Gene Flow and Migration

A highway cutting through a forest splits one population into two. If animals can no longer cross, no alleles move between the two sides anymore, and each side is free to drift or adapt on its own path. Before the highway existed, even a little bit of movement across the whole range kept the two sides genetically tied together. That's the core idea: gene flow is alleles moving between gene pools because individuals migrate and then reproduce in their new home, adding their alleles to that population's pool while removing them from the one they left.

The surprising part is how little movement it takes. Even one migrant per generation, on average, is often enough to prevent two populations from drifting apart genetically, because that single migrant's alleles get reshuffled into the new population every generation, continually resupplying variants that genetic drift or local selection might otherwise eliminate. Trace one migrating individual: it carries alleles out of population A's gene pool and, once it breeds in population B, those same alleles are now counted in population B's frequencies and no longer contribute to A's. Repeated over many migrants and generations, this traffic pulls allele frequencies in the two populations toward each other.

Gene flow doesn't aim at adaptation; it just mixes alleles. An allele that was favored in population A might be useless or even harmful in population B's environment, so an incoming migrant can just as easily drag a population away from its local optimum as toward one. This same mixing is what makes gene flow the opposite force from speciation: species formation needs populations to become genetically distinct and reproductively isolated, and gene flow homogenizes allele frequencies, erasing the divergence that isolation would otherwise build up.