Explain how a bottleneck event can alter allele frequencies in a population and affect its long-term evolutionary potential.
Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).
Model answer (4 marks)
A bottleneck event drastically reduces population size, leaving only a few survivors.
The survivors carry only a random subset of the original alleles, so allele frequencies in the post‑bottleneck population differ from those of the original population by chance (genetic drift).
Rare alleles are likely to be lost entirely, reducing genetic diversity and heterozygosity.
Reduced genetic diversity limits the population’s ability to adapt to future environmental changes, increasing its vulnerability to extinction or constraining the rate of evolution by natural selection.
The survivors carry only a random subset of the original alleles, so allele frequencies in the post‑bottleneck population differ from those of the original population by chance (genetic drift).
Rare alleles are likely to be lost entirely, reducing genetic diversity and heterozygosity.
Reduced genetic diversity limits the population’s ability to adapt to future environmental changes, increasing its vulnerability to extinction or constraining the rate of evolution by natural selection.
Examiner tips
- Use the word ‘bottleneck’ to show understanding of the event. Explain that the change is due to random sampling (genetic drift). Mention loss of rare alleles and reduced heterozygosity. Link loss of diversity to reduced adaptive potential and higher extinction risk.
Common mistakes
- Confusing a bottleneck with a founder effect. Failing to mention that allele changes are random. Over‑emphasising selection instead of drift.
Mark scheme (4 marks)
- A bottleneck event drastically reduces population size due to a catastrophic event, leaving only a small number of surviving individuals.
- The surviving individuals carry only a random subset of the original alleles, so allele frequencies in the post-bottleneck population differ from those of the original population by chance (genetic drift).
- Rare alleles are likely to be lost entirely from the population, reducing genetic diversity / heterozygosity.
- Reduced genetic diversity limits the population's ability to adapt to future environmental changes, increasing its vulnerability to extinction or constraining the rate of evolution by natural selection.
Key terms in this question
bottleneck event · evolutionary potential
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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