effective population size

A crowd of ten thousand people may, for genetic purposes, behave like a far smaller group if only a handful actually have children. Effective population size captures this gap: it is not the raw headcount but the number of individuals that effectively contribute genes to the next generation, and it governs how strongly genetic drift acts.

Technically, the effective population size, written Ne, is the size of an idealized population that would lose genetic variation by drift, or inbreed, at the same rate as the real population under study. It is the number that matters in population-genetics equations, and it is usually smaller—often much smaller—than the actual census size.

Many features pull Ne below the headcount: an unequal sex ratio among breeders, large variation in how many offspring individuals leave, fluctuations in population size over time, and overlapping generations. A single severe bottleneck in the past can depress Ne for a long time, because the harmonic mean weights small numbers heavily.

Effective population size is a cornerstone of conservation genetics. A species can be numerous yet have a small Ne, leaving it more exposed to drift, inbreeding, and the loss of adaptive variation than its census numbers would suggest.