If a population receives a large influx of migrants from genetically diverse sources, which outcome is most likely?

• A. Heterozygosity decreases
• B. Genetic variation within the population remains unchanged
• C. Differences between populations increase substantially
• D. Genetic variation within the population increases and differences between populations decrease

Answer: (D)

Gene flow from migrants introduces new alleles into the population, boosting the amount of genetic variation inside the population. When individuals from diverse sources mix, there are more different alleles available and more opportunities for heterozygotes, so genetic variation within the population increases. At the same time, those migrants carry alleles into other populations as well, making allele frequencies more similar across populations. That reduces differences between populations. So you get more variation within the population and less differentiation between populations. The idea that heterozygosity would drop, or that variation within would stay the same, or that differences between populations would rise, doesn’t fit the effect of substantial gene flow.