Wright-Fisher Model — Genetic Drift

Allele frequency trajectories under random sampling. Small populations drift to fixation faster. Each trajectory is one independent population replicate.

Pop size N = 100

Init freq p₀ = 0.50

Selection s = 0.00

Replicates = 20

Wright-Fisher: next generation has Binomial(2N, p̃) copies of allele A, where p̃ = p(1+s)/(1+sp). Fixation probability: u(p) ≈ p (neutral), u(p) ≈ (1−e^{−2Nsp})/(1−e^{−2Ns}) (selected). Effective time to fixation: ~4N generations (neutral) — large N slows drift dramatically.