Are we beyond the forces of evolution, or is natural selection still shaping us? A genomic study of modern Icelanders suggests modern humans are still a work in progress.
The finding comes from deCODE Genetics, a company based in Reykjavik, Iceland that is hunting for disease-associated genes. While analyzing DNA sequence from more than 29,000 Icelandic people, deCODE researchers discovered something intriguing. A section of chromosome 17, 900,000 base pairs long, is flipped into reverse order in about 20% of Europeans, but this flip is very rare among Africans and Asians. Inversions on a smaller scale happen frequently when chromosomes reshuffle themselves in the course of sexual reproduction. Usually, they flip back with the same frequency. Also striking was the fact that the two versions of the chromosomal section appear to have been around for some 3 million years, long before the appearance of modern Homo sapiens.
The fact that such an extended stretch of inverted DNA appears to have been maintained for so long and today is carried by 1 in 5 Europeans suggested that it must provide some kind of evolutionary advantage in the European environment. If natural selection played no role, one of the two versions would have likely dropped to a low frequency long ago.
So a team of deCODE geneticists led by Kari Stefansson, Augustine Kong, and Hreinn Stefansson took advantage of Iceland's extensive genealogical and medical records to look for clues. The results of the study, published in the February issue of Nature Genetics, are wowing population geneticists. Women who carry the inversion have slightly more children on average than those who don't, which would have a powerful effect over the long timeframe on which evolution acts. However, the team admits that exactly how the inversion results in larger families--and therefore more copies of itself--remains a mystery. The genes that lie in the section do not seem to be directly related to fertility. The genomes of women carrying the inversion also had slightly more shuffling of genes between copies of their chromosomes, a process known to be important for avoiding diseases.
"This could be the tip of several icebergs," says Peter Donnelly, a statistician at the University of Oxford, U.K. "If such inversions are common, then there isn't just one version of the human genome." He adds that "this shows natural selection is still acting on us."