In AP Biology, gene flow is the movement of alleles from one population to another (usually through migration), which changes allele frequencies, violates Hardy-Weinberg equilibrium, and keeps populations from diverging into separate species.
Gene flow is the transfer of genetic variation from one population to another. Picture two groups of the same species living apart. When individuals (or their gametes, like pollen) move between them and reproduce, they carry alleles along for the ride. That's gene flow, and it mixes the gene pools together.
If enough gene flow happens, the two populations end up with basically the same allele frequencies, which means they're effectively one population. The opposite is also true: cut off gene flow, and the populations are free to drift apart genetically. This is why gene flow is a big deal in Topic 7.4 (Population Genetics) and Topic 7.5 (Hardy-Weinberg Equilibrium). It's one of the forces that pushes allele frequencies around.
Gene flow lives in Unit 7 (Natural Selection) and connects directly to two learning objectives. Under AP Bio 7.5.A, one of the five Hardy-Weinberg conditions is "no migration," and gene flow IS migration, so it's literally one of the things that breaks H-W equilibrium and signals that evolution can occur. Under AP Bio 7.4.B, the essential knowledge spells it out: gene flow between two populations prevents them from diverging into separate species (EK 7.4.B.1.iii). That second point is the key idea. When you study speciation, gene flow is the thing working AGAINST it. The bigger theme here is that allele frequencies aren't fixed. Anything that adds, removes, or shuffles alleles can drive evolution, and gene flow is the "shuffling between groups" mechanism.
Keep studying AP Biology Unit 7
Hardy-Weinberg Equilibrium (Unit 7)
Hardy-Weinberg only holds if there's no migration. Since gene flow is migration carrying alleles, it's one of the five conditions that, when broken, tells you a population is evolving.
Genetic Drift (Unit 7)
These two are opposites in a tug-of-war. Drift makes small populations diverge by random chance, while gene flow pulls populations back together by mixing their alleles. More gene flow means less divergence.
Founder Effect (Unit 7)
The founder effect happens when a few individuals start a new, isolated population, which only matters because they're cut off from gene flow with the original group. No gene flow back home is exactly why their allele frequencies can drift away.
Speciation (Unit 7)
Speciation needs populations to stay genetically separate long enough to become incompatible. Gene flow stops that from happening, so reproductive isolation is really just the shutting off of gene flow.
Expect gene flow as one option among the evolutionary mechanisms (drift, mutation, selection, gene flow) on multiple-choice questions, where you pick which one explains a scenario. A classic setup: two previously isolated populations come back into contact, and you have to recognize that the mixing of their alleles is gene flow and that it violates Hardy-Weinberg. You'll also see it in geographic-variation questions, where an allele's frequency differs between separated groups precisely because gene flow between them is limited. On FRQs, gene flow shows up in speciation prompts. Released questions about isolated brook trout populations fragmented after glaciation (2022) and marine species split when the Caribbean and Pacific stopped connecting (2025) are both really asking about what happens when gene flow gets cut off. Your job is to argue that isolation stops gene flow, which lets the populations diverge.
Both change allele frequencies, but in opposite directions for divergence. Genetic drift is random change WITHIN one population (strongest when the population is small) and tends to make populations grow apart. Gene flow is the movement of alleles BETWEEN populations and tends to make them grow more similar. Quick tell: if alleles are moving from one group to another, it's gene flow; if it's just chance reshuffling inside one isolated group, it's drift.
Gene flow is the transfer of alleles from one population to another, usually through migration of individuals or their gametes.
Gene flow violates the "no migration" condition of Hardy-Weinberg equilibrium, so its presence means a population can evolve.
Gene flow makes populations more genetically similar, while genetic drift tends to make small populations diverge.
Gene flow between two populations prevents them from diverging into separate species (EK 7.4.B.1.iii).
Cutting off gene flow through geographic isolation is a key step that allows speciation to happen, which is why speciation FRQs often hinge on isolation.
Gene flow is the transfer of genetic variation from one population to another, typically when individuals or gametes migrate between groups and reproduce. It changes allele frequencies and is one of the conditions (no migration) that must be absent for Hardy-Weinberg equilibrium to hold.
No, it does the opposite. Gene flow keeps populations genetically connected and prevents them from diverging into separate species (EK 7.4.B.1.iii). Speciation needs gene flow to STOP, usually through geographic or reproductive isolation.
Gene flow is the movement of alleles BETWEEN populations, which makes them more alike. Genetic drift is random change in allele frequencies WITHIN one population, strongest in small populations, and it tends to make populations diverge. They pull in opposite directions.
One of the five Hardy-Weinberg conditions (AP Bio 7.5.A) is "no migration." Gene flow IS migration bringing new alleles in, so when it happens, allele frequencies shift and the population is no longer in equilibrium, meaning evolution can occur.
It appears on multiple-choice as one of the four evolutionary mechanisms you must distinguish, and in FRQs about speciation and isolated populations like the 2022 brook trout or 2025 Caribbean-Pacific marine species questions. You'll usually argue that isolation cuts off gene flow, allowing populations to diverge.