NOTE 23E: Recall that if the distribution of offspring number follows a Poisson distribution with mean 1, then the variance is also 1, and we have the standard **Wright–Fisher** formula, *pq*/*2N*. Also note that it is simplest to focus on the fitness of individual genes; Box 15.2 gives a formula for the rate of random drift in terms of the variance in fitness of diploid *individuals*, which is more complex.