Genetic Drift | Biology for SSS 3 PDF Download

What is 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 remain relatively stable because the genes are not affecting the fitness and do not have a natural selection pressure against the alleles.

Types of Genetic Drift

Bottleneck Effect
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 during natural disasters like volcanic eruptions, earthquake, etc. leading to the death of most of the population.

Founder Effect

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 effec.t. Eg., two birds of 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 observed in the following examples:

• The American Bison was once hunted to such an extent that it became endangered. The population which have recovered today show very little genetic variations.
• Consider a population of rabbits with brown fur and white fur, white fur being the dominant allele. Due to genetic drift, only the brown population might remain, with all the white ones eliminated.
• A couple with brown and blue eyes 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 is the movement of genes between the populations, species, or organisms. E.g., bacteria can transfer genes between different cells. On the contrary, genetic drift refers to the random selection of genes in a population. When individuals from one population migrate to some other population and breed there, gene flow occurs. Unlike genetic drift, gene flow does not evaluate the allele frequencies.