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Gene Flow: Two-Population Dynamics

When populations of the same species are separated geographically, genetic drift and differing selection pressures can act to differentiate the populations genetically. Migration among these populations has the opposite effect: it produces genetic similarity among the populations. For this reason, gene flow is said to be the great homoginizing 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.

Calculating the expected allelic frequency, q, given the proportion of the population that are new migrants, m, is as simple as calculating a weighted average:

where is the frequency of the allele in the native population and is the frequency of the migrants.

Gene Flow: Two-Population Dynamics

Adjust m1, m2, s1, and s2 to the right:

As m is increased, the rate of migration from one population to the other is increased. As s is increased, selection against migrants and hybrids is greatest. If both m values are set to the maximum value (while s is set at 0), both populations quickly become more similar until they are indistinguishable. If, however, selection against the migrants is increased, the populations maintain some degree of genetic differentiation.

Note: Pink individuals are hybrids (heterozygotes), green individuals are homozygous for the allele that originated in the upper population, and red individuals are homozygous for the allele that originated in the lower population. The selection coefficient for migrants in the lower population is s1, the selection coefficient for migrants in the upper population is s2, and the selection coefficients for the hybrids are 1/2 that of the migrants for each population.

Hint: If you are using a "slow" computer, begin with the Time setting at it's minimum and the Speed setting at it's maximum.