Which approach best determines whether observed trait divergence is due to selection or drift?

• A. Compare repeated patterns across independent populations; use statistics and simulations to test for selection versus drift.
• B. Use a single population and infer selection from the trait distribution.
• C. Rely on qualitative judgment with no statistical data.
• D. Examine only phenotypic data without genetic information.

Answer: (A)

To tell whether trait differences among populations come from selection or drift, you need information from multiple independent populations and a way to compare what you’d expect under neutral drift with what you actually observe. When populations facing different environments show consistent, parallel changes in a trait, and the size of that change is larger than what drift would typically produce given their genetic differences, it suggests natural selection is shaping the trait. Statistics and simulations are essential here: comparing the trait’s divergence (Qst) to the neutral genetic divergence (Fst) and running drift simulations under the same demographic history provide a neutral baseline. If the trait diverges more than the neutral expectation, selection is the likely driver; if it matches the neutral baseline, drift could explain it. Using just one population or relying on qualitative judgments without genetic data cannot distinguish these processes because drift can produce random patterns, and selection requires evidence that goes beyond a single case.