OUT OF AFRICA AND BACK AGAIN - NESTED CLADISTIC-ANALYSIS OF HUMAN Y-CHROMOSOME VARIATION

We surveyed nine diallelic polymorphic sites on the Y chromosomes of 1 ,544 individuals from Africa, Asia, Europe, Oceania, and the New World . Phylogenetic analyses of these nine sites resulted in a tree for 10 distinct Y haplotypes with a coalescence time of similar to 150,000 ye ars. The 10 haplotypes were unevenly distributed among human populatio ns: 5 were restricted to a particular continent, 2 were shared between Africa and Europe, 1 was present only in the Old World, and 2 were fo und in all geographic regions surveyed. The ancestral haplotype was li mited to African populations. Random permutation procedures revealed s tatistically significant patterns of geographical structuring of this paternal genetic variation. The results of a nested cladistic analysis indicated that these geographical associations arose through a combin ation of processes, including restricted, recurrent gene flow (isolati on by distance) and range expansions. We inferred that one of the olde st events in the nested cladistic analysis was a range expansion out o f Africa which resulted in the complete replacement of Y chromosomes t hroughout the Old World, a finding consistent with many versions of th e Out of Africa Replacement Model. A second and more recent range expa nsion brought Asian Y chromosomes back to Africa without replacing the indigenous African male gene pool. Thus, the previously observed high levels of Y chromosomal genetic diversity in Africa may be due in par t to bidirectional population movements. Finally, a comparison of our results with those from nested cladistic analyses of human mtDNA and b eta-globin data revealed different patterns of inferences for males an d females concerning the relative roles of population history (range e xpansions) and population structure (recurrent gene flow), thereby add ing a new sex-specific component to models of human evolution.