Explain how a new species can form from a single ancestral population that becomes geographically separated into two groups.
Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).
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.
Model answer (5 marks)
The river isolates the two groups, so there is no gene flow between them.
Each group retains its own genetic variation, with different alleles present.
The separated habitats impose different selection pressures on each group.
Natural selection therefore favours different alleles in each group, changing allele frequencies in opposite directions.
After enough genetic divergence, the groups can no longer produce fertile offspring, so they are separate species.
Each group retains its own genetic variation, with different alleles present.
The separated habitats impose different selection pressures on each group.
Natural selection therefore favours different alleles in each group, changing allele frequencies in opposite directions.
After enough genetic divergence, the groups can no longer produce fertile offspring, so they are separate species.
Examiner tips
- Use the word ‘isolated’ to show lack of gene flow. Mention ‘different selection pressures’ to link environment to evolution. Show that allele frequencies change in opposite directions. Finish with ‘no fertile offspring’ to justify speciation.
Common mistakes
- Forgetting to mention isolation or lack of gene flow. Confusing ‘speciation’ with ‘adaptation’. Not specifying that allele frequencies change in opposite directions.
Mark scheme (5 marks)
- The two groups are isolated and cannot interbreed / no gene flow between the two groups
- Each group has genetic variation / different alleles exist within each population
- Each group faces different selection pressures / different environments act on each group
- Natural selection favours different alleles in each group, so allele frequencies change differently over time
- After sufficient genetic divergence the two groups can no longer interbreed to produce fertile offspring, so they are now separate species
Related
- All Pearson Edexcel A-Level Biology A (9BI0) revision notes →
- How to answer a "Explain" question →
- Decode the mark scheme abbreviations →
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