Gene flow between populations can have which of the following combined effects on genetic variation?

• A. Increases variation within populations and increases differences between populations
• B. Decreases variation within populations and decreases differences between populations
• C. Increases variation within populations and decreases differences between populations
• D. Decreases variation within populations and increases differences between populations

Answer: (C)

Gene flow moves alleles between populations through migration and interbreeding. This tends to make populations more similar to each other, reducing genetic differences between them. At the same time, when migrants introduce new alleles into a population, the variety of alleles within that population increases, raising its genetic diversity. So the combined effect is that genetic variation within a population goes up, while differences between populations go down. This is why the best description is the one that states increased variation within populations and decreased differences between populations.