Effective population size

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The effective population size (N_e) is the size of an idealised population that would experience the same rate of genetic drift as the real population. The effective population size is normally smaller than the census population size N, partly because chance events prevent some individuals from breeding, and partly due to background selection and genetic hitchhiking. Idealised populations are based on unrealistic but convenient assumptions including random mating, rarity of natural selection such that each gene evolves independently, and constant population size.^[1]

The same real population could have a different effective population size for different properties of interest, such as genetic drift over one generation vs. over many generations, and a different effective population size at different sites in the genome. The effective population size is most commonly measured with respect to the coalescence time. In an idealised diploid population with no selection at any locus, the expectation of the coalescence time in generations is equal to twice the census population size. The effective population size is measured from genetic data as within-species genetic diversity divided by four times the mutation rate ${\displaystyle \mu }$, because in such an idealised population, the heterozygosity is equal to ${\displaystyle 4N\mu }$. In a population with selection at many loci and abundant linkage disequilibrium, the coalescent effective population size may not reflect the census population size at all, or may reflect its logarithm.

The concept of effective population size was introduced in the field of population genetics in 1931 by the American geneticist Sewall Wright.^[2][3] Some versions of the effective population size are used in wildlife conservation.