# Explain how variation in a population arises and why only heritable variation is relevant to natural selection.

> IB DP Biology Higher Level (2023 syllabus) — D4.1 Natural selection · Explain · 4 marks

> 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.

## Mark scheme (4 marks)

1. Variation arises through mutations, which introduce new alleles into a population.
2. Sexual reproduction (meiosis / independent assortment / crossing over and random fertilisation) shuffles existing alleles, producing additional phenotypic variation.
3. Environmentally induced variation (phenotypic plasticity) is not passed to offspring because it does not alter the DNA sequence / genotype.
4. 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

- [heritable variation](https://www.gradenine.co.uk/glossary/heritable-variation)
- [natural selection](https://www.gradenine.co.uk/glossary/natural-selection)

## Related

- [Revision notes for IB DP Biology Higher Level (2023 syllabus)](https://www.gradenine.co.uk/learn)
- [How to answer "Explain" questions](https://www.gradenine.co.uk/tools/command-word-cheatsheet)
- [Practice this with AI marking (free)](https://www.gradenine.co.uk/start)

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Source: [GradeNine](https://www.gradenine.co.uk/q/explain-how-variation-in-a-population-84b1149a) · Published by Druglandscape Ltd.