Explain how reproductive isolation leads to allopatric speciation in a population of organisms separated by a geographical barrier.
Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).
Model answer (4 marks)
A geographical barrier physically separates a population into two or more sub‑populations, preventing gene flow between them.
Each sub‑population is then subject to different selection pressures and/or genetic drift, so allele frequencies diverge independently over time.
After many generations the accumulated genetic differences result in reproductive isolation between the two sub‑populations – individuals can no longer interbreed to produce fertile offspring even if they meet.
Once reproductive isolation is complete, the two groups are classified as separate species (biological species concept), because they form distinct gene pools that can no longer exchange alleles.
Each sub‑population is then subject to different selection pressures and/or genetic drift, so allele frequencies diverge independently over time.
After many generations the accumulated genetic differences result in reproductive isolation between the two sub‑populations – individuals can no longer interbreed to produce fertile offspring even if they meet.
Once reproductive isolation is complete, the two groups are classified as separate species (biological species concept), because they form distinct gene pools that can no longer exchange alleles.
Examiner tips
- Use the word ‘prevent’ to show the barrier stops gene flow. Show the chain: barrier → no gene flow → divergent selection/drift → genetic differences → reproductive isolation → new species. Mention ‘biological species concept’ to link isolation to species status.
Common mistakes
- Confusing ‘geographical isolation’ with ‘reproductive isolation’ in the first sentence. Omitting the step that genetic differences accumulate over many generations. Failing to state that the new species are defined by the biological species concept.
Mark scheme (4 marks)
- Geographical barrier physically separates a population into two or more subpopulations, preventing gene flow between them.
- Each subpopulation is subject to different selection pressures and/or genetic drift, so allele frequencies diverge independently over time.
- Over many generations, the accumulated genetic differences result in reproductive isolation between the two subpopulations — individuals can no longer interbreed to produce fertile offspring even if they meet.
- Once reproductive isolation is complete, the two groups are classified as separate species (biological species concept), because they form distinct gene pools that can no longer exchange alleles.
Key terms in this question
allopatric speciation · reproductive isolation · geographical barrier
Related
- All IB DP Biology Higher Level (2023 syllabus) revision notes →
- How to answer a "Explain" question →
- Decode the mark scheme abbreviations →
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