 where \(x_{ij}\) is the probability of \(i\) \(A\) alleles in the next generation given \(j\) \(A\) alleles in this generation, and \(N\) is effective population size. A system governed by both the deterministic equations and the binomial sampling equation is said to undergo drift; all natural systems do so. (For more on drift, effective population size, and randomness in evolutionary theory, see entry on  genetic drift).  A system exhibiting heterozygote superiority whose dynamics are a function of the binomial sampling equation will not simply rest at its stable intermediate frequency but will hover around it, in some generations evolving toward it, more rarely evolving away, and in some generations exhibiting no evolution at all. Which of these cases are cases in which the system undergoes natural selection in the capacious sense? That is, which cases are cases in which the system falls within the purview of evolutionary theory? A natural answer is all of them. To answer in this way, however, we must not make evolution necessary for natural selection. If we do make evolution necessary, we issue the verdict that systems governed by the above systems of equations sometimes fall within this purview of evolutionary theory and sometimes fall outside it, on a generation to generation basis, despite being governed the whole time by a single same system of equations, ones undeniably developed as an outgrowth of Darwin’s initial theorizing.