'Evolution Is a Sloppy Artist'

A few weeks ago, the Gene Expression blog's Razib Khan reviewed a paper in Molecular Biology and Evolution that suggested that Crohn's disease may be the result of a selective sweep driven by adaptation to nutrient deficiencies incurred by European farmers switching to a grain-based diet. While that paper focused on one gene, Khan says, there is now a paper in PLoS Genetics that explores the hypothesis that "some deleterious mutations have hitchhiked [within the human genome] to high frequency due to linkage to sites that have been under positive selection." The study's results, the authors write, provide evidence that "hitchhiking has influenced the frequency of linked deleterious mutations in humans, implying that the evolutionary dynamics of advantageous and deleterious mutations may often depend on one another." In other words, Khan says, "Evolution is a sloppy artist … [and] there are often downsides to adaptation." But the paper's conclusions are tentative, and there are other processes that are responsible for the preservation of deleterious alleles, he adds. The authors of this paper haven't found the answer to the question of deleterious alleles, but rather one piece of the puzzle.