What Is Genetic Drift in Populations?
Genetic drift is the random fluctuation of allele frequencies in a population from one generation to the next, caused purely by chance rather than natural selection. Its effects are strongest in small populations.
Genetic drift is a random change in allele frequencies due to chance events in reproduction and survival; unlike natural selection, it is not driven by fitness differences and can even reduce a population's adaptedness.
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Step-by-step worked examples
A population of N = 25 beetles has allele frequency p = 0.5. Estimate the variance in allele frequency change next generation.
Var(Δp) = pq/(2N) = 0.5×0.5/(2×25) = 0.25/50 = 0.005 → SD ≈ √0.005 ≈ 0.071 (a ~7% swing is plausible)
A large population of N = 5,000 has the same p = 0.5. Compare its drift variance to the N = 25 population above.
Var(Δp) = 0.5×0.5/(2×5000) = 0.25/10000 = 0.000025 SD ≈ √0.000025 ≈ 0.005 (a ~0.5% swing) Conclusion: drift variance is ~200× weaker in the larger population (drift scales as 1/N)
After a flood, a population of 1,000 mice is reduced to 10 survivors by chance, unrelated to their genotype. What phenomenon does this illustrate, and what happens to genetic diversity?
A drastic, chance reduction in population size = a population bottleneck (a form of genetic drift) Rare alleles are likely lost entirely if none of the 10 survivors carry them Overall genetic diversity drops sharply Subsequent generations start from this smaller, less diverse gene pool
Flashcards
Quick quiz
Q1.Genetic drift has the strongest effect in…
Q2.Which formula estimates the variance in allele frequency change due to drift?
Q3.A population bottleneck is an example of…
Q4.Unlike natural selection, genetic drift is…
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Common mistakes
Genetic drift and natural selection are the same process. — Correct: Selection is fitness-driven and non-random; drift is chance-driven and unrelated to fitness.
Genetic drift only matters in huge populations. — Correct: Drift's effects are strongest in small populations; large populations are relatively buffered.
Drift always removes harmful alleles. — Correct: Drift is random — it can just as easily increase or fix a harmful allele as remove it.
The founder effect and bottleneck effect are unrelated to drift. — Correct: Both are specific cases of genetic drift caused by a small, non-representative sample of a gene pool.
FAQ
What is genetic drift?
Genetic drift is a random, chance-driven change in allele frequencies from one generation to the next, distinct from natural selection.
What is the formula for genetic drift?
Var(Δp) = pq/(2N) estimates the variance (spread) of allele frequency change per generation due to chance.
What are examples of genetic drift?
The founder effect (e.g., the Amish population, cheetah bottleneck) and population bottlenecks after disasters are classic examples.
How do you calculate genetic drift's effect on a population?
Use effective population size N and current allele frequency p in Var(Δp)=pq/2N; smaller N means bigger expected swings.