NOTE 19W: We have ignored the initial random establishment of the favorable allele. Given that it will be fixed, its expected frequency is higher than this simple deterministic prediction by a factor 2*s*; in effect, the initial frequency is 1/4*Ns,* rather than 1/2*N*. So, a better prediction would be (1/*s*)log_{e}(4*Ns*). However, this makes little difference to the estimate because the dependence is via a logarithm. See Problem 18.5.