A population of giraffes lives in a grassland habitat where food at ground level becomes increasingly scarce. Some giraffes have longer necks than others due to genetic variation. Explain how natural selection could lead to a population of giraffes with longer necks over many generations.
Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).
In a grassland habitat, vegetation at ground level is becoming increasingly scarce. Giraffes in the population vary in neck length due to mutations in their DNA.
Model answer (4 marks)
There is genetic variation in neck length within the population due to mutation. Giraffes with longer necks can reach more food and are better suited to the environment. Longer‑necked giraffes are more likely to survive and reproduce. The alleles for longer necks are passed on to offspring, so over many generations the frequency of the longer‑neck allele increases in the population.
Examiner tips
- Use the word ‘cause’ or ‘lead to’ to link variation to survival. Mention that longer necks give a feeding advantage. Show the chain: variation → differential survival → allele frequency change. Keep the answer concise and use the exact terms from the mark scheme.
Common mistakes
- Forgetting to state that the variation is genetic. Mixing up ‘survival’ with ‘reproduction’. Using vague phrases like ‘better suited’ without linking to food access.
Mark scheme (4 marks)
- There is genetic variation in neck length within the population (due to mutation)
- Giraffes with longer necks can reach more food / are better suited to the environment
- Longer-necked giraffes are more likely to survive and reproduce
- The alleles for longer necks are passed on to offspring, so over many generations the frequency of the longer-neck allele increases in the population
Key terms in this question
natural selection · genetic variation
Related
- Revision notes: Natural selection and evolution →
- How to answer a "Explain" question →
- Decode the mark scheme abbreviations →
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