Explain why natural selection does not lead to the evolution of a population when a trait is determined entirely by the environment rather than by genes.
Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).
Model answer (4 marks)
Natural selection can only change a population when the trait that is selected for is heritable. A trait that is determined entirely by the environment is not encoded in the genotype, so it cannot be passed on to the next generation. Natural selection acts on the phenotype, but if the phenotypic difference has no underlying allelic basis, differential survival or reproduction will not alter allele frequencies. Therefore, without a genetic basis the population cannot evolve – each generation’s phenotype is reset by the environment and the favourable phenotype is not inherited.
Examiner tips
- Mention that natural selection requires heritable variation
- Explain that environmental traits are not encoded in the genotype
- State that allele frequencies must change for evolution to occur
- Highlight that offspring do not inherit the phenotype
Common mistakes
- Confusing ‘phenotype’ with ‘genotype’ – claiming the trait is still heritable
- Forgetting to link the lack of heritability to no change in allele frequencies
- Over‑generalising that natural selection always works, ignoring the need for genetic variation
Mark scheme (4 marks)
- Natural selection requires heritable variation; environmentally determined traits are not encoded in the genotype and therefore cannot be inherited by offspring.
- Natural selection acts on phenotype, but if phenotypic differences have no underlying allelic basis, differential survival/reproduction does not change allele frequencies in the next generation.
- Evolution by natural selection requires a change in allele frequencies across generations; without a heritable genetic basis for the trait, no such change can occur.
- Offspring will not inherit the favourable phenotype because its expression depends on the environment, not on genes transmitted from parents; each generation's phenotype is reset by environmental conditions.
Key terms in this question
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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