GF-S Balance- Background
When populations of the same species are separated geographically, genetic drift and differing selection pressures can act to differentiate the populations genetically. Migration has the opposite effect: it produces genetic similarity among the populations. For this reason, gene flow is said to be the great homogenizing agent.
If interbreeding between the native and migrant individuals is common, and the hybrid offspring do not have decreased fitness compared to the native individuals, the populations can converge genetically. However, different selection pressures among the populations can lead to the maintenance of genetic differentiation. In this case, hybrid individuals have decreased fitness relative to the native individuals.