在群体遗传学中，美国遗传学家休厄尔·赖特在两篇标志性的论文（Wright 1931, 1938）中引入了有效群体大小（effective population size，又做有效种群大小）这一概念。他定义其为“在一个理想群体中，在随机遗传漂变影响下，能够产生相同的等位基因分布或者等量的同系繁殖的个体数量。”（"the number of breeding individuals in an idealized population that would show the same amount of dispersion of allele frequencies under random genetic drift or the same amount of inbreeding as the population under consideration".）有效群体大小是很多群体遗传学模型中的基本参量，通常小于绝对的种群大小（N）。参见小种群大小。

The effective population size is the number of individuals that an idealised population would need to have, in order to produce similar numbers of offspring as the real population of interest. In some simple cases, this effective population size is equal to the number of breeding individuals in the real population. Idealised populations are based on unrealistic simplifications such as random mating, simultaneous birth of each new generation, constant population size, and equal numbers of children per parent. The census population size N of a real population is usually larger than the effective population size N_e.