# Explain how natural selection can lead to the formation of a new species (speciation).

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

> A population of birds lives on a large continent. Over thousands of years, a mountain range rises up, splitting the population into two groups that can no longer interbreed. Eventually, the two groups become so different that they can no longer produce fertile offspring, even if they meet again.

## Mark scheme (5 marks)

1. Genetic variation exists within the original population due to mutations (and sexual reproduction)
2. The mountain range acts as a physical barrier, isolating the two groups so they cannot interbreed / preventing gene flow between the two populations
3. Each isolated population experiences different selection pressures / different environmental conditions
4. Individuals with alleles better suited to their local environment survive, reproduce and pass on their alleles (natural selection / survival of the fittest)
5. Over many generations the allele frequencies in each population change so much that the two groups can no longer interbreed to produce fertile offspring — they have become separate species (speciation)

## Key terms

- [natural selection](https://www.gradenine.co.uk/glossary/natural-selection)
- [speciation](https://www.gradenine.co.uk/glossary/speciation)

## 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)

---
Source: [GradeNine](https://www.gradenine.co.uk/q/explain-how-natural-selection-can-lead-eb255687) · Published by Druglandscape Ltd.