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.
Understanding Biology: Genetic Drift
Each generation is produced by a limited set of parents, not by every possible parent in a population. Imagine a gene with two alleles, one red and one blue. If both are equally common, the next generation will usually contain a similar mix, but it will not be exactly the same.
Some parents leave many offspring. Others leave none. By pure chance, more red copies may enter the next generation.
In another generation, blue copies may become more common. These small shifts can build up over many generations.
Eventually, one allele can reach fixation, meaning every copy in the population is that allele. The other allele is then lost unless mutation or migration brings it back.
Drift changes genetic diversity in a predictable direction even though each individual event is unpredictable. When an allele disappears, the population loses one possible version of a gene. Losing many alleles reduces heterozygosity.
Heterozygosity means that individuals carry two different alleles at a gene. A population with low heterozygosity has fewer genetic options when conditions change. For example, a new disease may spread through a group whose members share similar immune system genes.
Genetic variation does not guarantee survival, but it gives natural selection more material to work with. This is one reason conservation biologists pay close attention to population size and breeding patterns.
Real populations often experience drift after habitat loss, hunting, storms, fires, or disease outbreaks. A few survivors may carry an unusual set of alleles simply because they happened to survive. Their descendants can become numerous without those alleles being useful.
Island populations are often affected because they may begin with a small number of colonists and remain separated from the mainland. Different isolated groups can therefore become genetically distinct. In people, some inherited conditions occur more often in particular communities when an uncommon allele was present in a small founding group.
This pattern does not mean the allele was beneficial or harmful during the founding event. Its early frequency was shaped by who happened to have children.
Students often confuse drift with selection because both can change allele frequencies. The key evidence is the link to survival or reproduction. If individuals with one inherited trait consistently leave more offspring because that trait suits the environment, selection is a strong explanation.
If changes do not match a trait advantage, especially in a small isolated group, drift may be important. In real research, both processes can act at the same time. Scientists compare several populations, measure genetic variation, study family relationships, and test whether environmental conditions favor particular traits.
Simple coin or bead models help show the idea. Each draw represents the alleles passed to a new generation. Repeating the model produces different results, which reveals why chance has such a large effect in small groups.
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.