Moran Process: Genetic Drift & Fixation

In finite populations neutral mutations fix with probability 1/N — drift overpowers selection when Ns≪1.

N = 50
s = +0.020
p₀ = 0.50
8
Fixation prob π = —  |  Theory π_theory = —  |  Fixed A: 0  |  Fixed B: 0  |  Running: —
Moran process: at each step, select one individual proportional to fitness (1+s for A, 1 for B), and replace a random individual.
Fixation probability: π(p₀) = (1 − e^{−2Nsp₀}) / (1 − e^{−2Ns}) for s≠0, else 1/N for one mutant.
Neutral drift (s=0): all alleles fix with prob proportional to frequency. Selection (s>0) favors A but drift can still overpower when Ns≪1.