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
Every generation is a sampling event: only some gametes become zygotes, so survivor frequencies wobble by luck alone. That wobble is genetic drift, and it has no preferred direction. In huge populations the sample is vast and the wobble hides; in small ones the same luck swings frequencies hard.
A neutral allele in a tiny group can drift to fixation or vanish with no selection involved at all. A crash can leave a few survivors whose quirks become the new normal. A handful of founders can start an island colony already unlike the mainland.
Headcounts lie, so geneticists count effective size instead. One male fathering most offspring, skewed sex ratios, wild family size swings, and boom bust cycles all shrink the genetic size below the census. Effective size sets the pace of drift, which is why a seemingly large group can still lose variation fast.
Drift is sampling noise, while selection pushes steadily toward higher fitness. Saving a species means keeping its effective size large enough that drift does not eat its variation.
Small samples wander, crashes and founders jump, and effective size sets the speed.
2 · Watch
Take it off screen
Where it sits
8 questions wait behind this lesson, each with its answer explained. Every answer feeds the sky: stars light as they are learned, and dim when it is time to come back.