Explain how natural selection could lead to a change in the colour of a population of pebble crabs over many generations, after a new predatory fish begins hunting in the area.
Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).
Pebble crabs live on a rocky seabed. Their shell colour varies — some crabs have shells that closely match the grey rock, whilst others have brightly coloured shells. A new predatory fish arrives in the habitat and hunts the crabs by sight.
Model answer (4 marks)
There is genetic variation in shell colour caused by different alleles. The new fish hunts by sight, so brightly coloured crabs are more visible and are eaten more often, reducing their survival. Grey, camouflaged crabs are less likely to be caught, so they survive and reproduce, passing the grey‑shell allele to their offspring. Over many generations the frequency of the grey‑shell allele rises, and the population becomes predominantly grey.
Examiner tips
- Use the word ‘variation’ and mention alleles; link visibility to predation; state survival/reproduction of grey crabs; show allele frequency change over generations
Common mistakes
- Failing to mention genetic variation or alleles; confusing cause and effect (e.g., saying fish prefer grey crabs); not explaining the increase in allele frequency
Mark scheme (4 marks)
- There is genetic variation / variation in shell colour within the crab population due to mutations / different alleles
- Brightly coloured crabs are more easily seen / eaten by the predatory fish, so they are less likely to survive
- Grey/camouflaged crabs survive and reproduce, passing the allele(s) for grey shell colour to their offspring
- Over many generations the frequency of the allele for grey shell colour increases in the population / the population becomes predominantly grey
Key terms in this question
Related
- Revision notes: Natural selection and evolution →
- How to answer a "Explain" question →
- Decode the mark scheme abbreviations →
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