Figure 1

Fig. 1. Spatial simulations are consistent with observed clines in allele frequency. The Old World was modeled as a two-dimensional stepping-stone divided into 9226 demes of 100 by 100 km, each with a carrying capacity of 50 diploid individuals (100 genes). Each generation, each gene had a 25% probability of migrating to an adjacent deme. We modeled a range expansion out-of-Africa through the Sinai Peninsula that started 1400 generations ago (40,000 years), as in (7). At that time, we assumed that Africa was fully occupied, with all demes being at carrying capacity and harboring an allele with a uniform initial frequency p₀ of 0.05, 0.1, or 0.2. We simulated the stochastic evolution of this allele in Africa and in Eurasia until today, keeping only simulations where the allele has persisted until today somewhere in the Old World. (The proportion of simulations kept was equal to 0.042, 0.886, and 1.000 for p₀ values of 0.05, 0.1, or 0.2, respectively, as reported in the legend of the histogram.) The histogram (lower right) reports the frequency distribution of the allele in non-African populations, demonstrating that the allele can reach very high frequencies by drift and surf (4) outside of Africa. The distributions were obtained from 1000 simulations, except for p₀ = 0.05, where only 424 simulations out of 10,000 were successful. For each parameter value, the maps show two spatial frequency distributions where alleles reached a minimum average frequency of 50% outside Africa, illustrating how alleles with low initial frequencies in Africa can reach high frequency by colonization and drift in non-African populations, while remaining at low frequencies within Africa. [View Larger Version of this Image (134K JPEG file)]

 

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