# Explain how variation in beak depth within a population of finches can lead to changes in allele frequency over several generations when the available seed size changes.

> IB DP Biology Standard Level (2023 syllabus) — D4.1 Natural selection · Explain · 4 marks

> In the Galápagos Islands, a prolonged drought caused large, hard seeds to become the dominant food source in one finch population. Finches with deeper beaks were better able to crack these seeds.

## Mark scheme (4 marks)

1. Heritable variation in beak depth exists within the finch population due to differences in alleles.
2. Finches with deeper beaks have a selective advantage / higher fitness when large, hard seeds are the main food source, as they can crack and consume them more successfully.
3. Finches with deeper beaks are more likely to survive, reproduce, and pass on the alleles for deeper beaks to offspring (differential reproductive success).
4. Over successive generations, the frequency of alleles associated with deeper beaks increases in the population, shifting the mean beak depth.

## Key terms

- [allele frequency](https://www.gradenine.co.uk/glossary/allele-frequency)

## Related

- [Revision notes for IB DP Biology Standard Level (2023 syllabus)](https://www.gradenine.co.uk/learn)
- [How to answer "Explain" questions](https://www.gradenine.co.uk/tools/command-word-cheatsheet)
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Source: [GradeNine](https://www.gradenine.co.uk/q/explain-how-variation-in-beak-depth-098f85e6) · Published by Druglandscape Ltd.