A species of freshwater fish lives in a large river. Due to a prolonged drought, the river splits into two completely separate lakes. The fish populations in the two lakes are isolated from each other for thousands of years. Explain how this geographical isolation could eventually lead to the two populations becoming separate species.
Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).
Geographical isolation is a key driver of speciation. When populations of the same species are separated, they may eventually become so different that they can no longer interbreed to produce fertile offspring.
Model answer (5 marks)
1. Random mutations introduce new alleles into each isolated population.
2. The two lakes provide different environmental conditions, creating distinct selection pressures.
3. Natural selection favours different alleles in each lake, so those alleles increase in frequency.
4. Over many generations the populations accumulate many genetic and phenotypic differences.
5. When the differences are great enough, interbreeding produces infertile offspring, so the two populations are now separate species.
2. The two lakes provide different environmental conditions, creating distinct selection pressures.
3. Natural selection favours different alleles in each lake, so those alleles increase in frequency.
4. Over many generations the populations accumulate many genetic and phenotypic differences.
5. When the differences are great enough, interbreeding produces infertile offspring, so the two populations are now separate species.
Examiner tips
- Use the word ‘mutations’ and ‘alleles’ for point 1. Mention ‘different selection pressures’ for point 2. Show that natural selection changes allele frequencies for point 3. Explain accumulation of differences over many generations for point 4. State loss of fertility on interbreeding for point 5.
Common mistakes
- Mixing up mutation with natural selection. Failing to mention that the lakes are completely isolated. Using vague terms like ‘changes’ instead of ‘allele frequencies’ or ‘phenotypic differences’.
Mark scheme (5 marks)
- Mutations occur randomly in each isolated population, producing new alleles / genetic variants
- Each population experiences different selection pressures / environmental conditions in their separate lakes
- Natural selection favours different alleles / phenotypes in each lake, so different alleles increase in frequency in each population
- Over many generations, the two populations accumulate so many genetic differences that their genotypes / phenotypes become very different
- Eventually the two populations can no longer interbreed to produce fertile offspring, so they are now separate species
Related
- All Edexcel A-Level Biology A: Salters-Nuffield (9BN0) revision notes →
- How to answer a "Explain" question →
- Decode the mark scheme abbreviations →
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