- Genetic drift changes allele frequencies by chance, not fitness.
- Small populations lose genetic diversity fastest.
- Wright formalized drift theory in 1931.
Genetic drift is a change in allele frequency within a population caused by random sampling rather than natural selection. It affects every finite population but reshapes small ones fastest, sometimes fixing or eliminating gene variants in just a few generations.
Why It Matters
In any population, each generation represents a sample of its parents' genes. When that sample is small, chance alone can shift allele frequencies far from their starting point.
Over time, drift can erase genetic variation entirely, leaving a population with fewer raw materials for adaptation.
Key figure
1931
Year Sewall Wright formalized genetic drift theory
This matters beyond textbooks. Conservation biologists tracking endangered species, from cheetahs to California condors, measure drift's effects directly. A species reduced to a few dozen breeding individuals can lose alleles that might have helped it survive disease, climate shifts, or habitat change. The bottleneck effect describes exactly this scenario.
Drift also explains patterns in human genetics. Small founder groups that colonized islands or crossed land bridges carried only a fraction of their parent population's variation. The founder effect accounts for why certain genetic conditions appear at unusually high rates in isolated communities, from Afrikaners in South Africa to Ashkenazi Jewish populations.
How It Works
Picture a jar holding 50 red and 50 blue marbles. Draw 20 at random to start a new jar. The new jar will rarely hold exactly 10 of each color.
Repeat the draw from the new jar, and the imbalance grows. Given enough rounds, one color disappears. This is drift: random sampling error compounding across generations.
Key figure
4Ne
Generations to fixation (on average) for a neutral allele
The mathematics, first laid out by the American geneticist Sewall Wright in his 1931 paper "Evolution in Mendelian Populations," shows that drift's strength is inversely proportional to population size. In a population of effective size Ne, a neutral allele fixes (reaches 100% frequency) in an average of 4Ne generations.
For a population of 500, that is 2,000 generations. For a population of 50, just 200.
The Japanese geneticist Motoo Kimura extended this framework in 1968 with the neutral theory of molecular evolution. Kimura proposed that most mutations at the molecular level are selectively neutral. Their fate in the population is governed by drift, not selection.
His criterion: when an allele's selective advantage or disadvantage (s) is smaller than 1/(2Ne), drift dominates. For large populations, only alleles very close to neutral drift freely. For small populations, even mildly beneficial or harmful alleles behave as though neutral.
Key Context
Wright's work on drift fed into the modern evolutionary synthesis of the 1930s and 1940s, alongside contributions from R.A. Fisher and J.B.S. Haldane.
Wright and Fisher disagreed sharply on drift's importance. Fisher argued selection overwhelmed drift in most natural populations. Wright countered that subdivided populations with limited migration created pockets where drift dominated, fueling his shifting balance theory of evolution.
A study published in Nature Reviews Genetics by Brian Charlesworth confirmed that drift's effects scale predictably with effective population size across species, from bacteria to vertebrates. Organisms with smaller effective population sizes carry proportionally less neutral genetic variation, exactly as Wright and Kimura predicted.
FAQ
What is the difference between genetic drift and natural selection?
Natural selection consistently favors alleles that improve survival or reproduction. Genetic drift changes allele frequencies through chance alone, without regard to whether an allele is beneficial, harmful, or neutral. Both are mechanisms of evolution, but selection is directional while drift is random.
Can genetic drift affect large populations?
It can, but its effects are negligible in large populations because the law of large numbers keeps allele frequencies close to their expected values. Drift becomes a significant evolutionary force only when effective population size drops below a few thousand individuals.
How does genetic drift reduce genetic diversity?
Each generation, random sampling removes some low-frequency alleles from the gene pool. Over many generations, alleles are randomly lost or fixed at 100% frequency. Once an allele disappears, it cannot return unless reintroduced by mutation or migration. Small populations lose diversity fastest because fewer copies of each allele exist.
Is genetic drift always harmful?
Not necessarily. Drift is directionless, so it can sometimes increase the frequency of beneficial alleles by chance. However, in small populations, drift more often removes beneficial variation and allows mildly harmful alleles to persist, reducing a population overall fitness. This effect, called genetic load, concerns conservation biologists managing endangered species.
Related Reading
Sources
- Primary Research: Evolution in Mendelian Populations (Sewall Wright, 1931)
- Primary Research: Evolutionary Rate at the Molecular Level (Motoo Kimura, 1968)
- Additional Context:
- Genetic Drift (NHGRI Glossary)
- Genetic Drift (UC Berkeley Evolution 101)
- Genetic Drift and Effective Population Size (Nature Scitable)
Fact Check: Claim-by-Claim Verification Verified
All six major claims verified against multiple independent sources. Wright's 1931 paper, Kimura's 1968 neutral theory, fixation time formula, and Wright-Fisher debate all confirmed.
Sources used for verification
- Genetic Drift - genome.gov
- Evolution in Mendelian Populations - Oxford Academic
- Evolutionary Rate at the Molecular Level - Nature
- Genetic Drift - UC Berkeley
- Genetic Drift and Effective Population Size - Nature Scitable
