Fig. 1.
Time-varied selective pressures on an evolutionary tree. We model the evolutionary history of six representative human populations as a binary tree. Here, si (i=1, 2, …, 10) denotes the selection coefficient of the i-th epoch and can be obtained by estimating selection differences between paired populations. Divergence times between populations are based on previous studies (Schaffner et al., 2005; Oppenheimer, 2012; Schiffel and Durbin, 2014; Mondal et al., 2016). We assume one generation time is ∼30 years. We obtain the optimal solution deviated least from neutral evolution using a probabilistic approach. The numbers (×10−3/generation) on the branches are the optimal solution. In the solution, numbers larger than zero suggest natural selection favoured light pigmentation, while those less than zero indicate natural selection preferred dark pigmentation.