This is the best fit found among several exact least-squares evaluations of the similar trees found by a heuristic search algorithm, improving on the neighbor-joining tree. Under the assumption of random genetic drift and no migration or selection (clearly not applicable to African Americans), the branch lengths are proportional to t/2N_e (ref. 16). Because the time from the root (in Africa) to all terminals (modern populations) is the same, the increasing distance from populations in Africa to those in East Asia and the Americas represents increasing drift caused by decreasing effective population sizes. Within a geographic region, the genetic clustering of populations often parallels the linguistic clustering because stochastic factors affect both as populations diverge¹⁹. SF, San Francisco; TW, Taiwan; AZ, Arizona; MX, Mexico; R, Rondonian. The symbols represent bootstrap values (based on 1,000 replicates): circles, >95%; diamonds, 90–95%; triangles, 85–90%.