# Explain how a new species can form from a single ancestral population that becomes geographically separated into two groups.

> Pearson Edexcel A-Level Biology A (9BI0) — Topic 7 Genetics, Populations, Evolution and Ecosystems · Explain · 5 marks

> Two groups of the same mammal species became separated millions of years ago when a large river formed, dividing their habitat. Today, the two groups are so different that they can no longer interbreed successfully.

## Mark scheme (5 marks)

1. The two groups are isolated and cannot interbreed / no gene flow between the two groups
2. Each group has genetic variation / different alleles exist within each population
3. Each group faces different selection pressures / different environments act on each group
4. Natural selection favours different alleles in each group, so allele frequencies change differently over time
5. After sufficient genetic divergence the two groups can no longer interbreed to produce fertile offspring, so they are now 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)

---
Source: [GradeNine](https://www.gradenine.co.uk/q/explain-how-a-new-species-can-1984a678) · Published by Druglandscape Ltd.