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.
Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).
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.
Model answer (5 marks)
1. The mountain range is a physical barrier that geographically isolates the two groups, preventing interbreeding and gene flow.
2. Random mutations arise in each isolated group, creating new alleles and genetic variants.
3. Each side of the mountain has different selection pressures, so different alleles become advantageous in each group.
4. Natural selection favours individuals with advantageous alleles, so allele frequencies change differently over many generations.
5. After long periods the populations become genetically distinct enough that they can no longer interbreed to produce fertile offspring, so they are separate species.
2. Random mutations arise in each isolated group, creating new alleles and genetic variants.
3. Each side of the mountain has different selection pressures, so different alleles become advantageous in each group.
4. Natural selection favours individuals with advantageous alleles, so allele frequencies change differently over many generations.
5. After long periods the populations become genetically distinct enough that they can no longer interbreed to produce fertile offspring, so they are separate species.
Examiner tips
- Use the word ‘isolated’ to show geographic isolation. Mention ‘random mutations’ and ‘different selection pressures’. Explain how natural selection changes allele frequencies. Finish with ‘speciation has occurred’ to link to the final point.
Common mistakes
- Confusing ‘physical barrier’ with ‘behavioural barrier’. Failing to mention natural selection as the mechanism. Not stating that the two groups can no longer produce fertile offspring.
Mark scheme (5 marks)
- The mountain range acts as a physical barrier that geographically isolates the two groups, preventing interbreeding / gene flow between them.
- Random mutations arise in each group, introducing new alleles / genetic variants into each isolated population.
- Each environment (on either side of the mountain) has different selection pressures, so different alleles/variants are advantageous in each group.
- 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.
- 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
- All Edexcel A-Level Biology A: Salters-Nuffield (9BN0) revision notes →
- How to answer a "Describe" question →
- Decode the mark scheme abbreviations →
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