Which of the following is a situation in which directional selection may be observed?

• A. A population of fish where rare color patterns are favored over common ones.
• B. A population of birds maintains two distinct beak sizes, both favored by different food sources.
• C. A population of rabbits with intermediate fur length survives better than those with very short or very long fur.
• D. A population of moths becomes darker in color over time due to increased industrial pollution.

Answer: (D)

Verified step by step guidance

1. Step 1: Understand the concept of directional selection. Directional selection occurs when one extreme phenotype is favored over others, leading to a shift in the population's traits over time. This is often observed in response to environmental changes.
2. Step 2: Analyze the given scenarios to identify which one demonstrates a shift toward an extreme phenotype. For example, consider how environmental factors might favor one specific trait over others.
3. Step 3: Evaluate the scenario involving moths. Increased industrial pollution can darken the environment, making darker moths less visible to predators. This would favor the darker phenotype, causing a shift in the population's coloration over time.
4. Step 4: Compare this scenario to the others provided. The other examples describe stabilizing selection (favoring intermediate traits) or disruptive selection (favoring multiple distinct traits), which are different from directional selection.
5. Step 5: Conclude that the correct answer is the scenario where a population of moths becomes darker in color over time due to increased industrial pollution, as it demonstrates a clear example of directional selection.