The Pith: Evolution is a sloppy artist. Upon the focal zone of creative energy adaptation can sculpt with precision, but on the margins of the genetic landscape frightening phenomena may erupt due to inattention. In other words, there are often downsides to adaptation.

A few weeks ago I reviewed a paper which suggests that Crohn’s disease may be a side effect of a selective sweep. The sweep itself was possibly driven by adaptation to nutrient deficiencies incurred by European farmers switching to a grain based diet. The reason for this is a contingent genomic reality: the positively selected genetic variant was flanked by a Crohn’s disease risk allele. The increment of fitness gain of the former happens to have been greater than the decrement entailed by the latter, resulting in the simultaneous increase in the frequency of both the fit and unfit variants. You can’t always have one without the other.

But that’s just focusing on one gene, though the authors did indicate that this may be a genome-wide feature. A new paper in PLoS Genetics argues that that is the case, at least to some extent. Evidence for Hitchhiking of Deleterious Mutations within the Human Genome:

Deleterious mutations reduce ...