 Importantly, the equality between an allele’s current frequency and its probability of becoming fixed assumes that the allele in question, and other alleles at the same locus, are selectively neutral—meaning that random drift is the sole determinant of the changes in frequency. If this assumption is relaxed, matters become more complicated. The fate of an allele then depends on both drift and on its selective advantage or disadvantage. This takes us beyond the confines of the simple Wright-Fisher model since now there are two evolutionary factors at work—random drift and natural selection. In a finite population, an allele that is selectively advantageous, so has a positive selection coefficient, will have a higher probability of fixation that a neutral allele; and conversely for one that is selectively disadvantageous. This is fairly obvious. What is less obvious, but still true, is that even if a novel variant arises that confers a significant selective advantage, it is still more likely to be eliminated by drift than to become fixed. To quantitatively study the combined effects of selection and drift, population geneticists use an advanced probabilistic technique known as diffusion analysis, which lies beyond the scope of this article (see Rice 2004 ch.5, Hartl 2020, ch.6. or Otto and Day 2007, ch.15). But one key result deserves mention, which is that the eventual fate of an allele depends on the relative magnitude of two quantities, namely \(4N_e\) and \(s\). Here \(N_e\) denotes the “effective population size” (which corrects the actual population size to take account of deviations from the idealized assumptions of the Wright-Fisher model), and \(s\) is the selection coefficient, which is a measure of the relative fitness of organisms with the allele compared to organisms without, where \(0 \leq s \leq 1\). It turns out that if \(4N_e s \gg 1\) then natural selection will determine the fate of an allele, while if \(4N_e s \ll 1\) then drift will determine its fate.