What is genetic drift?

Genetic drift is a change in allele frequencies brought about by chance sampling of alleles between generations rather than by selection, and its effect is much greater in small populations.

What is the founder effect?

A small number of individuals leaves a population and establishes a new one, carrying only a small and unrepresentative sample of the original gene pool, so the new population has different allele frequencies and less genetic diversity.

What is a genetic bottleneck?

A genetic bottleneck occurs when a population is drastically reduced in size by an event such as a famine, an epidemic or a natural disaster, so the survivors carry only a fraction of the original alleles and the recovered population is far less diverse.

Why is genetic drift a much weaker force in a large population?

In a large population the alleles passed on are a large sample of the gene pool, so chance deviations from the parental frequencies are small and tend to cancel one another out.

Ellis-van Creveld syndrome is very rare worldwide but common in one Amish community in Pennsylvania. Explain why.

The community was founded by a very small group of settlers, at least one of whom carried the recessive allele, and it has grown in isolation with very little marriage outside it, so the allele frequency has remained far higher than in the general population.

OCR A-Level Biology B H422

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Population Genetics and Epigenetics

Genetic drift, the founder effect and speciation

3 things to know
Components 01, 02 and 03

Overview

Recall questions on genetic drift, the founder effect and genetic bottlenecks, Ellis-van Creveld syndrome in the Amish of Pennsylvania, ABO frequencies across populations, the risks of a reduced gene pool, and geographical and reproductive isolation in speciation, African primates and humans.

What you need to know

Answer each one before you reveal it.

Your recallNot started

Key term

Why does a reduced gene pool put a population at risk?

Key term

Modern humans are spread across the whole world yet remain one species. Why?

Process

Name two processes other than natural selection that change allele frequencies in a population.

Questions from this subtopic

Each one with the model answer underneath.

1
Name two processes other than natural selection that change allele frequencies in a population.
Model answer
The founder effect and genetic bottlenecks, both forms of genetic drift, change allele frequencies by chance rather than by selection.
2
What is genetic drift?
Model answer
Genetic drift is a change in allele frequencies brought about by chance sampling of alleles between generations rather than by selection, and its effect is much greater in small populations.
3
What is the founder effect?
Model answer
A small number of individuals leaves a population and establishes a new one, carrying only a small and unrepresentative sample of the original gene pool, so the new population has different allele frequencies and less genetic diversity.
4
What is a genetic bottleneck?
Model answer
A genetic bottleneck occurs when a population is drastically reduced in size by an event such as a famine, an epidemic or a natural disaster, so the survivors carry only a fraction of the original alleles and the recovered population is far less diverse.
5
Why is genetic drift a much weaker force in a large population?
Model answer
In a large population the alleles passed on are a large sample of the gene pool, so chance deviations from the parental frequencies are small and tend to cancel one another out.
6
Ellis-van Creveld syndrome is very rare worldwide but common in one Amish community in Pennsylvania. Explain why.
Model answer
The community was founded by a very small group of settlers, at least one of whom carried the recessive allele, and it has grown in isolation with very little marriage outside it, so the allele frequency has remained far higher than in the general population.

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