Genetic Drift and Effective Population Size
Allele frequencies wander from generation to generation because gametes are a sample, and the smaller the population the wider that wandering. The effective size, which counts breeding individuals unevenly, is usually well below the number of individuals present.
What a learner can do afterwards
- Explains drift as sampling error rather than as a force pushing in a direction.
- Predicts the fate of a neutral allele in a small population over many generations.
- Says why the effective size is smaller than the census size, with one cause named.
1 · Read
Genetic drift is a change in allele frequency caused by random sampling. It has no preferred direction. A small population makes a smaller sample, so chance usually causes wider swings than it does in a large population. When family sizes vary widely, a few parents supply most of the genes, so the genetic sample is smaller and drift is stronger.
A neutral allele has no fitness difference. Over many generations it may reach fixation, meaning frequency 1, or disappear, meaning frequency 0, by chance alone.
A disaster that randomly leaves a few survivors creates a bottleneck. The survivors' alleles become the starting sample for the next population. A few founders starting a colony create a founder effect.
A random disaster resamples the gene pool.
Selection changes frequencies because heritable variants differ in reproductive success. Drift changes frequencies because the sample is random. Both can act in the same population.
Effective population size describes how many individuals contribute genes in a way that matters for drift. One male fathering most offspring, an uneven sex ratio, or large family-size differences can make effective size much smaller than the headcount.
For an uneven sex ratio, a standard estimate of effective size is Ne = 4 × males × females ÷ (males + females). With 20 males and 80 females, Ne is 4 × 20 × 80 ÷ 100, or 64, well below the headcount of 100. A smaller effective size lets drift remove or fix neutral variation faster than the headcount suggests.
Drift is directionless sampling change. Small or unevenly breeding populations have smaller effective size, so neutral variation can be lost or fixed faster.
Original lesson, assessment, and flashcard text authored for this topic in the Open Source Taxonomy project.
Rewritten readings were written for this dataset; the video is a Khan Academy embed, with publisher terms unknown. The reading on effective population size was written for this topic in the Open Source Taxonomy project.
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Where it sits
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