Effective population size

Effective population size ($N_e$) is a concept in population genetics that measures the size of an idealized population that would experience the same magnitude of genetic drift as the actual population under study. The actual population may be larger than $N_e$ due to factors that reduce genetic diversity: unequal sex ratios, variation in family size, population bottlenecks, and non-random mating.

The effective population size determines the balance between natural selection and genetic drift. When $N_e$ is large, selection dominates and advantageous alleles spread reliably. When $N_e$ is small, drift can overpower selection and drive alleles to fixation regardless of their fitness effects. This is why balancing selection can maintain polymorphisms in large populations but may fail in small ones.

The concept was developed by Sewall Wright and remains central to understanding how population structure shapes evolutionary outcomes. In conservation biology, $N_e$ is the key parameter for predicting the viability of endangered populations; in evolutionary theory, it sets the threshold for when selection becomes effective.