Wright-Fisher model: Each generation samples N offspring from binomial distribution with
p' = (p(1+s)+μ(1-p)) / (1 + sp).
Drift dominates when 4Ns ≪ 1, selection dominates when 4Ns ≫ 1.
Kimura's fixation probability: P_fix ≈ (1-e^{-2s})/(1-e^{-4Ns}).
Mutation-selection balance: q̂ ≈ μ/s.