Genetic drift is also known as allelic drift or the Sewall Wright effect. It is the change in allelic frequency of a population over generations due to random chance.
It may occur in all population sizes but particularly its effect is strongest in small populations. This is because, in a large population, there is less chance of losing an entire allele, as many organisms contain alleles and all alleles cannot be wiped away.
Genetic drift causes a severe reduction in the gene pool. It is seen in population after population bottlenecks. Population bottlenecks can be defined as the sharp reduction in the size of the population due to environmental activities (like earthquakes, diseases, predation, flood, etc) or human activities (like hunting).
There are two effects that cause genetic drift.
Bottleneck effect
This effect occurs when the size of the population is reduced for at least one generation due to natural calamities or anthropogenic causes.
Bottleneck effects are evolutionary occurrences that cause founder populations with the potential to experience genetic drift by stochastically reducing the genetic diversity of a population.
Example:
- Northern elephant seals’ genetic diversity has likely decreased as a result of a population bottleneck due to hunting in the 1890s. The population was reduced to about 20 individuals at the end of the 19th century. Although the population has now increased to around 30,000, the bottleneck effect can be seen as they have far less genetic diversity than southern elephant seal populations that were not hunted intensely.
Founder Effect
In this effect, a small population is founded in a new location from a large original population due to geographical barriers. This new population now cannot interact and mate with the original population.
Over a long time, the allelic frequencies of the new population will be different from the original population. These alleles will dominate and due to mutation, a new species will be formed that will no longer breed with the original population.
Example:
The majority of the Afrikaner Dutch settlers in South Africa are descended from a small number of colonists. The Afrikaner population has an unusually high frequency of the gene that causes Huntington’s disease because those original Dutch colonists just happened to carry that gene with unusually high frequency.
Effects of genetic drift
- Loss of genetic variation.
- Reduces the robustness or fitness of the population.
- Chance of extinction or recolonization.
- Loss of rare alleles and decreased gene pool.
- Loss of ability to adapt to severe environmental conditions.
- Due to the small population, the chance of inbreeding increases.
- Increase in genetic homogeneity.
- Inbreeding depression in species.
- Formation of the new population which may be distinct from the original population.
- Evolution of new species.
