The bigger the population size the smaller the effect of drift on allele frequencies. Small populations are very prone to drift and alleles can go to fixation quickly. Population Bottlenecks can cause the same effect, i.e. a bottleneck is a reduction in genetic variation. Migration is another term used for this, but only covers some aspects of gene flow. People need not permanently move, only their genes.
Visitors can leave their genes behind in the form of children.
Gene Flow Gene flow between two populations, over time, will make them more similar in allele frequencies. Gene flow almost always decreases with increased geographic distance. Gene Flow Gene Flow between different Yanomama villages. Evolutionary Forces Mutations tend to be rare on a generational time scale. Evolutionary Forces Non-random mating, especially inbreeding, can lead to a change in genotype and selection can act to change allele frequencies.