Explain how sexual selection can drive speciation even in the absence of geographic isolation.
Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).
Model answer (4 marks)
Sexual selection is a form of natural selection where individuals of one sex choose mates based on specific heritable traits.
In a single population, different sub‑groups can develop distinct mate preferences, so that each group preferentially mates within itself.
Because these groups mate assortatively, gene flow between them is reduced and the allele frequencies in each group diverge over successive generations.
Eventually the genetic differences become so great that the groups can no longer interbreed successfully, producing reproductive isolation and thus a new species.
In a single population, different sub‑groups can develop distinct mate preferences, so that each group preferentially mates within itself.
Because these groups mate assortatively, gene flow between them is reduced and the allele frequencies in each group diverge over successive generations.
Eventually the genetic differences become so great that the groups can no longer interbreed successfully, producing reproductive isolation and thus a new species.
Examiner tips
- Use the term ‘sexual selection’ first, then explain mate choice and heritable traits. Show the chain: mate preference → assortative mating → reduced gene flow → allele frequency change → reproductive isolation. Keep the answer concise – 4 marks allow a short paragraph with 4 points.
Common mistakes
- Confusing natural selection with sexual selection. Failing to mention assortative mating or reduced gene flow. Using vague phrases like ‘differences arise’ without linking to reproductive isolation.
Mark scheme (4 marks)
- Sexual selection involves individuals of one sex choosing mates based on specific heritable traits / characteristics.
- Different subpopulations / groups within the same area may develop different mate preferences, leading to assortative mating within each group.
- Differential mating reduces gene flow between the groups, so allele frequencies diverge over generations.
- Over time, the groups become so genetically distinct that they can no longer interbreed successfully / are reproductively isolated, fulfilling the criterion for separate species.
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 →
More Evolution and speciation questions
- Explain how the concept of gradualism differs from punctuated equilibrium as mod…
- Explain how reproductive isolation leads to allopatric speciation in a populatio…
- Explain how natural selection acts on variation within a population to bring abo…
- Explain how the founder effect can lead to speciation in a small population that…
- Explain how sympatric speciation can occur within a single population occupying …
- Explain how hybridisation followed by polyploidy can result in the formation of …