Genetic drift and bottle neck effect
Joe Wolfensohn
Teacher
Recall Questions
This topic requires prior knowledge of allele frequency, population genetics, and the principles of evolution through natural selection. You can test your knowledge on these below.
What is a gene pool?
The complete set of alleles present in a population at a given time.
What causes allele frequency to change in a population?
Mutation, natural selection, migration, genetic drift, and genetic bottlenecks.
What is natural selection?
The process where individuals with advantageous alleles are more likely to survive and reproduce, passing these alleles on to the next generation.
Topic Explainer Video
Check out this @JoeDoesBiology video that explains hardy weinberg or read the full notes below. Once you've gone through the whole note, try out the practice questions!
What is a Genetic Bottleneck?
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A genetic bottleneck is an event that drastically reduces the size of a population and its gene pool.
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Often caused by:
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Natural disasters (e.g., floods, volcanic eruptions)
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Disease outbreaks
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Human activities (e.g., hunting, habitat destruction)
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As a result:
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Many alleles are lost from the gene pool.
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The new population has reduced genetic diversity.
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Rare alleles may be lost entirely or become more common by chance (if the surving members of the population had these rare alleles).
Example
- Cheetahs have undergone severe bottlenecks in the past. Today, they have extremely low genetic diversity, making them more vulnerable to disease and environmental change.
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What is Genetic Drift?
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Genetic drift is a random change in allele frequencies within a population over time.
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It occurs due to chance rather than selection, especially in small populations.
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Alleles can:
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Become more common
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Disappear completely
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Unlike natural selection, genetic drift is not driven by a selective advantage.
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Genetic drift has a stronger effect in small populations
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It can lead to reduced genetic variation as some alleles may be lost from the population.
Example
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In a small island population of birds, if 3 out of 10 birds with the only copy of a rare allele die by chance, that allele may be lost completely- even if it had no negative impact.
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Key Terms
- Genetic bottleneck: A sharp reduction in population size that reduces genetic diversity.
- Genetic drift: Random changes in allele frequencies in a population over time.
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Exam Tips
Make sure to distinguish random genetic drift from directional natural selection.
Identify that genetic drift has a greater effect in small populations.
If asked about consequences, include loss of genetic diversity, increased risk of extinction, and reduced adaptability.
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In 1982, a volcanic eruption on a Pacific island caused a drastic reduction in the population of a flightless bird species. Fewer than 20 individuals survived. Over time, researchers noticed that the new population had a much lower genetic diversity and a higher frequency of a rare inherited leg deformity.
Explain how a genetic bottleneck and genetic drift could account for the increased frequency of the leg deformity in this population. (4 marks)
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The volcanic eruption caused a genetic bottleneck, reducing population size dramatically.
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This led to a loss of alleles from the gene pool, including potentially beneficial alleles.
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The leg deformity allele may have been present by chance in the surviving individuals.
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Genetic drift in the small population increased the frequency of the deformity allele over generations.
Practice Question
Try to answer the practice question from the TikTok on your own, then watch the video to see how well you did!