Describe how a genetic bottleneck can reduce the genetic diversity of a population. Include in your answer an explanation of why reduced genetic diversity makes a species more vulnerable to extinction.
Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).
Cheetahs are thought to have experienced a genetic bottleneck thousands of years ago. Today, cheetahs show very little genetic variation compared to most other mammals.
Model answer (5 marks)
A genetic bottleneck occurs when a population is drastically reduced in size, for example by a natural disaster or disease. During this reduction many alleles are lost from the gene pool because only a few individuals survive. The surviving individuals therefore carry only a small range of alleles, so the remaining population has low genetic diversity.
If the environment changes or a new disease appears, individuals with alleles that provide resistance or adaptation may not exist in the population. Natural selection cannot act on variation that does not exist, so the population cannot adapt, increasing the risk of extinction.
If the environment changes or a new disease appears, individuals with alleles that provide resistance or adaptation may not exist in the population. Natural selection cannot act on variation that does not exist, so the population cannot adapt, increasing the risk of extinction.
Examiner tips
- Use the term "genetic bottleneck" and explain the size reduction first. Include the loss of alleles and the resulting low diversity. Then link low diversity to vulnerability by citing lack of adaptive alleles and inability of natural selection to act. Keep each point concise to fit 5 marks.
Common mistakes
- Confusing a bottleneck with a founder effect; they are different events.
- Failing to explain why low diversity makes a species vulnerable – e.g., missing the point about lack of adaptive alleles and natural selection.
Mark scheme (5 marks)
- A genetic bottleneck occurs when the population size is drastically reduced
- Many alleles are lost from the gene pool
- The surviving individuals only carry a small range of alleles, so the remaining population has low genetic diversity
- If the environment changes or a new disease arises, individuals with alleles that provide resistance / adaptation may not exist in the population
- Natural selection cannot act on variation that does not exist, so the population cannot adapt, increasing the risk of extinction
Key terms in this question
genetic bottleneck · genetic diversity
Related
- All AQA A-Level Biology (7402) revision notes →
- How to answer a "Describe" question →
- Decode the mark scheme abbreviations →