When inferring historical demography, how do neutral markers compare to adaptive traits in sticklebacks?

• A. Neutral markers show no pattern.
• B. Adaptive traits show directional selection and parallel changes.
• C. Neutral markers show demographic history.
• D. Adaptive traits show directional selection and parallel changes; neutral markers reflect demographic history.

Answer: (D)

Neutral genetic markers are largely shaped by genetic drift, population size changes, splits, and migration, so they carry information about historical demography. Adaptive traits, in contrast, respond to natural selection; in sticklebacks, repeated freshwater colonizations produce directional changes in traits like armor plating and body form, and these changes often occur in parallel across independent populations facing similar environments. So, to infer history, neutral markers reveal the demographic events that shaped populations, while adaptive traits illuminate the selective forces and show parallel evolution. This combination is best captured by noting that adaptive traits show directional selection and parallel changes, while neutral markers reflect demographic history.