Gene flow links populations by moving alleles with migrating individuals or gametes. In AP Biology, it is a core population-level mechanism that can rapidly shift allele frequencies and reshape genetic variation across locations.

Core idea: migration produces gene flow

Migration is the movement of individuals (or their gametes) between populations.

This diagram summarizes several standard population-structure models used to conceptualize gene flow: a one-way continent–island model, a fully connected “island” model, and 1D/2D stepping-stone models. The arrows represent migration pathways among demes, illustrating how connectivity patterns shape opportunities for alleles to move between populations. Source

When migrants reproduce, they create gene flow, meaning alleles are transferred into or out of a population’s gene pool.

Gene flow can occur in two directions:

• Immigration (into a population): can add alleles or increase the frequency of alleles already present.

• Emigration (out of a population): can remove alleles or lower their frequencies.

How gene flow changes allele frequencies

Gene flow is evolution at the population level because it changes allele frequencies (the proportion of each allele in the population). Its impact depends on:

• the migration rate (how many migrants enter/leave per generation)

• the allele frequencies in migrants compared with the recipient population

• whether migrants survive and reproduce (gene flow requires genetic contribution, not just movement)

A useful way to model allele-frequency change from immigration is:

This equation highlights that allele frequencies after migration become a weighted average of resident and immigrant allele frequencies.

The plot shows allele frequency over time in multiple subpopulations exchanging migrants each generation, with trajectories converging toward a common equilibrium. This illustrates that repeated migration pulls each population’s allele frequency toward the shared migrant pool, reducing differences among populations over successive generations. Source

Effects of gene flow on genetic variation

Within a population

Gene flow can increase genetic variation within a population when immigrants bring in alleles not previously present, or reintroduce alleles lost locally. This can:

• increase the number of alleles in the gene pool

• raise heterozygosity if new alleles enter at appreciable frequency

Gene flow can also decrease within-population variation in some cases, such as when:

• a small population receives many immigrants with a narrow genetic sample

• a previously diverse local gene pool is replaced by alleles common in the source population

Between populations

Because populations exchange alleles, gene flow tends to make populations more genetically similar over time when migration is ongoing. In general:

• high gene flow reduces genetic differences between populations

• low gene flow allows populations to diverge genetically because alleles are not being shared

Biological factors that influence migration and gene flow

Gene flow is not guaranteed just because organisms move. Key determinants include:

• Dispersal ability: flying seeds, planktonic larvae, or highly mobile animals often show more gene flow than sedentary species.

• Geographic barriers: mountains, rivers, habitat fragmentation, and distance can reduce movement and mating between populations.

• Timing and behaviour: migrants must arrive during the breeding period and be able to court/mate successfully.

• Population size and density: migrants entering a small population can change allele frequencies more strongly than the same number entering a large population.

• Mode of gene movement: gene flow can occur via whole individuals (animals relocating) or gametes (pollen carried by wind or pollinators).

Why gene flow matters in interpreting population patterns

When studying differences among populations, gene flow provides a mechanistic explanation for why allele frequencies may shift without requiring local birth of new alleles. In syllabus terms, migration can cause gene flow, which:

• adds alleles through immigration or removes alleles through emigration

• alters genetic variation within populations (by introducing/removing alleles)

• alters genetic variation between populations (by homogenising allele frequencies when migration is substantial)

Because gene flow can change allele frequencies quickly, it is especially important for understanding rapid genetic changes following:

This aerial map highlights protected habitat patches and the corridor-like open spaces that connect them, illustrating how landscape structure can facilitate organism movement. Increased connectivity can raise migration rates between populations, strengthening gene flow and counteracting genetic divergence caused by isolation. Source

• habitat corridors opening/closing

• translocations and reintroductions

• colonisation of nearby habitats where individuals regularly move back and forth