Gene flow is the movement of genes within a population. This movement of genes is the product of mating and gene exchange within populations. A genetically isolated population is known as a deme, gene flow can occur within demes or between demes when populations meet. On a limited scale gene flow can occur across species barriers. This occurrence is one of the arguments against releasing genetically modified organisms into the environment, particularly those that have been modified to include antibiotic resistance for screening purposes. Because of gene flow, these genes can spread through the wild population of that species and antibiotic resistance can be spread between species.

Gene flow is a method by which advantageous genes or alleles can be spread throughout a population, making them more common and more likely to remain within the population. If genes or alleles do not spread within a population, they are very quickly lost and, in evolutionary terms, they are a dead end. Gene flow creates new combinations of genes in individuals that can then be tested against the environment. If the new combination is more able to survive in the given environment, that gene and combination of genes is more likely to spread within the population.