In a finite population, gene frequencies wander purely by chance — who happens to reproduce. This DRIFT eventually pushes an allele all the way to 0% or 100% (extinction or fixation), even with no selection at all. The smaller the population, the faster it happens; genetic diversity leaks away at a rate of 1/(2N) per generation. It’s evolution’s coin-flip. Slide the population size.
The Wright–Fisher model: each generation of N diploid individuals draws its 2N alleles by binomial sampling from the parents’ frequency, with no selection. Expected allele frequency is unchanged, but VARIANCE accumulates — heterozygosity (diversity) decays geometrically as Hₜ = H₀·(1 − 1/2N)ⁿ, so an allele is eventually lost or FIXED with probability equal to its current frequency. Smaller N drifts faster. It is the null model against which selection is detected. A fail-loud self-check throws unless heterozygosity decays to near zero (fixation) over many generations. ◆ real population genetics, node-verified.
Idealised (non-overlapping generations, constant N, no selection/migration/mutation) — the complement to [[the-hardy-weinberg]]’s infinite-population no-drift case. The 1/2N drift rate and inevitable fixation are the exact content; real populations add the other forces back.