Question

The increase in the beak depth of Galápagos finches with a relative abundance of large seeds, is an example of

• A. Disruptive selection
• B. Directional selection
• C. Stabilizing selection
• D. Balancing selection

Hint:

Visualize the population's trait distribution as a bell curve.

\textbf{Directional:} The whole curve shifts to the left or right.
\textbf{Stabilizing:} The curve becomes narrower and taller in the middle.
\textbf{Disruptive:} The curve splits into two peaks at the ends, with a valley in the middle. \end{itemize}

Answer 1

The Correct Option is B

Step 1: Concept Overview:
This question illustrates natural selection, exemplified by Peter and Rosemary Grant's Galápagos Islands research. The task is to identify the selection mode where environmental pressure favors one phenotypic extreme.

Step 2: Detailed Explanation:
- The situation describes an increase in beak depth due to abundant large seeds. Finches with deeper, stronger beaks (one extreme) could better crack seeds, increasing their survival and reproduction.
- This pattern, favoring one phenotypic extreme, defines directional selection. It shifts the population's average trait value over time.
- Disruptive selection would favor both extremes (e.g., very small and very large beaks) while disfavoring intermediate forms.
- Stabilizing selection would favor the intermediate phenotype (e.g., average beaks) while disfavoring extremes.
- Balancing selection maintains genetic diversity by stabilizing multiple alleles or phenotypes.

Step 3: Conclusion:
This is directional selection, where the population's average beak depth evolves directionally due to environmental changes. Therefore, option (B) is correct.