Genetic drift
Genetic drift is evolutionary change caused by chance shifts in allele frequencies. It is strongest in small populations and can reduce genetic variation without making a population better adapted.
What genetic drift is
Genetic drift is a change in allele frequencies caused by chance. In every generation, only some individuals reproduce and only some gene copies are passed on. That sampling can make alleles become more common, rarer, fixed, or lost even when they do not affect survival or reproduction.
Why chance matters
Imagine drawing a small handful of colored beads from a jar to represent the next generation. The sample may not match the original mix exactly. In large populations those random differences are often diluted, but in small populations chance can shift the genetic mix sharply from one generation to the next.
Allele loss and fixation
An allele is fixed when every individual in a population carries that version, and it is lost when no copies remain. Genetic drift can push either outcome without a trait being useful or harmful. Once variation is lost, mutation or gene flow is needed to introduce it again.
Population size
Drift affects all finite populations, but its effects are much stronger when the effective population size is small. Effective size is not always the same as head count; it depends on how many individuals actually contribute genes to future generations and how evenly they do so.
Bottlenecks and founder effects
A bottleneck happens when a population is sharply reduced, leaving survivors that may carry only part of the original genetic diversity. A founder effect happens when a new population begins from a small number of individuals. Both events can make chance a major driver of future allele frequencies.
Drift and natural selection
Natural selection and genetic drift often act at the same time. Selection can increase alleles that improve reproductive success, while drift can randomly raise or lower allele frequencies. In small populations, drift can sometimes overpower weak selection, allowing harmful alleles to persist or useful alleles to disappear.
Conservation and health
Genetic drift matters in conservation because small or isolated populations can lose variation, become more inbred, and have less capacity to adapt. It also helps explain why some inherited disorders are unusually common in populations shaped by founder events or long periods of isolation.
Why it matters
Genetic drift matters because evolution is not only adaptation. Chance can shape genomes, especially in small populations, and can leave lasting traces in species history. Understanding drift helps scientists interpret DNA data, manage endangered populations, and avoid telling every evolutionary story as if it were driven by selection.