# Explain how reproductive isolation leads to allopatric speciation in a population of organisms separated by a geographical barrier.

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

## Mark scheme (4 marks)

1. Geographical barrier physically separates a population into two or more subpopulations, preventing gene flow between them.
2. Each subpopulation is subject to different selection pressures and/or genetic drift, so allele frequencies diverge independently over time.
3. Over many generations, the accumulated genetic differences result in reproductive isolation between the two subpopulations — individuals can no longer interbreed to produce fertile offspring even if they meet.
4. Once reproductive isolation is complete, the two groups are classified as separate species (biological species concept), because they form distinct gene pools that can no longer exchange alleles.

## Key terms

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

## 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-leads-to-e451ee04) · Published by Druglandscape Ltd.