# Explain how reproductive isolation between two allopatric populations can arise and ultimately lead to the formation of two distinct species.

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

## Mark scheme (4 marks)

1. Allopatric populations are geographically separated by a physical barrier, preventing gene flow between them.
2. Each isolated population is subjected to different selection pressures and/or genetic drift, causing allele frequencies to diverge independently over time.
3. Over many generations, the populations accumulate sufficient genetic differences that, if they were to meet, they would no longer be able to interbreed successfully / produce fertile offspring.
4. Because the two populations can no longer exchange genes / interbreed successfully, they are defined as separate species according to the biological species concept.

## Key terms

- [reproductive isolation](https://www.gradenine.co.uk/glossary/reproductive-isolation)

## 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)
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Source: [GradeNine](https://www.gradenine.co.uk/q/explain-how-reproductive-isolation-between-two-77807b39) · Published by Druglandscape Ltd.