Which statement best describes the distribution of effect sizes among loci that control adaptive traits in sticklebacks?

• A. All loci have equal effect sizes.
• B. Environmental factors determine all variation.
• C. A few large-effect QTL and many small-effect loci.
• D. A single gene controls all adaptive traits.

Answer: (C)

Adaptive traits in sticklebacks are shaped by a mix of genetic effects across many loci: a few major-effect loci drive large, quick changes, while a larger number of loci contribute smaller effects that fine-tune the phenotype. This combination creates a distribution where big shifts can arise from a handful of QTL, and the rest of the genome lines up subtle adjustments to fit the environment. That’s why the best description is that there are a few large-effect QTL and many small-effect loci. The other ideas don’t fit this real pattern: not all loci contribute equally, environment isn’t the sole driver of these genetic effects, and a single gene cannot account for the diversity of adaptive traits observed.