genetic leakage

Meaning: Genetic leakage describes gene flow across a boundary that normally separates two gene pools, such as between closely related species with incomplete reproductive barriers. A fertile hybrid mates with one parent group (backcrossing), so alleles from the other group enter that gene pool. Migration of individuals between populations of one species has the same effect and is usually just called gene flow. The result is new alleles in the receiving population and smaller allele-frequency differences between the two groups. Strong, continued flow can merge populations, while weak flow may be offset by natural selection.

On the MCAT: A passage describes two populations or species with overlapping ranges, a hybrid zone, or hybrids of reduced fertility. You may be asked whether allele frequencies will converge, whether speciation will be slowed, or which prezygotic or postzygotic barrier is leaky. Predict that more leakage means less divergence, and that stronger isolation (geographic barrier, sterile hybrids) means less leakage and more divergence.

Common confusion: Leakage is not mutation or genetic drift, because it moves existing alleles between gene pools rather than creating new ones or changing frequencies by chance. Forming hybrids is also not enough. A sterile hybrid, like a mule, usually passes no alleles on, so little leakage occurs. Alleles must reach the next generation through fertile offspring.