What is directional selection?

• A. Favors intermediate variants
• B. Occurs when natural selection favors one of the extreme variations of a trait
• C. Keeps variation constant
• D. Reduces variation equally

Answer: (B)

Directional selection is when natural selection favors individuals at one end of the trait spectrum, causing the average value of that trait to shift over generations. This happens because those extreme individuals have higher fitness and leave more offspring, so their traits become more common in the population over time.

For example, if environment changes make larger beaks advantageous for accessing food, birds with larger beaks will tend to survive and reproduce more, gradually increasing the population’s average beak size. This process can also reduce genetic variation for that trait since alleles associated with other variants are selected against, though new variation can arise through mutation.

This contrasts with stabilizing selection, which favors intermediate phenotypes and keeps the average trait value around a middle point, and disruptive selection, which favors both extremes and can increase variation. The key idea is that one extreme is consistently favored, pushing the population toward that trait value.