Gene Flow and Migration

Gene flow is the transfer of alleles from one population to another through the movement and reproduction of migrating individuals. By introducing alleles from outside, gene flow tends to make connected populations genetically more similar to one another and counteracts the differentiation produced by drift and local selection.

Definition

Gene flow (or migration) is the movement of alleles between populations that occurs when individuals or their gametes move from one population to another and reproduce, changing the recipient population's allele frequencies toward those of the source.

Scope

The entry covers how migration moves alleles between populations, the homogenising effect of gene flow on allele frequencies, its role as a counterweight to drift and selection, and the way population differentiation is summarised. It is a conceptual and methodological topic within population genetics, not clinical guidance.

Core questions

• How does the movement of individuals change allele frequencies in connected populations?
• How does gene flow counteract differentiation caused by drift and local selection?
• How is the degree of differentiation between populations measured?

Key concepts

• Migration of individuals
• Allele transfer between populations
• Homogenisation of allele frequencies
• Migration-drift balance
• Isolation by distance
• Population differentiation (FST)
• Admixture

Key theories

Migration-drift balance and isolation by distance

Sewall Wright described how gene flow homogenises populations while drift differentiates them, and showed that limited dispersal produces an isolation-by-distance pattern in which nearby populations are genetically more similar than distant ones.

Mechanisms

When migrants move into a recipient population and reproduce, they contribute their alleles to its gene pool, shifting its allele frequencies toward those of the source population. Even modest, recurring migration is a strong homogenising force: it opposes the differentiation that drift and locally varying selection would otherwise generate, so the genetic structure observed among populations reflects a balance between gene flow and these diversifying forces. The resulting differentiation is commonly summarised by the fixation index FST, which quantifies how allele-frequency variance is partitioned among populations relative to the total.

Clinical relevance

Historical migration and admixture have shaped how disease alleles are distributed across human populations and underlie the admixed ancestry of many individuals, which is relevant to choosing appropriate population references when interpreting genetic variation. It describes how variation is distributed and is not a basis for individual diagnostic or treatment decisions.

Epidemiology

Patterns of human migration and admixture have spread some alleles across regions and produced populations of mixed ancestry, so allele frequencies often vary along geographic gradients rather than in sharply bounded groups.

History

Sewall Wright introduced migration as one of the principal evolutionary forces in the 1930s and, in his 1943 isolation-by-distance work, formalised how limited dispersal structures populations geographically. The fixation index he developed became the standard way to measure differentiation, and modern reviews have refined its definition, estimation, and interpretation.

Debates

How should the fixation index FST be defined and interpreted?

FST has several related definitions and estimators, and its interpretation depends on assumptions about population history; commentators stress that a single index can conflate different demographic processes.

Key figures

• Sewall Wright
• Brian Charlesworth
• Bruce Weir
• Kent Holsinger

Seminal works

• wright-1943
• holsinger-weir-2009

Frequently asked questions

What effect does gene flow have on differences between populations?

It tends to reduce them: by transferring alleles from one population to another, gene flow makes connected populations more genetically similar and counteracts the divergence caused by drift and local selection.

What does the fixation index FST measure?

It summarises how genetic variation is partitioned among populations relative to the total, indexing the degree of differentiation between them; low values indicate populations that are genetically similar and high values indicate strong differentiation.