Mutation and sexual reproduction as sources of variationSpec D4.1.2
In short
Variation is the raw material of natural selection. Mutation generates new alleles by changing the base sequence of a gene. Sexual reproduction generates new combinations of alleles through crossing over and random orientation in meiosis, and random fertilisation. Without variation, all individuals would be equally fit and there would be nothing to select.
Natural selection can only act if individuals differ. Two processes generate the heritable variation it acts on.
| Process | What it does | How |
|---|---|---|
| Mutation | Generates new alleles | A random change in the base sequence of DNA. Only mutations in cells that give rise to gametes are passed to offspring. Most are neutral or harmful; a few are beneficial. |
| Sexual reproduction | Generates new combinations of alleles | Crossing over in prophase I and random orientation of bivalents in metaphase I of meiosis give genetically different gametes. Random fertilisation of one gamete by another adds further combinations. |
Mutation is the only source of genuinely new alleles. Sexual reproduction does not create alleles, but it shuffles existing ones into combinations that may never have existed before, so every offspring is genetically unique (apart from identical twins).
In asexual reproduction, offspring are genetically identical to the parent except where a mutation occurs, so variation is much more limited.
Keep the guide's distinction exact: mutation = new alleles; sexual reproduction = new combinations of alleles. Swapping them loses the mark.
Written and checked against the IB Biology SL specification · Updated October 2026