# Explain how hybridisation followed by polyploidy can result in the formation of a new plant species.

> IB DP Biology Higher Level (2023 syllabus) — A4.1 Evolution and speciation · Explain · 4 marks

## Mark scheme (4 marks)

1. Hybridisation occurs when two different species interbreed, producing offspring with one chromosome set from each parent species
2. The hybrid is typically sterile because the two chromosome sets are non-homologous and cannot pair during meiosis
3. Polyploidy (chromosome doubling) restores fertility by providing a homologous partner for each chromosome, allowing normal meiosis
4. The new polyploid is reproductively isolated from both parent species because crosses with either parent produce sterile offspring, satisfying the biological species concept

## Key terms

- [hybridisation](https://www.gradenine.co.uk/glossary/hybridisation)
- [polyploidy](https://www.gradenine.co.uk/glossary/polyploidy)

## 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-hybridisation-followed-by-polyploidy-126647e6) · Published by Druglandscape Ltd.