Genetic Drift

Genetic drift is an evolutionary change in allelic frequencies of a population as a matter of chance. It occurs in very small populations but its effects are strong. It occurs due to an error in selecting the alleles for the next generation from the gene pool of the current generation. It does not occur due to any environmental influences.In large populations allele frequency of the genes remains relatively stable because the genes are not affecting fitness and do not have a natural selection pressure against the alleles.

Types of Genetic Drift

Bottleneck Effect

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In the bottleneck effect, the population size severely decreases due to competition, predators, or diseases.

The frequency of certain alleles in a population change because the organisms that carry them are eliminated. The others increase in number because they are the only alleles left. This is observed when people do not complete the course of antibiotics.

When a person takes an antibiotic, it kills all the harmful bacteria. If the person stops taking the dose, a small population of the bacteria is left in the body which might be different from the original one. The new alleles will be the dominant ones in the population now and the gene pool will be determined by the organisms which did not die.

Founder Effect

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In the founder effect, a new population is founded in a new location due to physical or geographical barriers.

The new population formed does not interact and mate with the original population.

As a result, the allelic frequencies of the new population will be different from the original population.

There are many species that are found only on a particular island. This is due to the founder effect. For eg., two birds belonging to the same species reach an island. Their alleles will be responsible for the diversity on that island.

These alleles will dominate and mutations in the population will lead to the formation of new species. The new population will diverge to such an extent that they will no longer interbreed.

What Causes Genetic Drift?

Genetic drift usually occurs in smaller populations. In a small population with many alleles, any of the alleles can become extinct. In a population with many organisms, there is less chance of losing an entire allele. This is because many organisms contain the alleles and all the alleles cannot be wiped away.

If the allele affects the organism such that it causes more reproduction of DNA, the allele frequency increases. If the allele harms the organism, the allele frequency decreases. When the allele frequency increases or decreases because of its presence in some random organism that survived, it is known as genetic drift.

Genetic Drift Example

Genetic drift can be understood well with the following examples:

• The American Bison was once hunted to such an extent that it became endangered. The population which has recovered today shows very little genetic variation.

• Let's take an example of a group of rabbits with brown fur and white fur, white fur being the dominant allele. Due to the process of genetic drift, only the brown population might remain, with all the white ones eliminated.

• It should be noted that a child will have blue or brown eyes if either of the parents has blue or brown eyes. A couple with brown and blue eyes will have children with brown or blue eyes. Even if there is a 50% chance of having blue eyes, brown eyes being the dominant allele, all the children might have brown eyes in the future generations as a matter of chance.

• A bird has an allele for two different sizes of beaks. Genetic drift might eliminate one of the beak sizes from the population, thus reducing the genetic variations of the gene pool of birds.

• Hypothesize a plant that produces blue or yellow flowers. If the yellow flowers are destroyed in a fire and the blue allele is the dominant one, the plant will produce only blue flowers.

Genetic Drift vs Gene Flow

Gene flow can be defined as the movement of genes between populations, species, or organisms. For eg., bacteria can transfer genes between different cells. On the other hand, genetic drift can be defined as the random selection of genes in a population. The gene flow occurs when one individual from one population migrates to another and breeds there. Gene flow does not evaluate the allele frequencies, unlike genetic drift.