Drift with weak selective advantage

Drift & Selection on rare alleles with weak selective advantage in finite populations
(N = 100, f(B₀) = 0.005, W0 = W1 = 0.99, W2 = 1.0)

Trajectories of a weakly advantageous (s = -0.01) mutant allele in multiple populations (N = 100 @). From a single copy of such an allele (initial f(B) = 1/200 = 0.005), almost all populations go to f(B) = 0.0 within 25 generations; two populations remain polymorphic at 50 ~ 100 generation, and only one goes to f(B) = 1, despite the selective advantage of B in the BB genotype. Note that once f(B) = 0.1, f(BB) = 0.01, so that one BB homozygote is expected in a population of N = 100. For f(B) = 0.2, f(BB) = 0.04, with the expectation of four BB homozygotes. This is the approximate threshold for f(B) to go to fixation, as seen here.

Stochastic genetic drift in a population of moderate size has a greater effect than does weak directional deterministic selection. Note that s > 1/2N.