# A species of freshwater fish lives in a large river. Due to a prolonged drought, the river splits into two completely separate lakes. The fish populations in the two lakes are isolated from each other for thousands of years. Explain how this geographical isolation could eventually lead to the two populations becoming separate species.

> Edexcel A-Level Biology A: Salters-Nuffield (9BN0) — 3.4 Evolution and speciation · Explain · 5 marks

> Geographical isolation is a key driver of speciation. When populations of the same species are separated, they may eventually become so different that they can no longer interbreed to produce fertile offspring.

## Mark scheme (5 marks)

1. Mutations occur randomly in each isolated population, producing new alleles / genetic variants
2. Each population experiences different selection pressures / environmental conditions in their separate lakes
3. Natural selection favours different alleles / phenotypes in each lake, so different alleles increase in frequency in each population
4. Over many generations, the two populations accumulate so many genetic differences that their genotypes / phenotypes become very different
5. Eventually the two populations can no longer interbreed to produce fertile offspring, so they are now separate species

## Related

- [Revision notes for Edexcel A-Level Biology A: Salters-Nuffield (9BN0)](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/a-species-of-freshwater-fish-lives-0c2d5e96) · Published by Druglandscape Ltd.