.” Humans and Neanderthals interbred tens of thousands of years ago, but today, Neanderthal DNA makes up only 1-4% of the genomes of modern non-African people. To understand how modern humans lost their Neanderthal genetic material and how humans and Neanderthals remained distinct, Juric and colleagues developed a novel method for estimating the average strength of natural selection against Neanderthal genetic material. They found that natural selection removed many Neanderthal alleles from the genome that might have had mildly negative effects. The scientists estimate that these gene variations were able to persist in Neanderthals because Neanderthals had a much smaller population size than humans. Once transferred into the human genome, however, these alleles became subject to natural selection, which was more effective in the larger human populations and has removed these gene variants over time.The study is one of the first attempts to quantify the strength of natural selection against Neanderthal genes. It enhances the understanding of how Neanderthals contributed to human genomes (along with Harris and Nielsen, Genetics 2016). It also confirms previous reports that East Asian people had somewhat higher initial levels of Neanderthal ancestry than Europeans.
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