# A population of freshwater fish lives in a large lake. After a period of volcanic activity, a lava flow divides the lake into two smaller, completely separate lakes. Explain how the two isolated fish populations could eventually become two separate species.

> Pearson Edexcel A-Level Biology A (9BI0) — 3.4 Evolution and speciation · Explain · 5 marks

> A population of freshwater fish lives in a large lake. After a period of volcanic activity, a lava flow divides the lake into two smaller, completely separate lakes. The two smaller lakes have different water temperatures, food sources and levels of dissolved oxygen.

## Mark scheme (5 marks)

1. The two populations are separated and can no longer interbreed / exchange genetic material (gene flow is prevented)
2. Random mutations occur in each population, producing genetic variation / different alleles in each population
3. Natural selection acts differently in each lake because the environments are different
4. Individuals with advantageous characteristics are more likely to survive, reproduce and pass on their alleles / genes to offspring
5. Over many generations the two populations become so genetically different that they can no longer interbreed to produce fertile offspring, so they are now two separate species

## Related

- [Revision notes for Pearson Edexcel A-Level Biology A (9BI0)](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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