What happens to allele frequencies when migrants introduce new alleles into a population?

• A. Allele frequencies become more different from donor population
• B. Allele frequencies shift toward those of the donor population
• C. Allele frequencies stay completely the same
• D. Allele frequencies become fixed in all populations

Answer: (B)

The main idea here is gene flow. When individuals move into a population, they bring their alleles with them, adding to the existing gene pool. This makes the overall allele frequencies in the recipient population become a blend of the resident frequencies and the donor’s frequencies. In other words, the new frequencies shift toward those of the donor population, because the migrants’ alleles are now part of the population’s gene pool. The bigger the number of migrants, the larger the shift toward the donor frequencies. This also means populations become more similar to each other over time. Frequencies don’t stay the same, and migration alone isn’t enough to fix an allele in all populations.