gene flow

Meaning: Gene flow is the movement of alleles from one population into another, usually when individuals migrate and then reproduce, or when gametes such as pollen are carried between populations. It can add new alleles and shift allele frequencies in the receiving population toward those of the source. Over time it tends to reduce genetic differences between populations. When gene flow is low, genetic drift, mutation and different selection pressures can make populations diverge, which can eventually lead to speciation.

On the MCAT: A passage may describe mainland and island populations, or two fields exchanging pollen, and ask how allele frequencies change. Expect to predict convergence when migration is high and divergence when a barrier (river, mountain range, distance) cuts it off. In Hardy-Weinberg questions, gene flow is one of the conditions that violates equilibrium, along with mutation, nonrandom mating, genetic drift and natural selection. The new frequency in the receiving population falls between its old value and the source population's value, and it moves closer to the source as more migrants contribute.

Common confusion: Moving to a new population is not enough. Migrants change the gene pool only if they reproduce there and pass on alleles. Gene flow is also not genetic drift. Drift is random change in allele frequency, strongest in small populations, while gene flow brings in alleles from another population. Gene flow usually refers to exchange within a species, though hybridization between closely related species can also transfer alleles.