It is the introduction of genetic material in and out of a population by emigration or immigration.
This variable flow of individuals in and out of the population not only changes the gene structure of the population, but can also introduce new genetic variation to populations in different locations and habitats. .
Measure of gene flow:
Gene flow can be measured by comparing allele frequencies of populations. If the two populations has increased variation in their gene pool, it shows lesser gene flow because gene flow increases homogeneity among different populations.
One of the direct method is to observe the dispersal of individuals and record their reproductive success. But this method of estimating level of gene flow is only suitable for some species.
Gene flow can also be measured by using the effective population size and the net migration rate per generation.