A population of giraffes living on the African savanna feeds on leaves from trees. Over many generations, the average neck length of giraffes in the population has increased. Explain how natural selection could have caused this change.
Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).
In a population of giraffes, individuals vary in neck length. Giraffes with longer necks are able to reach leaves higher up in trees, giving them access to more food.
Model answer (4 marks)
Variation in neck length exists in the population, caused by mutation.
Longer‑necked giraffes can reach more leaves, giving them a survival advantage – the selection pressure is competition for food.
Because they survive better, they are more likely to reproduce and pass the advantageous allele for longer necks to their offspring.
Over many generations the frequency of the longer‑neck trait rises, increasing the average neck length of the population.
Longer‑necked giraffes can reach more leaves, giving them a survival advantage – the selection pressure is competition for food.
Because they survive better, they are more likely to reproduce and pass the advantageous allele for longer necks to their offspring.
Over many generations the frequency of the longer‑neck trait rises, increasing the average neck length of the population.
Examiner tips
- Use the exact terms: variation, selection pressure, survival advantage, reproduction, allele frequency.
- Show the causal chain: variation → advantage → reproduction → change in allele frequency.
- Keep the answer concise – 4 points, one sentence each.
- Mention the specific selection pressure (competition for food).
Common mistakes
- Failing to state that variation is due to mutation.
- Confusing natural selection with genetic drift or gene flow.
- Using vague language such as "more likely" without linking to reproduction.
Mark scheme (4 marks)
- There is variation in neck length within the population (due to mutation)
- Giraffes with longer necks can reach more food / have a better chance of surviving (named selection pressure: competition for food)
- Giraffes with longer necks are more likely to reproduce and pass on the advantageous allele/trait to offspring
- Over many generations, the frequency of the longer-neck trait increases in the population
Key terms in this question
Related
- Revision notes: Evolution by natural selection →
- How to answer a "Explain" question →
- Decode the mark scheme abbreviations →
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