Effective population size

Effective population size is the size of an idealized population that would experience the same genetic drift as a real population. It can differ greatly from the number of individuals counted.

Genetic drift: Random changes in allele frequencies caused by chance sampling across generations. Effective population size measures the strength of this random change.
Census population size: The number of individuals present in a population at a specified time. It counts bodies directly, unlike effective size, which reflects genetic drift.
Conservation genetics: The use of genetic principles and data to preserve biological diversity and manage populations. Effective size helps assess whether small populations can retain genetic variation.
Effective number of breeders: The number of individuals contributing offspring to a single cohort or reproductive episode. It concerns a particular breeding event, whereas effective population size often summarizes longer-term drift.
Wright–Fisher model: A population-genetic model in which each generation is sampled from a fixed-size parental population. Its idealized assumptions provide a standard baseline for drift.
Allele frequency: The proportion of gene copies in a population that carry a particular allele. Drift is observed as random change in these proportions.
Population bottleneck: A sharp temporary reduction in population size that can eliminate genetic variation by chance. A bottleneck can depress effective size long after census numbers recover.
Coalescent effective population size: An effective size inferred from the rate at which sampled gene copies share a common ancestor. Its estimate depends on genealogical patterns and can differ from variance-based measures.
Variance effective population size: An effective population size defined by the rate of increase in genetic variance across generations. It quantifies drift through changes in allele-frequency variance.
Inbreeding coefficient: The probability that two alleles at a locus are identical by descent. Its increase provides one route to estimating effective size.

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