Temporal trails of natural selection in human mitogenomes

The human mitochondrial genealogy displays signatures of purifying selection, with comparatively few non-synonymous changes along the basal branches but an excess towards the tips (Elson et al. 2004). This pattern suggests that many of the mutations in the younger branches of the tree have slightly deleterious fitness effects, and are gradually removed from the population over time. The presence of these transient polymorphisms has important implications for analyses of adaptation, population genetics and the chronology of human prehistory. In a recent investigation of this matter, Subramanian (2009) estimated the ratios of non-synonymous to synonymous rates of change (dN/dS) in proteincoding genes across 18 human mitochondrial haplogroups (represented by 1512 mitogenomes). He found a strong negative correlation between dN/dS and haplogroup age, which is consistent with the action of purifying …