Evolution does not happen only because organisms develop useful traits. It happens when the genetic composition of a population changes across generations. Two important forces behind those changes are gene flow and genetic drift.
Both can alter the frequencies of genetic variants in a population, but they do so in fundamentally different ways. Gene flow moves genetic variation between populations through the movement and reproduction of individuals or their reproductive cells. Genetic drift changes genetic frequencies through random sampling, with especially strong effects in small populations.
Understanding the difference matters because the two processes can push populations in opposite directions. Gene flow tends to make populations more genetically similar, while genetic drift often makes separate populations more genetically different. Neither process requires a trait to be beneficial, and neither is driven by an organism’s needs.
What is gene flow?
Gene flow is the movement of genetic variants from one population to another when individuals, or their reproductive cells, move between populations and contribute genes to the next generation.
For example, imagine two populations of the same species living in neighboring areas. If individuals from one population migrate into the other and successfully reproduce, genetic variants carried by those migrants can become part of the recipient population’s gene pool.
Gene flow can occur through migration, but migration alone is not enough. An individual that moves into a new population does not necessarily change its genetic composition unless that individual or its reproductive cells contribute genetically to future generations.
The process can occur in many ways. Animals may move between populations and mate. Plants can exchange genes when pollen travels between populations. Aquatic organisms may disperse between connected habitats. In each case, the key event is the transfer of genetic material between populations.
How gene flow changes populations
Gene flow can change the frequency of particular alleles—the different forms of a gene—in a population.
Suppose a population contains mostly allele A and relatively little allele B. If individuals carrying allele B enter from another population and reproduce successfully, allele B may become more common in the receiving population.
The effect depends on several factors, including how many migrants arrive, how genetically different they are from the existing population, and how successfully they reproduce.
Gene flow also has an important effect at the level of populations rather than just individual alleles. Continued exchange of genes tends to reduce genetic differences between populations. If two populations repeatedly exchange individuals, their gene pools become more connected, making it harder for them to diverge genetically.
This does not mean gene flow always makes populations genetically identical. Natural populations may remain different when migration is limited, selection favors different traits in different environments, or other evolutionary forces counteract the effects of gene flow.
What is genetic drift?
Genetic drift is a change in allele frequencies caused by random differences in which individuals survive and reproduce.
Every generation is, in effect, a sample of the genetic variation present in the previous generation. That sample is not perfectly representative simply because reproduction and survival involve chance. An allele can become more common or less common because its carriers happen to leave more or fewer offspring—not because the allele itself provides an advantage.
Genetic drift is therefore a random evolutionary process.
Consider a population in which two alleles are present at similar frequencies. If, by chance, individuals carrying one allele produce more surviving offspring in a particular generation, that allele may become more common in the next generation. In another population with the same starting frequencies, chance could produce the opposite result.
Over many generations, drift can cause an allele to become very common or disappear entirely.
Why population size matters so much for genetic drift
Genetic drift is strongest in small populations.
In a large population, random differences among individuals tend to have relatively small effects on overall allele frequencies because many individuals contribute genes to the next generation. In a small population, the genetic contribution of just a few individuals can have a much larger effect.
This is why chance events can have lasting evolutionary consequences in small populations.
Two situations are especially important: population bottlenecks and the founder effect.
Population bottlenecks
A population bottleneck occurs when a population experiences a sharp reduction in size. The surviving individuals carry only a portion of the genetic variation that existed in the larger population.
The reduction itself can therefore change allele frequencies by chance and can eliminate genetic variants from the surviving population.
The crucial point is that the survivors are not necessarily a genetically representative sample of the original population. If certain alleles happen to be absent among the survivors, those alleles may disappear from the population even if they had been common before the bottleneck.
The founder effect
The founder effect occurs when a new population is established by a small number of individuals from a larger population.
Because the founders carry only a subset of the original population’s genetic variation, the new population may have unusual allele frequencies simply because of which individuals happened to establish it.
A founder population can therefore differ genetically from its source population even without natural selection causing the initial difference.
Gene flow and genetic drift can produce opposite patterns
The clearest distinction between the two processes is what they tend to do to populations.
| Feature | Gene flow | Genetic drift |
|---|---|---|
| Basic mechanism | Movement of genetic material between populations | Random change in allele frequencies |
| Requires movement between populations? | Yes | No |
| Role of chance | Not the defining feature | Central |
| Strongest effect | When populations exchange migrants or reproductive cells | In small populations |
| Effect within a population | Can introduce new alleles or change existing allele frequencies | Can increase or decrease allele frequencies randomly |
| Effect on differences between populations | Generally reduces genetic differences | Often increases genetic differences |
| Can eliminate alleles? | It can alter frequencies, but its typical effect is to introduce or redistribute variation | Yes, through random loss or fixation |
The distinction is particularly important when thinking about populations that are geographically separated.
Imagine two populations that begin with similar genetic variation. If they remain isolated, genetic drift can cause their allele frequencies to change independently. Over time, the populations may become increasingly different.
If individuals continue moving and reproducing between them, gene flow connects their gene pools. The incoming genetic material can counter some of the divergence produced by drift.
Gene flow does not necessarily increase genetic diversity
It is tempting to think of gene flow simply as a process that “adds diversity.” Often it does introduce genetic variants that were previously uncommon or absent in a population, but the broader effect depends on where the genes come from.
Gene flow can increase genetic variation within a population when migrants introduce alleles that were not previously present. But gene flow can also reduce differences among populations by spreading the same alleles between them.
In other words, gene flow redistributes genetic variation as well as introducing it. Its effects have to be considered both within populations and between populations.
Genetic drift has a different relationship with variation. Because random sampling can eliminate alleles, drift can reduce genetic variation within a population over time. At the same time, independent drift in separate populations can make those populations increasingly different from one another.
Neither process requires natural selection
Gene flow and genetic drift are sometimes confused with natural selection, but they operate differently.
Natural selection occurs when heritable differences among individuals affect their reproductive success, causing some genetic variants to become more common because they are associated with traits that improve reproductive success in a particular environment.
Genetic drift does not favor variants because they are useful. A beneficial allele can be lost through drift, particularly in a small population, while a harmful allele can increase in frequency by chance.
Gene flow also does not inherently favor useful traits. An allele can enter a population because its carrier migrated there, regardless of whether that allele is advantageous, disadvantageous, or effectively neutral in the new environment.
These processes can operate simultaneously. A population might experience natural selection favoring one allele, genetic drift changing its frequency randomly, and gene flow continually introducing copies of alleles from neighboring populations.
How the forces interact
Real populations are rarely shaped by a single evolutionary mechanism.
Suppose two populations occupy different environments. Natural selection may favor different traits in each environment, causing their allele frequencies to diverge. Genetic drift can reinforce that divergence by randomly changing frequencies, particularly if the populations are small. Gene flow can work in the opposite direction by moving alleles between the populations.
The eventual genetic composition of each population reflects the combined effects of these forces.
The balance can matter especially when gene flow brings alleles into an environment where they are not favored by local selection. In that situation, migration can oppose local adaptation by continually introducing genetic variants from elsewhere. Conversely, gene flow can provide genetic variation that gives a population additional material on which natural selection can act.
Genetic drift can also interact with gene flow. Strong drift in a small population may rapidly change allele frequencies, while even modest gene flow can introduce variants that would otherwise have been lost. The relative strength of these processes depends on population size, migration, reproductive patterns, and the amount of genetic difference among populations.
A simple way to tell them apart
When looking at an evolutionary change, ask what caused the genetic variants to change frequency.
If genetic material moved from one population to another, the process is gene flow.
If allele frequencies changed because chance affected which individuals or genes were represented in the next generation, the process is genetic drift.
The difference can be summarized in one sentence: gene flow connects populations by moving genes between them, while genetic drift changes populations through random sampling of genes from one generation to the next.
Together, these processes help explain why populations can become more alike, become more different, gain or lose genetic variation, and change genetically over time.
