A population of Galápagos finches living on an island with large, hard seeds includes individuals with different beak sizes. Explain how natural selection could lead to finches with larger, stronger beaks becoming more common in this population over many generations.
Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).
The Galápagos finches feed mainly on seeds found on the island. Individuals in the population show variation in beak size and strength. Only large, strong beaks can crack open the hard seeds efficiently.
Model answer (5 marks)
There is variation in beak size and strength within the population.
Finches with larger, stronger beaks can crack open the hard seeds more efficiently, giving them more food and energy.
Because they obtain more food they are more likely to survive and reach breeding age – the principle of survival of the fittest.
These survivors are also more likely to reproduce, passing on the alleles for larger, stronger beaks to their offspring.
Over many generations the proportion of individuals with larger, stronger beaks increases in the population.
Finches with larger, stronger beaks can crack open the hard seeds more efficiently, giving them more food and energy.
Because they obtain more food they are more likely to survive and reach breeding age – the principle of survival of the fittest.
These survivors are also more likely to reproduce, passing on the alleles for larger, stronger beaks to their offspring.
Over many generations the proportion of individuals with larger, stronger beaks increases in the population.
Examiner tips
- Show the chain of cause and effect – variation → better performance → survival → reproduction → allele frequency change.
- Use the exact terms ‘alleles’, ‘genes’, ‘frequency’ and ‘survival of the fittest’.
- Include the word ‘over many generations’ to indicate the time scale.
- Keep the answer concise – 5 marks, so one sentence per point.
Common mistakes
- Using vague terms like ‘more likely’ without linking to survival and reproduction.
- Omitting the genetic inheritance step – failing to mention alleles or genes.
- Writing the answer as a list of points rather than a cohesive explanation.
Mark scheme (5 marks)
- There is variation in beak size and strength within the population
- Finches with larger, stronger beaks are better adapted to crack open the hard seeds and so obtain more food / energy
- Finches with larger, stronger beaks are more likely to survive (and reproduce) — survival of the fittest
- Finches with larger, stronger beaks are more likely to reproduce and pass on the alleles / genes for larger, stronger beaks to their offspring
- Over many generations, the proportion / frequency of finches with larger, stronger beaks increases in the population
Key terms in this question
Related
- All Edexcel A-Level Biology A: Salters-Nuffield (9BN0) revision notes →
- How to answer a "Explain" question →
- Decode the mark scheme abbreviations →
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