The evolutionary effects of genetic drift are greatest when:

• A. natural selection is very strong
• B. the population size is small
• C. gene flow is extensive
• D. mutation rates are high

Answer: (B)

Verified step by step guidance

1. Understand the concept of genetic drift: Genetic drift refers to random changes in allele frequencies within a population, which occur due to chance events rather than natural selection. These changes are more pronounced in smaller populations because random events have a larger proportional impact.
2. Analyze the role of population size: In small populations, the effect of random events (e.g., a few individuals failing to reproduce) can significantly alter allele frequencies. In contrast, in large populations, the impact of such random events is diluted.
3. Evaluate the other options: Natural selection is a non-random process that favors certain traits, so it does not amplify the effects of genetic drift. Extensive gene flow (movement of alleles between populations) reduces the impact of genetic drift by introducing genetic variation. High mutation rates introduce new alleles but do not directly amplify the random changes caused by genetic drift.
4. Focus on the relationship between genetic drift and small population size: Small populations are more susceptible to genetic drift because there are fewer individuals to buffer against random changes in allele frequencies. This can lead to the fixation or loss of alleles more quickly than in larger populations.
5. Conclude that the evolutionary effects of genetic drift are greatest when the population size is small, as this is the condition under which random changes in allele frequencies have the most significant impact.