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.
Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).
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.
Model answer (4 marks)
Heritable variation in beak depth exists within the finch population because different alleles determine beak size.
When large, hard seeds dominate the diet, finches with deeper beaks can crack the seeds more efficiently and therefore obtain more food.
Finches with deeper beaks are more likely to survive, reproduce and pass those alleles on to their offspring, giving them a selective advantage.
Over successive generations the alleles for deeper beaks increase in frequency, raising the mean beak depth of the population.
When large, hard seeds dominate the diet, finches with deeper beaks can crack the seeds more efficiently and therefore obtain more food.
Finches with deeper beaks are more likely to survive, reproduce and pass those alleles on to their offspring, giving them a selective advantage.
Over successive generations the alleles for deeper beaks increase in frequency, raising the mean beak depth of the population.
Examiner tips
- Use the word ‘heritable’ to show genetic basis. Mention ‘selective advantage’ or ‘higher fitness’. Explain differential reproductive success. Show that allele frequency changes over generations.
Common mistakes
- Confusing phenotypic variation with environmental change. Forgetting to link deeper beaks to higher fitness. Not stating that the change is in allele frequency, not just phenotype.
Mark scheme (4 marks)
- Heritable variation in beak depth exists within the finch population due to differences in alleles.
- 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.
- Finches with deeper beaks are more likely to survive, reproduce, and pass on the alleles for deeper beaks to offspring (differential reproductive success).
- Over successive generations, the frequency of alleles associated with deeper beaks increases in the population, shifting the mean beak depth.
Key terms in this question
Related
- All IB DP Biology Standard Level (2023 syllabus) revision notes →
- How to answer a "Explain" question →
- Decode the mark scheme abbreviations →
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