Explain how variation in a population arises and why only heritable variation is relevant to natural selection.
Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).
Populations of the peppered moth (Biston betularia) contain individuals with a range of wing coloration, from pale speckled to almost entirely melanic. This variation has a genetic basis, though environmental factors can also influence phenotype.
Model answer (4 marks)
Variation in a population arises mainly from mutations, which create new alleles in the gene pool. Sexual reproduction also generates variation: meiosis, independent assortment, crossing‑over and random fertilisation shuffle existing alleles, producing new combinations of traits. Some variation is environmentally induced (phenotypic plasticity); this does not change the DNA sequence and therefore is not inherited. Natural selection can only act on heritable variation because only changes that are passed to offspring alter allele frequencies in the next generation, allowing evolution to occur.
Examiner tips
- State the four sources of variation in the order given by the mark scheme
- Explain why non‑heritable variation is irrelevant to evolution
- Use the term ‘allele frequency’ to show understanding of selection’s effect
- Keep the answer concise – 4 marks only
Common mistakes
- Confusing phenotypic plasticity with genetic variation
- Lacking the term ‘allele frequency’ when describing selection
- Over‑expanding the answer with unnecessary detail
Mark scheme (4 marks)
- Variation arises through mutations, which introduce new alleles into a population.
- Sexual reproduction (meiosis / independent assortment / crossing over and random fertilisation) shuffles existing alleles, producing additional phenotypic variation.
- Environmentally induced variation (phenotypic plasticity) is not passed to offspring because it does not alter the DNA sequence / genotype.
- Natural selection acts only on heritable variation because differential survival/reproductive success must result in a change in allele frequency in subsequent generations for evolution to occur.
Key terms in this question
heritable variation · natural selection
Related
- All IB DP Biology Higher Level (2023 syllabus) revision notes →
- How to answer a "Explain" question →
- Decode the mark scheme abbreviations →
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