Genetic drift is a random change in allele frequencies from one generation to the next. It matters because it can change a population even when no allele gives an advantage. The effect is strongest in small populations, where chance events can remove or increase alleles quickly.
Over time, drift can reduce genetic variation and make populations more different from each other.
Drift happens because only some individuals survive and reproduce, and the alleles they pass on are a random sample of the gene pool. A bottleneck occurs when a population is suddenly reduced, while a founder effect occurs when a few individuals start a new population. Both events can make allele frequencies very different from the original population.
Unlike natural selection, genetic drift is not guided by fitness or adaptation.
Key Facts
- Genetic drift = random change in allele frequencies across generations.
- Allele frequency = number of copies of an allele / total copies of the gene in the population.
- Drift is strongest in small populations because random sampling error is larger.
- A bottleneck effect occurs when a population crash leaves a small, unrepresentative gene pool.
- A founder effect occurs when a new population begins from a few individuals with limited alleles.
- Natural selection changes allele frequencies because of fitness differences, while drift changes them by chance.
Vocabulary
- Allele
- An allele is a version of a gene, such as one version for dark color and another for light color.
- Allele frequency
- Allele frequency is the proportion of all gene copies in a population that are a particular allele.
- Genetic drift
- Genetic drift is random change in allele frequencies caused by chance events in survival and reproduction.
- Bottleneck effect
- The bottleneck effect is genetic drift that happens after a population is sharply reduced in size.
- Founder effect
- The founder effect is genetic drift that happens when a small group starts a new population with only part of the original gene pool.
Common Mistakes to Avoid
- Saying genetic drift always improves a population is wrong because drift is random and can increase harmful, neutral, or helpful alleles by chance.
- Confusing genetic drift with natural selection is wrong because selection depends on differences in survival or reproduction, while drift does not.
- Ignoring population size is wrong because the same random event has a much larger effect in a small population than in a large population.
- Assuming a bottleneck preserves the original allele frequencies is wrong because the survivors may be a random, unrepresentative sample of the original population.
Practice Questions
- 1 A beetle population has 40 copies of allele A and 60 copies of allele a. What are the allele frequencies of A and a?
- 2 After a storm, only 10 birds survive from a population. Among them, 16 of 20 total gene copies are allele B. What is the frequency of allele B in the surviving population?
- 3 Two populations have the same allele frequencies today, but one has 20 individuals and the other has 2,000. Explain which population is more likely to show large genetic drift over the next few generations and why.