In inferring historical demography, what pattern would you expect in neutral markers versus adaptive traits in sticklebacks?

• A. Neutral markers show demographic history (bottlenecks, drift); adaptive traits show directional selection and parallel changes across populations.
• B. Neutral markers show adaptation; adaptive traits show drift.
• C. Neutral markers show no pattern; adaptive traits show random changes.
• D. Neutral markers and adaptive traits both show only drift.

Answer: (A)

Neutral genetic markers reflect population history because they aren’t shaped by natural selection. Their allele-frequency changes mainly track drift, bottlenecks, and migration, so the patterns you see in these markers can reveal when populations split, shrink, or come back together. In sticklebacks, adaptive traits tell a different story: they respond to local environmental pressures. Across many freshwater populations, you see similar, directional changes in traits that improve survival in freshwater conditions, a hallmark of parallel evolution driven by natural selection.

Put together, neutral markers map out demographic history—how populations have grown, shrunk, or moved—while adaptive traits reveal the selective forces acting on those populations and the repeated, parallel changes that occur under similar ecological pressures. That combination is why the correct description is that neutral markers show demographic history, and adaptive traits show directional selection with parallel changes across populations.