What is Genetic Drift?
Genetic drift occurs as an evolutionary change in the allele frequencies of a population. It occurs in very small areas, but the effects are powerful.
This is due to an error in the selection of alleles from the genome of the current generation to the next generation. This is not due to any environmental factors.
In large populations allele frequencies of genes remain constant because genes do not affect fitness and there is no natural selection pressure on the allele.
Types of Genetic Drift
- In the bottleneck effect, population densities decrease sharply due to competition, predators and disease.
- The frequency of certain alleles in a population changes because the organisms that carry them are removed. The number of others increases because they are the only remaining alleles.
- This is seen in natural disasters such as volcanic eruptions and earthquakes, which kill most people.
- Population establishment in a backyard occurs due to a physical or geographic barrier to the founder’s action.
- The newly created population is inconsistent and incompatible with the original population.
- As a result, the allele frequencies of the new population will be different from the original population.
- Many species are found only on one island. This is because of the foundation effect. For example, two birds of the same species find an island. Their alleles will be responsible for the mutation on that island.
- Those alleles will become dominant, and population changes will create new species. The new population will diverge to the extent that it will not reproduce among itself.
What causes Genetic Drift?
Genetic drift usually occurs in small numbers. In a subpopulation with many alleles, each allele may be extinct. In a population of many organisms, loss of an entire allele is rare. Because many organisms have alleles, not all alleles can be acquired.
If the allele affects the organism so that the allele causes more reproduction, the allele frequency increases. If the allele is harmful to the organism, the allele frequency decreases. When the frequency of an allele increases or decreases in one rare living organism, it is known as genetic drift.
Genetic Drift Example
- The American bison was once hunted to extinction. The population found today shows very little genetic variation.
- Consider a group of rabbits, hairy and white, where white is the dominant allele. Due to genetic drift, only the brown population can survive, all whites are eliminated.
- The brown- and blue-eyed couple have brown- and blue-eyed children. Even with a 50% chance of blue, the dominant allele for brown eyes, all children can have brown eyes by chance in future generations.
- That bird has an allele for two different sized beaks. Genetic drift can eliminate beak size from a population, thus reducing genetic diversity in the birds’ genome.
- Imagine a plant that produces blue or yellow flowers. If the yellow flowers are destroyed in a fire and the blue allele is dominant, the plant will produce only blue flowers.