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generations, corresponding to a mean effective population size of 6,450–26,800 individuals.
There is one more factor to take into account in the calculations just made. We have used mean estimates to coalescence time, but these estimates have large variances. When the sample of genes is large, the standard deviation of the mean for nuclear genes is larger than 2N (Nei, 1987, eq. 13.74); for mitochondrial DNA it is larger than N/2. The 95% confidence interval for the number of generations to coalescence will correspondingly extend at the upper end to more than 2N (53,600) generations for mitochondrial DNA.
Thus the mitochondrial DNA sequence data are consistent with a mean effective population size between somewhat less than 10,000 and more than 50,000 individuals throughout the Pleistocene. This population size is, in turn, consistent with the estimate based on the HLA DRB1 polymorphism of a mean population size of 100,000 individuals for human ancestors over the last 30–40 Myr. The mitochondrial DNA data are also consistent with the result that no bottleneck smaller than several thousand individuals could have happened in hominid history.
The multiregional model emphasizes regional continuity in the evolution from H. erectus to archaic H. sapiens, and later to anatomically modern humans. Several variations of this model have been formulated (Aiello, 1993) that differ in the relative role given to two factors: (i) the prevalence of Africa in the gradual emergence of modern features, and (ii) the amount of migration between regions. At one extreme, the "African hybridization and replacement model" proposes that modern humans first evolved in Africa, but their dispersal throughout the world was accompanied by certain amount of hybridization with indigenous premodern populations (Braüer, 1984, 1992). A variant that might be called the "Levantine hybridization and replacement model" is statistically supported by a correlation analysis of morphological traits that favors the Middle East over Africa as the origin of modern human traits (Waddle, 1994). At the other extreme, some authors deny a recent African origin for modern humans while emphasizing regional continuity and gene exchange throughout Africa, Europe, and Asia (Wolpoff et al., 1988; Wolpoff, 1989). Other authors have proposed an intermediate "assimilation model'' that gives preponderance to Africa in the emergence of anatomically modern traits but attributes major importance to gene flow and differential selection pressures resulting in morphological differentiation among regions (Smith et al., 1989; Smith, 1992; Spuhler, 1993).
The paleontological data favor some form of the multiregional model, but one that gives preponderance to Africa (or the Levant, according to Waddle, 1994) as the locus where modern human traits first appear