Explain how reproductive isolation between two allopatric populations can arise and ultimately lead to the formation of two distinct species.
Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).
Model answer (4 marks)
Allopatric populations are separated by a physical barrier that prevents gene flow. Each isolated group experiences different selection pressures and/or genetic drift, so allele frequencies diverge independently. Over many generations the populations accumulate enough genetic differences that, if they were to meet, they would no longer interbreed successfully or would produce infertile offspring. Because the two populations can no longer exchange genes, they are defined as separate species under the biological species concept.
Examiner tips
- Mention the barrier first, then divergence, then reproductive isolation, then species definition
- Use terms: allopatric, gene flow, selection, genetic drift, allele frequency, reproductive isolation, biological species concept
Common mistakes
- Confusing sympatric with allopatric; not stating the barrier
- Lacking the step that genetic differences lead to infertility or non‑interbreeding
- Using vague terms like ‘different’ without specifying selection or drift
Mark scheme (4 marks)
- Allopatric populations are geographically separated by a physical barrier, preventing gene flow between them.
- Each isolated population is subjected to different selection pressures and/or genetic drift, causing allele frequencies to diverge independently over time.
- Over many generations, the populations accumulate sufficient genetic differences that, if they were to meet, they would no longer be able to interbreed successfully / produce fertile offspring.
- Because the two populations can no longer exchange genes / interbreed successfully, they are defined as separate species according to the biological species concept.
Key terms in this question
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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