What is one way that genetic drift can occur?

Genetic drift is a mechanism of evolution that refers to random changes in the frequency of alleles (gene variants) in a population, particularly in small populations. One significant way that genetic drift can occur is through chance events that influence which individuals survive and reproduce. For instance, if a natural disaster randomly eliminates a portion of a population, the alleles carried by the surviving individuals may not represent the original population's allele frequencies. This randomness can lead to an increase or decrease in certain alleles purely by chance, which is a hallmark of genetic drift.

In contrast, natural selection, which is mentioned in the first choice, operates through the differential survival of individuals based on advantageous traits rather than random chance. Environmental changes affecting all individuals do not specifically lead to genetic drift, as they typically influence selection rather than random allele frequency changes. Finally, gene flow refers to the transfer of alleles between populations, which can introduce new alleles but is not a process of genetic drift. Thus, the key factor in genetic drift is the role of chance events in altering allele frequencies, making it a distinct evolutionary mechanism compared to the others listed.