Answer the following question given this paragraph: 

11.2 Mechanisms of Evolution There are four factors that can change the allele frequencies of a population. Natural selection works by selecting for alleles that confer beneficial traits or behaviors, while selecting against those for deleterious qualities. Mutations introduce new alleles into a population. Genetic drift stems from the chance occurrence that some individuals have more offspring than others and results in changes in allele frequencies that are random in direction. When individuals leave or join the population, allele frequencies can change as a result of gene flow.


Q: Out of the four ways that allele frequencies of a population can be changed, which way is caused by individuals leaving or joining the population?


A:
gene flow