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: Understanding the Concept:
This question describes a classic example of natural selection observed by Peter and Rosemary Grant in the Galápagos Islands. The key is to identify which mode of selection is at play when an environmental pressure favors one extreme of a phenotypic range.

Step 2: Detailed Explanation:
- The scenario states that an abundance of large seeds leads to an increase in beak depth. This means that finches with deeper, stronger beaks (one extreme of the beak depth spectrum) were better able to crack the large seeds and thus had higher survival and reproductive rates.
- This pattern, where selection favors individuals at one end of the phenotypic range, is the definition of directional selection. It causes the average value of the trait in the population to shift in that direction over time.
- Disruptive selection would favor both extremes (e.g., very small and very large beaks) but select against the intermediate form.
- Stabilizing selection would favor the intermediate phenotype (e.g., average-sized beaks) and select against both extremes.
- Balancing selection maintains genetic diversity by keeping multiple alleles or phenotypes stable in a population.

Step 3: Final Answer:
The scenario is a textbook case of directional selection, where the population's average beak depth evolves in a specific direction in response to environmental changes. Thus, option (B) is correct.