Fig. 6

Populations evolve blindness in the face of immigration only with the help of strong selection. a The equilibrium frequency of the blindness allele (q) for an infinite population, and b–d average frequencies of the allele after t generations in finite populations with either constant or episodic migration. For each combination of selection (s) and migration (m) we conducted 100 replicate simulations with fixed values of the mutation rate (u=10⁻⁶), frequency of the blindness allele in the surface population (Q=0.01), and q ₀=Q. Colors correspond to the frequency of the blindness allele for a given combination of s and m, where blue is high frequency (blindness evolved) and red is low (blindness did not evolve). The solid white line corresponds to the degree of selection required in the infinite population (a) to result in q _∞ >0.5 (\(s^{*}_{0.5}\)). The area between the solid and dashed lines corresponds to the region where three equilibria exist. If 2N s≪1, drift is stronger than selection, and if 4N m≪1, drift is stronger than migration. If m Q≪u, mutation is the primary force introducing copies of the blindness alleles to the cave population