Genetic gradients derived from pure first and second Paleolithic range expansions from western Alaska along the American continent. (Left) Illustrative example of PC1 map obtained from a simulated old single first expansion. (Right) Illustrative example of PC1 map obtained from a simulated recent pure second expansion. (Below) PC1 maps for single first and second expansions derived from 100 computer simulations. Each black line represents the PC1 gradient (connecting positive and negative centroids) from an independent computer simulation. The green line represents the median of slopes and intercepts among the 100 simulations. These gradients can be compared with gradients derived from real data (Cavalli-Sforza et al. ; Cavalli-Sforza et al. )
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