How can selection be distinguished from drift using allele frequency data in sticklebacks?

• A. Look for greater divergence in neutral markers than trait-associated alleles
• B. Only examine the oldest allele frequencies
• C. Look for greater divergence in trait-associated allele frequencies than neutral markers; consistent allele frequency changes across independent populations imply selection.
• D. Compare mean trait values without genetic data

Answer: (C)

Distinguishing selection from drift using allele frequencies relies on comparing how much divergence occurs at trait-associated loci versus neutral markers. If selection is acting on a trait, populations in different environments will show larger differences in the frequencies of alleles that affect that trait than in neutral markers, because selection pushes those alleles in directions that improve fitness in each habitat. You’ll also often see consistent, parallel shifts in the same trait-associated alleles across independent populations facing similar selective pressures, something drift would make unlikely. In sticklebacks, repeated freshwater versus marine contrasts lead to predictable changes in specific trait genes, while neutral markers don’t show the same parallel pattern. Merely looking at the oldest allele frequencies or at mean trait values without genetic data doesn’t reveal the selective versus random-drift dynamics.