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 · View as Markdown

Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).

Model answer (4 marks)

Hybridisation occurs when two different species interbreed, producing an offspring that contains one chromosome set from each parent species.
The hybrid is usually sterile because the two chromosome sets are non‑homologous and cannot pair during meiosis.
If the hybrid undergoes chromosome doubling (polyploidy), each chromosome now has a homologous partner, restoring normal meiosis and fertility.
The resulting polyploid is reproductively isolated from both parent species, as crosses with either parent give sterile offspring, thereby satisfying the biological species concept.

Examiner tips

Use the exact terms ‘hybridisation’, ‘polyploidy’, ‘chromosome doubling’, ‘reproductive isolation’
Show the sequence: hybridisation → sterility → chromosome doubling → fertility and isolation
Mention that the new species is a ‘polyploid’ and that it is isolated by sterility of hybrids with parents

Common mistakes

Confusing ‘polyploidy’ with ‘hybridisation’ or vice versa
Failing to explain why the hybrid is sterile (non‑homologous chromosomes)
Not stating that the new polyploid is reproductively isolated from both parents

Mark scheme (4 marks)

Hybridisation occurs when two different species interbreed, producing offspring with one chromosome set from each parent species
The hybrid is typically sterile because the two chromosome sets are non-homologous and cannot pair during meiosis
Polyploidy (chromosome doubling) restores fertility by providing a homologous partner for each chromosome, allowing normal meiosis
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 in this question

hybridisation · polyploidy