 The fact that \(E(\Delta p) = 0\) does not imply that drift will have no evolutionary effect. For \(\Delta p\) may have a substantial variance around the mean of zero (depending on the value of \(N\)), so it may be quite probable that \(\Delta p\) deviates from zero by a substantial amount. (Similarly, if one flips a fair coin 20 times, the expected number of heads is 10, but the probability that the actual number of heads is 8 or less is quite substantial – approximately 25%). In the Wright-Fisher model, the variance of \(\Delta p\) turns out to be \(Var(\Delta p) = \frac{1}{2N}p(1-p)\). Thus as the population size increases, the variance of \(\Delta p\) gets smaller and smaller, which illustrates the point that random drift is more important in small than in large populations.