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Question #46475
The Question
In a heavily industrialized region, a very small, isolated population of Biston betularia (peppered moth) is continuously exposed to soot-covered tree trunks. Despite strong directional selection favoring the melanic (dark) phenotype, biologists observe that the maladaptive light-colored phenotype becomes fixed in the population over several generations. Which of the following best explains this evolutionary outcome?
Options
- A.
The absence of gene flow prevented the dominant melanic allele from being expressed in the population.
- B.
Genetic drift overpowered the directional selection pressure, leading to the stochastic fixation of the maladaptive allele.
- C.
The population reached Hardy-Weinberg equilibrium, which inherently stabilizes recessive alleles in small populations.
- D.
Directional selection in isolated populations always shifts towards the recessive allele to maintain the genetic load.
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Correct Answer
Step-by-Step Solution
Explanation
How the Examiner Thinks
Evolutionary_Exception
Tackle problems where standard rules evolve or change based on specific environmental constraints.
Concepts Behind This Question
Biston betularia|Allele frequency|Genetic drift
Why This Question Is Tricky
Common Mistakes
Conceptual Pivot: Students often treat Natural Selection as an absolute, deterministic force (e.g., 'polluted environment always equals dark moths').
They fail to synthesize this with the Hardy-Weinberg condition that in small populations, Genetic Drift (stochasticity) acts as a dominant evolutionary force that can override selection, causing even maladaptive alleles to reach fixation.
Quick Facts About This Question
Question Information
| Subject | Biology |
| Chapter | Evolution - Class 12 |
| Concept | Industrial Melanism |
| Difficulty | 8 |
| Cognitive Load | 5 |
| Target Solve Time | 120 sec |
| Access | Free |