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Difficulty: MediumDarwin's Theory of Natural Selection

During a prolonged drought on an isolated island, small soft seeds became scarce, while large hard-shelled seeds remained abundant. Researchers observed that birds with deeper, stronger beaks survived and reproduced at higher rates than those with shallower beaks. Over subsequent generations, the average beak depth in the population increased. According to Charles Darwin's theory of natural selection, which mechanism best accounts for this evolutionary change?

  1. Natural selection acted on pre-existing variation in beak depth, favoring individuals with advantageous traits to reproduce.Answer
  2. B
    Individual birds stretched and enlarged their beaks to crack hard seeds, passing these acquired larger beaks to their offspring.
  3. C
    The environmental stress of the drought directly induced target mutations causing birds to develop larger beaks.
  4. D
    Birds intentionally altered their anatomical structures to adapt to the new food source and transmitted the habit.

Answer

Natural selection acted on pre-existing variation in beak depth, favoring individuals with advantageous traits to reproduce.
According to Darwin's theory of natural selection, populations possess pre-existing variation. When environmental conditions change (such as a drought reducing soft seeds), individuals possessing advantageous variations (deeper beaks suitable for hard seeds) exhibit higher survival and reproductive rates. Consequently, the advantageous trait becomes more prevalent in subsequent generations.

Step-by-Step Solution

1
Identify the initial population condition in the scenario.
The initial population already contained variation in beak depth (some birds had shallow beaks, others deeper beaks).
Darwinian natural selection requires pre-existing heritable variation in the population before selective pressure is applied.
2
Analyze the environmental change and selective pressure.
Drought changed the food availability to predominantly large, hard seeds, creating a survival challenge.
Environmental factors act as selective forces that determine which phenotypes have higher fitness.
3
Evaluate the differential survival and reproduction.
Birds with deeper beaks survived and reproduced more effectively, passing the alleles for deeper beaks to the next generation.
Differential reproduction leads to an shift in allele frequencies across generations toward the favorable trait.

Key Concept

Natural Selection via Differential Survival and Pre-existing Variation
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Topics

Darwin's Theory of Natural Selection

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