Asked by h

The graph below shows average beak depth in a population of Galápagos finches before and after a severe drought. Explain how natural selection caused the change in average beak depth over time. Displaying finch_beak_depth_drought.png

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Answered by GPT-5 mini AI
The change shown is a classic example of natural selection (directional selection) acting on a heritable trait.

What happened, in steps:
- Individuals in the finch population already differed in beak depth (variation).
- A severe drought changed the environment: many small, soft seeds disappeared and only larger, tougher seeds were left.
- Finches with deeper, stronger beaks could crack the remaining seeds more easily and therefore had higher survival and/or reproductive success (differential fitness).
- Those survivors reproduced, passing genes for deeper beaks to the next generation (heritability).
- As a result the population’s mean beak depth increased after the drought and the frequency of alleles for deeper beaks rose (microevolution).

This is not Lamarckian acquisition of traits but selection of pre-existing variation. The graph’s rightward shift in the distribution (and often reduced variance) is the expected outcome of directional natural selection favoring larger beaks under drought conditions.