Compensatory evolution and fitness valleys

Negative selection prevents the fixation of deleterious alleles. Thus, although some deleterious alleles can segregate in the population, conventional wisdom holds that deleterious mutations are evolutionary dead ends. In an letter to Nature published today online we show that for mt-tRNAs this is not always the case. Our data indicate, that compensatory switches between different Watson-Crick pairs in the secondary stem structures occur by a combination of two individually deleterious alleles that are benign when combined together in the same haplotype. Thus, in the case of mt-tRNAs even deleterious mutations do not always impair evolution.