If a freshwater population shows strong plate reduction, what study design would best test if this trait is under directional selection?

• A. Compare allele frequencies at EDA and nearby regions across populations and use selection statistics or sweep analyses.
• B. Measure allele frequencies in a single population.
• C. Crossmarine and freshwater and look at offspring.
• D. Look only at morphological phenotype without genetics.

Answer: (A)

Directional selection on a trait is most convincingly shown when you compare genetic variation across populations that face different selective pressures and look for signals of selection around the gene that controls the trait. In freshwater sticklebacks, plate number is linked to the EDA region. If this trait is being shaped by directional selection in freshwater environments, the allele that reduces plates should become more common in freshwater populations compared with marine populations. You’d also expect to see genomic signs of a selective sweep near EDA, such as reduced genetic diversity and patterns of extended linkage disequilibrium around that region.

That’s why measuring allele frequencies across populations and applying selection statistics or sweep analyses is the best approach here: it combines changes in frequency with direct evidence of selection in the genome, which is what confirms that the trait is being acted on directionally in the wild.

Looking at just one population won’t tell you whether selection has shifted allele frequencies or if changes could be due to drift. Crossing marine and freshwater individuals and examining offspring helps understand inheritance but doesn’t directly show whether natural selection is currently acting in natural populations. Focusing only on morphology without genetics misses the genetic basis and the selection signals that would indicate directional selection.