# A population of lizards lives on a large island. Over thousands of years, a mountain range forms across the middle of the island, dividing the population into two groups. After a very long period of time, scientists discover that the two groups can no longer interbreed to produce fertile offspring. Describe how the two groups of lizards became separate species.

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

> A population of lizards lives on a large island. Over thousands of years, a mountain range forms across the middle of the island, dividing the population into two groups. After a very long period of time, scientists discover that the two groups can no longer interbreed to produce fertile offspring.

## Mark scheme (5 marks)

1. The mountain range acts as a physical barrier that geographically isolates the two groups, preventing interbreeding / gene flow between them.
2. Random mutations arise in each group, introducing new alleles / genetic variants into each isolated population.
3. Each environment (on either side of the mountain) has different selection pressures, so different alleles/variants are advantageous in each group.
4. Natural selection means individuals with advantageous alleles are more likely to survive and reproduce, passing those alleles on to offspring; over many generations the allele frequencies in each population change differently.
5. The two populations become so genetically different that they can no longer interbreed to produce fertile offspring — they are now separate species (speciation has occurred).

## Related

- [Revision notes for Edexcel A-Level Biology A: Salters-Nuffield (9BN0)](https://www.gradenine.co.uk/learn)
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