Which is NOT a commonly cited mechanism contributing to stickleback adaptive radiation?

• A. Natural selection on standing variation
• B. Major-effect loci such as EDA and Pitx1_Pel
• C. Ecological opportunity
• D. Random genetic drift as the sole driver

Answer: (D)

The main idea is how stickleback adaptive radiations arise through selective and genetic influences in new environments. The strongest drivers are natural selection acting on existing genetic variation and the action of major-effect loci, within the context of ecological opportunity provided by new habitats. Random genetic drift as the sole driver is not a commonly cited mechanism because drift alone is unlikely to produce the repeated, directionally adaptive changes seen across many populations. Drift can shift allele frequencies, but the parallel shifts toward reduced armor or altered pelvic fins point to selection acting on standing variation and on large-effect genes like EDA (armor plating) and Pitx1_Pel (pelvic spine). Ecological opportunity explains why diversification happens in the first place—new lakes or habitats create niches that selection can exploit. So, drift isn’t considered a primary mechanism driving the adaptive radiation; the other factors together better account for the patterns observed in sticklebacks.