Final answer:
Gene flow occurs when individuals move into or out of a population through migration, resulting in the exchange of genetic material between populations.
Step-by-step explanation:
Gene flow occurs when individuals move into or out of a population. This movement can happen through migration, where members of one population migrate to another population. Migration allows the exchange of genetic material between populations and can lead to changes in allele frequencies in both populations.
For example, let's consider a population of birds that live on two separate islands. If some birds from one island migrate to the other island and mate with the birds there, they bring new genetic material into the population. This can introduce new genetic variation to the population and change the gene structure of the population.
In summary, for gene flow to occur, members of one population must migrate to another population, resulting in the exchange of genetic material between the populations.