Directional selection is best described as

• A. Favors one extreme phenotype over the others
• B. Favors intermediate phenotype
• C. Favors all phenotypes equally
• D. Acts randomly to change allele frequencies

Answer: (A)

Directional selection occurs when individuals at one end of a trait spectrum have higher fitness and leave more offspring than others. Over generations, the population’s average trait value shifts in that direction, and the distribution moves toward that extreme. This often reduces variation for that trait if one extreme remains consistently favored. An example is when a pest in a crop becomes larger because larger size improves survival, so the population mean shifts upward over time. This is different from stabilizing selection, which favors the intermediate form and keeps the average trait around a single value, and from disruptive selection, which favors both extremes. It’s also not random; it’s driven by differential survival and reproduction.