Effective population size

What is it? An aid to calculation in population genetics, introduced by Sewall Wright. The rate of change of the genetic make-up of a population by genetic drift — fluctuations in allele frequencies caused by random sampling — is inversely related to the effective population size, Ne. Ne is measured with reference to an ideal ‘Wright–Fisher’ population. This has a fixed number of N diploid breeding individuals, with no distinction of sex. Each new generation is formed by sampling genes randomly from the parents of the previous generation. The rate at which selectively neutral variability is lost from the population is then equal to 1/(2N). In the messier real world, there may be two different sexes, population size may change, individuals may differ in their reproductive success and there may be overlapping generations. If you are clever enough, you can write down an expression for the rate of drift per generation in such cases. This is then equated to 1/(2Ne). Ne is usually substantially smaller than the census number of breeding individuals.