Which evolutionary force can make a rare allele become common purely through chance?

Genetic drift can make a rare allele become common purely through chance.

Drift is the random change of allele frequencies from one generation to the next. It is strongest in small populations, where chance differences in who survives and reproduces can substantially alter the gene pool. An allele can become fixed, meaning it reaches a frequency of 100 percent, or disappear entirely.

Two important examples are the founder effect and the population bottleneck. A founder effect occurs when a small group establishes a new population. A bottleneck occurs when a population is sharply reduced and the survivors carry only part of the original variation.

Drift differs from natural selection, which changes frequencies because inherited traits affect reproductive success. In real populations, drift and selection can operate simultaneously, while gene flow changes frequencies by moving alleles between populations.