Final answer:
Gene flow refers to the transfer of genetic material, specifically alleles, between populations through the migration of individuals or gametes. It can significantly impact genetic diversity and allele frequencies within populations.
Step-by-step explanation:
During gene flow, what flows is the flow of alleles in and out of a population. This happens due to the migration of individuals or gametes, which leads to a mixing of genetic material between populations. For example, if an individual from one population moves to another and reproduces there, they can introduce new alleles to the gene pool of the population they enter. This process is critical for maintaining genetic diversity, as it can prevent populations from becoming too genetically similar and therefore more susceptible to certain diseases or environmental changes. Gene flow can lead to changes in allele frequencies, which can have significant effects on the populations involved. For instance, many plants send their pollen far and wide, by wind or animals, and wild animals like lions experience immigration and emigration that affects their population's genetic structure by introducing new genetic variation into different locations and habitats.