Testing Wright’s Intermediate Population Size Hypothesis – When Genetic Drift is a Good Thing

Abstract

In his 1931 monograph, Sewall Wright predicted genetic drift would overwhelm selection in small populations, and selection would dominate in large ones, but he also concluded drift could facilitate selection in populations of intermediate size. The idea that drift and selection would act together in populations of intermediate size has been almost completely ignored even as empirical evidence of rapid evolution associated with population bottlenecks has continued to accumulate. I used forward-time simulations with random mating and discrete generations to test the hypothesis that drift can facilitate selection. I find that selection generates biased distributions of Δq, and this bias is greatest for population sizes between 20 and 200, resulting in drift facilitation. Drift facilitation reduces the accumulation of drift load and segregation load for populations of intermediate size. Small populations accumulated higher levels of drift load and large populations maintained high levels of segregation load. Fixation of beneficial mutations is facilitated in intermediate populations when dominance is low and selection is weak. Accumulation of beneficial mutations over time (fixation flux) was higher across small to intermediate size populations and declined rapidly for large populations. Compared to predictions of fixation time for codominant beneficial mutations from diffusion equations, drift facilitation accelerates fixation in populations of intermediate size, but fixation time is slower in large populations when selection is weak. These results suggest drift facilitation in small and intermediate populations promote purging of genetic load and fixation of beneficial mutations, and may result in relatively rapid adaptation compared to large populations.