How is gene flow different from genetic drift?

Focus on what causes the allele-frequency change. Gene flow is about movement between populations: alleles enter or leave when individuals or gametes move. Genetic drift is about chance: allele frequencies change because of random outcomes. [S1]

A quick memory aid: flow means movement between populations; drift means chance changing frequencies. Neither term simply means “a population changed.” To classify a change, look for the process behind it.

Imagine two fictional groups of beetles, Meadow A and Meadow B. Suppose they differ in how common a particular allele is.

1. Gene flow: A beetle carrying an allele from Meadow A moves to Meadow B and reproduces there. The allele has moved between populations via an organism, so this illustrates gene flow. [S1]
2. Genetic drift: Meadow B is isolated. By chance, the beetles that reproduce in one season carry the allele more often than the group did before. No beetle or gamete came from Meadow A; the frequency shift is attributed to chance, so this illustrates drift. [S1] [S2]

These are fictional examples, not reports about real beetles. In a real population, a change could involve multiple forces at once. [S2] The examples show how to distinguish the processes; they do not establish what caused any change in nature.

Ask first: Did an allele move into or out of this population through migration of individuals or gametes? If yes, gene flow is involved. If not, ask: Is the frequency change attributed to chance? If yes, that points to genetic drift. [S1] [S2]

Here is the comparison side by side:

• Gene flow — main idea: Alleles move between populations. Diagnostic question: “Did individuals or gametes carry alleles into or out of this population?”
• Genetic drift — main idea: Chance changes allele frequencies. Diagnostic question: “Is the frequency change attributed to chance rather than to alleles moving between populations?”

Use these questions to identify the process described, not to assume it is the only force operating. Drift and natural selection, for example, may occur simultaneously. [S2]

Use this copyable explanation, filling in only the parts your example supports:

“[Process] is the best description because [what happened]. The key clue is [alleles moved between populations / chance changed allele frequencies].”

For the fictional Meadow B drift example: “Genetic drift is the best description because, by chance, the beetles reproducing in one season carried the allele more often. The key clue is that no beetles or gametes arrived from another population.”

Keep the explanation precise: if an example only says that frequencies changed, it may not give enough information to identify the cause. State what the example shows, and avoid claiming that one process explains everything unless the information supports that conclusion.