Explain how natural selection can lead to a change in the frequency of a heritable trait in a population of organisms over several generations.
Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).
A population of mice lives in a grassland habitat that becomes progressively drier and paler in colour over many decades. The mice show heritable variation in fur colour, ranging from dark brown to pale sandy.
Model answer (4 marks)
Individuals in the population show heritable variation in fur colour, ranging from dark brown to pale sandy.
Pale‑furred mice are better camouflaged against the increasingly dry, pale habitat and are therefore less likely to be predated.
Because they survive longer and reproduce more, they pass the alleles for pale fur to a larger proportion of their offspring.
Over many generations the frequency of the allele(s) for pale fur rises, so the population becomes predominantly pale‑furred.
Pale‑furred mice are better camouflaged against the increasingly dry, pale habitat and are therefore less likely to be predated.
Because they survive longer and reproduce more, they pass the alleles for pale fur to a larger proportion of their offspring.
Over many generations the frequency of the allele(s) for pale fur rises, so the population becomes predominantly pale‑furred.
Examiner tips
- Use the word ‘heritable’ to show genetic basis. Explain the selective advantage in terms of predation. Show the link between survival, reproduction and allele frequency. Mention that the change occurs over many generations.
Common mistakes
- Failing to state that the variation is heritable. Confusing ‘frequency of the trait’ with ‘frequency of the allele’. Omitting the role of predation as the selective pressure. Not recognising that the change is gradual over many generations.
Mark scheme (4 marks)
- Individuals within the population show heritable variation in fur colour (from dark to pale).
- Pale-furred mice are better camouflaged against the paler habitat and so have a selective advantage, being less likely to be predated.
- Pale-furred individuals survive longer and reproduce more, passing the alleles for pale fur to a greater proportion of offspring.
- Over many generations, the frequency of the allele(s) for pale fur increases in the population, so the population becomes predominantly pale-furred.
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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