Final answer:
Gene flow is the movement of genes between populations, causing allele frequency changes, primarily through migration and mating between individuals of adjacent populations.
Step-by-step explanation:
Gene flow is a crucial evolutionary force that involves the movement of genes from one population to another, leading to changes in allele frequencies. This movement can happen through various means including the migration of individuals or the transfer of gametes, like pollen or seeds, between populations. Besides migration, gene flow can also occur due to mating between individuals of adjacent populations, which can introduce new genetic variations. For instance, animals like lions can experience immigration and emigration, where developing males leave their pride to join another with genetically unrelated females. Likewise, plants can send their pollen or seeds over large distances, either by the wind or carried by animals, which can integrate new alleles into different populations.