Gene flow is the transfer of genetic material from one population into another. In sexually reproducing organisms, it occurs when individuals or reproductive material move between populations and contribute to subsequent generations. It can also occur between distinct species through interbreeding. A central process in population genetics, gene flow redistributes alleles—alternative forms of genetic sequences—and helps determine how populations remain connected or diverge during evolution. Unlike mutation, which generates new variants, gene flow primarily moves existing variants between populations. (pressbooks.umn.edu)
Mechanisms and biological scope
Among animals, gene flow commonly results from dispersal followed by reproduction in a different breeding population. Physical movement alone is insufficient: an animal that visits another population but leaves no descendants contributes no gene flow through reproduction. The genetic contribution of immigrants therefore depends on survival, mating success, and offspring reproduction, rather than simply on the number of individuals arriving. (nature.com)
In plants, genetic material can move through pollen or seeds. Pollination between populations transfers paternal genetic material, whereas a dispersed seed carries an offspring’s genetic material. Consequently, pollen movement and seed movement can connect populations in different ways and over different distances. Gene flow may be strongly asymmetric when one population supplies substantially more reproductive material than another. (pressbooks.umn.edu)
In bacteria, horizontal gene transfer provides another route for genetic exchange. Rather than depending on mating and offspring production, it transfers DNA between organisms or lineages outside ordinary parent-to-offspring inheritance. Research on microbial communities shows that ecological similarity can strongly influence which organisms exchange genes, so physical proximity is not the only determinant of microbial gene flow. (nature.com)
Changes in allele frequencies
A simple migration model illustrates the effect of gene flow. Let (p) be an allele’s frequency in a recipient population, (p_m) its frequency among immigrants, and (m) the fraction of the post-migration gene pool supplied by immigrants. With no additional evolutionary forces acting during this step,
[ p'=(1-m)p+mp_m, ]
so
[ \Delta p=m(p_m-p). ]
Migration therefore shifts the recipient frequency toward the immigrant frequency. If both frequencies are identical, genetic exchange occurs without changing the frequency of that allele. These equations describe a simplified mixing process, not every aspect of successful reproduction or selection after immigration. (people.wou.edu)
For illustration, if (p=0.20), (p_m=0.80), and (m=0.10), the resulting frequency is (0.26). The model distinguishes the proportion of immigrant genetic contributions from the absolute number of migrants; these quantities need not have the same biological effect in populations of different sizes.
Diversity, drift, and adaptation
Gene flow can introduce alleles absent from a recipient population and increase its genetic diversity. Repeated exchange generally reduces differentiation among populations, counteracting divergence caused by genetic drift. The balance depends on migration, effective population size, and population history. Similar allele frequencies, however, can reflect either continuing exchange or recent shared ancestry, so similarity alone does not establish ongoing gene flow. (pmc.ncbi.nlm.nih.gov)
Its relationship with natural selection is more complex. Immigrants can introduce variation that facilitates adaptation, but they can also introduce alleles poorly suited to local conditions. Local adaptation persists when selection maintains locally advantageous variants despite immigration. The outcome depends on migration intensity, environmental differences, and genetic linkage among relevant loci. There is therefore no universal migration rate that always promotes or always prevents adaptation. (sciencedirect.com)
Species boundaries and introgression
Gene flow often opposes speciation by maintaining genetic connections between diverging populations. Nevertheless, divergence can proceed while some exchange continues. Reproductive isolation comprises barriers that reduce genetic exchange, including differences in mating or reproductive compatibility. Such barriers need not block all parts of the genome equally: some regions can cross population boundaries more readily than others. (onlinelibrary.wiley.com)
Hybridization is interbreeding between genetically differentiated groups. When hybrid descendants reproduce with a parental population, genetic material can become incorporated into that population through introgression. Hybridization therefore does not necessarily imply extensive or lasting gene flow. Studies of fruit flies and house mice demonstrate that exchange can vary markedly across genomic regions associated with reproductive barriers. Hybridization can also contribute to the formation of new evolutionary lineages rather than simply merging existing ones. (nature.com)
Measurement and inference
Researchers investigate gene flow using movement observations, pedigrees, parentage assignment, and genetic comparisons. Observed dispersal measures movement, whereas parentage data can establish reproductive contributions. DNA sequencing supplies information for demographic models that estimate population separation, migration, or episodes of admixture. Different methods capture different timescales, making recent reproductive exchange distinct from historical connectivity. (sciencedirect.com)
The fixation index, commonly written (F_{ST}), describes genetic differentiation among populations. It is not a direct migration meter. Converting it into a migration estimate requires restrictive assumptions about population size, migration structure, neutrality, and equilibrium between migration and drift. Selection, historical separation, unequal population sizes, and marker properties can violate those assumptions; consequently, the same differentiation pattern may support multiple demographic histories. (nature.com)
Conservation applications
In conservation biology, gene flow is relevant to isolated populations experiencing loss of variation and inbreeding. A documented example is the Florida panther restoration program, which introduced eight female pumas from Texas in 1995. A 2010 study reported increased heterozygosity, improved survival measures, and reduced indicators associated with inbreeding after genetic mixing. The study also identified continuing habitat and population-management challenges, distinguishing genetic restoration from the removal of all threats to population persistence. (pubmed.ncbi.nlm.nih.gov)