In population genetics, what does Fst measure?

• A. Genetic differentiation among populations.
• B. Mutation rate within a population.
• C. Initial allele frequency.
• D. Mean fitness of individuals.

Answer: (B)

Fst measures how much genetic variation is partitioned among populations versus within them, i.e., genetic differentiation among populations. It quantifies how different populations are at the genetic level, not how often mutations occur, what the starting allele frequencies were, or how fit individuals are. In simple terms, a low Fst means populations are genetically similar and gene flow is high; a high Fst means they are genetically distinct and gene flow is limited. It's often understood through heterozygosity: Fst compares the total expected genetic diversity if all populations were one group to the average diversity found within subpopulations. This makes Fst a tool for understanding population structure and the balance between drift and migration.