Final answer:
Migration is the key factor that causes gene flow, as it involves the transfer of alleles between populations, affecting their allele frequencies.
Step-by-step explanation:
The movement of alleles from one population to another, which causes gene flow, is primarily due to migration. Migration allows for the flow of alleles in and out of a population through the movement of individuals or gametes, which can result in significant changes to allele frequencies both in the population they leave and the one they enter. Plants and animals both contribute to gene flow; plants may send seeds or pollen to new locations, and animals can migrate or be dispersed across different geographical regions.