---
title: "Genetic Drift and Effective Population Size"
description: "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 une"
canonical: https://lightmysky.com/learn/science/genetic-drift-and-effective-population-size-mt_lbB1GbKv_t
source: https://lightmysky.com/learn/science/genetic-drift-and-effective-population-size-mt_lbB1GbKv_t.md
retrieved: 2026-11-12
---

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# 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.

Subject: Science · Area: Genetics & Evolution · Ages 18 to 19
Page: https://lightmysky.com/learn/science/genetic-drift-and-effective-population-size-mt_lbB1GbKv_t

## Ready when they can

- 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.

## Lesson: Sampling error that evolves

*(drawing: Drift changes frequencies because each generation is a random sample.)*

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.

**Example.** 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.

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.

**Example.** 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.

**Recap.** 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.*

## Practice

8 questions on this page, each with its working shown.

## Needs first

- [Selection Coefficients and Predicting Allele Frequency Change](https://lightmysky.com/learn/science/selection-coefficients-and-predicting-allele-frequency-change-mt__JS2XJxOXV)

## Opens up

- [Mutation, Gene Flow and the Balance That Holds Variation](https://lightmysky.com/learn/science/mutation-gene-flow-and-the-balance-that-holds-variation-mt_A2UgDE0ORv)
- [Measuring Genetic Variation: Heterozygosity and Population Structure](https://lightmysky.com/learn/science/measuring-genetic-variation-heterozygosity-and-population-structure-mt_Prn7vgkyLl)
- [Phylogenomics: Gene Trees, Species Trees and Why They Disagree](https://lightmysky.com/learn/science/phylogenomics-gene-trees-species-trees-and-why-they-disagree-mt_rg7feIg1D4)
- [Conservation: Small Populations, Fragmentation and Reserve Design](https://lightmysky.com/learn/science/conservation-small-populations-fragmentation-and-reserve-design-mt_yOl2NGoLeb)