Gene flow, also known as migration or allele flow, is the transfer of genetic material from one population to another. It can occur through various events, such as pollen being blown to a new destination or people moving to new cities or countries. Gene flow can introduce new genetic variants to a population where they previously did not exist, making it an important source of genetic variation. The level of gene flow among populations can be estimated by observing the dispersal of individuals and recording their reproductive success. Gene flow can have both positive and negative consequences depending on the population and the environment in which they reside.

Some key points about gene flow are:

• Gene flow can constrain speciation and prevent range expansion by combining the gene pools of different groups, thus preventing the development of differences in genetic variation that would have led to differentiation and adaptation.
• Dispersal resulting in gene flow may also result in the addition of novel genetic variants under positive selection to the gene pool of a species or population, a process known as adaptive introgression.
• Although gene flow does not change allele frequencies for a species as a whole, it can alter allele frequencies in local populations.
• Gene flow can greatly enhance the fitness of a population, but it can also have negative consequences depending on the population and the environment in which they reside.
• Gene flow is important in shaping the genetic variation in modern human populations. For example, ancient humans and Neanderthals interbred, and this gene flow introduced new genetic variation to the human population.

Overall, gene flow is an important mechanism that contributes to genetic variation within and among populations.