THE EFFECTS OF SOCIAL-STRUCTURE, GEOGRAPHICAL STRUCTURE, AND POPULATION-SIZE ON THE EVOLUTION OF MITOCHONDRIAL-DNA - II - MOLECULAR CLOCKS AND THE LINEAGE SORTING PERIOD

Evolutionary geneticists have increasingly used sequence variation in mitochondrial DNA (mtDNA) as a source of historical information, Howev er, conclusions based on these data remain tentative because a suffici ently clear understanding of the evolutionary dynamics of mtDNA has ye t to be developed. In this paper we present the results of computer si mulations designed to illustrate the effects of social structure, geog raphical structure, and population size on the rate of nucleotide subs titution and lineage sorting of mtDNA. The model is based in part on t he social structure of macaque monkeys. Simulated populations of femal es were divided into 25 social groups; the animals in each were distri buted in a hierarchy of four dominance rank categories. The probabilit ies for offspring survivorship were varied among dominance ranks to re flect the fitness consequences of social structure. Population size wa s varied across runs from 100 to 300 females. The pattern of female mi gration was also varied to mimic either the island model or the steppi ng-stone model. All these variables are shown to affect the lineage so rting period (LSP), and certain combinations of parameter values can c ause the retention of mtDNA polymorphisms for a very long time. In add ition, the simulations exhibited a negative relationship between the L SP and substitution rate over a modest and realistic range of LSP valu es. An important implication of these results is that estimates of tim e since isolation based on the assumption of a constant molecular cloc k may be biased and unreliable.