---
title: "Mutation, Gene Flow and the Balance That Holds Variation"
description: "Mutation makes new alleles slowly and migration moves existing alleles between populations, while selection and drift can raise or lower their frequencies. What is observed is the balance of these inp"
canonical: https://lightmysky.com/learn/science/mutation-gene-flow-and-the-balance-that-holds-variation-mt_A2UgDE0ORv
source: https://lightmysky.com/learn/science/mutation-gene-flow-and-the-balance-that-holds-variation-mt_A2UgDE0ORv.md
retrieved: 2026-11-12
---

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# Mutation, Gene Flow and the Balance That Holds Variation

Mutation makes new alleles slowly and migration moves existing alleles between populations, while selection and drift can raise or lower their frequencies. What is observed is the balance of these inputs and changes rather than any one force alone.

Subject: Science · Area: Genetics & Evolution · Ages 18 to 20
Page: https://lightmysky.com/learn/science/mutation-gene-flow-and-the-balance-that-holds-variation-mt_A2UgDE0ORv

## Ready when they can

- Explains why a harmful recessive allele persists at low frequency instead of disappearing.
- Predicts what steady migration does to the difference between two populations.
- Ranks mutation and migration by how fast each changes a frequency.

## Lesson: Forces that hold variation in balance

*(drawing: The Hardy-Weinberg equations give a baseline when the model conditions hold.)*

Under Hardy-Weinberg conditions, allele frequencies and genotype frequencies stay constant. The conditions are no mutation, random mating, no gene flow, a very large population, and no selection.

Mutation is the source of new alleles. It usually changes a frequency slowly because new mutations are rare. Migration moves existing alleles between populations, and its effect depends on the number of migrants compared with the population size.

**Example.** A harmful recessive allele can remain at low frequency when unaffected heterozygotes carry most copies. Under Hardy-Weinberg conditions, affected people are aa, so their frequency is q squared. To find q, take the square root of that affected frequency. Selection removes affected homozygotes, but it does not remove copies hidden in unaffected carriers.

**Example.** When two similarly sized populations exchange migrants every generation, their allele frequencies usually move toward each other. A larger migrant fraction creates a faster change than a smaller one.

Mutation is usually a slow source of new variation. Gene flow can change frequencies faster when the migrant input is large enough compared with the population. The comparison depends on rates and population sizes.

A harmful allele may stay at a similar level when removal by selection is balanced by input from migration or mutation. The observed frequency is the result of the forces acting together.

**Recap.** Mutation adds new alleles, gene flow moves existing ones, and selection or drift can change their frequencies. Hidden recessives and a balance of forces can maintain variation.

*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

- [Genetic Drift and Effective Population Size](https://lightmysky.com/learn/science/genetic-drift-and-effective-population-size-mt_lbB1GbKv_t)

## Opens up

- [Speciation: Isolation, Hybrid Zones and Where the Boundary Blurs](https://lightmysky.com/learn/science/speciation-isolation-hybrid-zones-and-where-the-boundary-blurs-mt_ngZxpFlwo1)
- [The Neutral Theory and Reading a Molecular Clock](https://lightmysky.com/learn/science/the-neutral-theory-and-reading-a-molecular-clock-mt_yIidVpa703)