Gene flow can counteract the effects of genetic drift in small populations by:

• A. Introducing new alleles and reducing differentiation
• B. Increasing drift
• C. Decreasing gene flow
• D. Immediately causing speciation

Answer: (A)

Gene flow helps maintain genetic variation in populations by introducing new alleles from other populations, which can counteract the random changes of genetic drift in small populations. In small groups, drift can cause allele frequencies to fluctuate and some alleles to become fixed or lost, reducing variation and increasing differences between populations. When individuals or their gametes move between populations, they bring in new genetic material, increasing the overall variation within the population and making different populations more similar to each other. This influx of alleles offsets the random, directionless shifts of drift.

The other options don’t fit: increasing drift would amplify the random changes drift causes, not counteract them. Decreasing gene flow would reduce the mixing that brings in new alleles, allowing drift to have a greater effect. Immediate speciation isn’t a direct outcome of gene flow; in fact, gene flow tends to homogenize populations and slow divergence unless other forces like strong selection or geographic isolation act separately.