PAIRWISE COMPARISONS OF MITOCHONDRIAL-DNA SEQUENCES IN SUBDIVIDED POPULATIONS AND IMPLICATIONS FOR EARLY HUMAN-EVOLUTION

We consider the effect on the distribution of pairwise differences bet ween mitochondrial DNA sequences of the incorporation into the underly ing population genetics model of two particular effects that seem real istic for human populations. The first is that the population size was roughly constant before growing to its current level. The second is t hat the population is geographically subdivided rather than panmictic. In each case these features tend to encourage multimodal distribution s of pairwise differences, in contrast to existing, unimodal datasets. We argue that population genetics models currently used to analyze su ch data may thus fail to reflect important features of human mitochond rial DNA evolution. These may include selection on the mitochondrial g enome, more realistic mutation mechanisms, or special population or mi gration dynamics. Particularly in view of the variability inherent in the single available human mitochondrial genealogy, it is argued that until these effects are better understood, inferences from such data s hould be rather cautious.