What phenomenon describes a change in allele frequency due to random chance events?

The phenomenon that describes a change in allele frequency due to random chance events is genetic drift. This process occurs in small populations where random events can significantly influence which alleles are passed down to the next generation, often leading to a decrease in genetic variation over time.

For example, if a natural disaster randomly eliminates a portion of a population, the alleles of the surviving individuals may not represent the original population's genetic diversity, resulting in changes to the allele frequency purely by chance rather than by environmental pressures or adaptation.

In contrast, gene flow involves the movement of alleles between populations, often through migration, and is not solely based on random chance. Natural selection is a non-random process that preferentially favors the survival and reproduction of individuals with advantageous traits. Genetic variation, while an important concept, refers to the diversity of alleles within a population and does not describe a process related to chance events affecting allele frequencies.