Which scenario illustrates directional selection?

Which scenario illustrates directional selection?

• A. A population’s average trait shifts toward one extreme due to selective pressure
• B. Average trait remains constant due to balanced selection
• C. Extreme variants are favored equally
• D. Diversity increases due to disruptive selection

Answer: (A)

Directional selection happens when a persistent selective pressure favors individuals at one end of the trait spectrum, causing the population’s average trait to move toward that extreme over generations. In this scenario, the pressure makes one extreme advantageous, so the mean shifts in that direction, which is exactly what directional selection describes. The other patterns don’t fit: balanced selection tends to keep the average constant by equal fitness across variants, and if extreme variants are favored equally, the distribution may broaden without a steady move toward a single extreme. Disruptive selection increases diversity by favoring both extremes and reducing the middle, which also does not produce a steady shift of the mean toward one end.

Directional selection happens when a persistent selective pressure favors individuals at one end of the trait spectrum, causing the population’s average trait to move toward that extreme over generations. In this scenario, the pressure makes one extreme advantageous, so the mean shifts in that direction, which is exactly what directional selection describes. The other patterns don’t fit: balanced selection tends to keep the average constant by equal fitness across variants, and if extreme variants are favored equally, the distribution may broaden without a steady move toward a single extreme. Disruptive selection increases diversity by favoring both extremes and reducing the middle, which also does not produce a steady shift of the mean toward one end.