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Gene flow is the transfer of genes among populations, resulting from either the dispersal of gametes or from the migration of individuals.

This phenomenon plays a significant evolutionary role in all organisms, and depending on the rates of gene flow, the mechanism either induces genetic diversity or generates genetic homogeneity among populations. When the rate of gene flow is low, the introduction of new alleles into a population generates genetic diversity. On the other hand, a high rate of gene flow reduces the genetic variation between the populations, increasing homogeneity.

Mobility is a critical factor that affects the rate of gene flow between populations. More mobile organisms have higher chances of contributing to gene flow through migration. Sedentary organisms like plants can also facilitate gene flow through their pollen and seeds that get carried to distances by animals or wind. However, migration itself does not always guarantee gene flow – it needs to be accompanied by the exchange of genes or genetic material between the resident and the migrant individuals.

Physical and reproductive barriers can impede gene flow. For example, allopatric speciation occurs when geographic barriers isolate populations of the same species, restricting gene flow. When these barriers are removed, if the populations have diverged enough while separated that they can no longer produce viable offspring with a member of the other population, they may be classed as separate species.

Human-assisted gene flow can help in genetic rescue. A high rate of inbreeding in small populations lowers diversity, reduces fitness, and increases extinction risk. The introduction of unrelated individuals or organisms through human interference can reduce the extent of inbreeding, improve diversity, and thus increase overall fitness.