Genetic drift

Genetic drift helps in the process of speciation (formation of new species) as well as in evolution. It is a mechanism of evolution responsible for random changes in a population.

The concept of Genetic Drift is given by Sewall Wright. Genetic drift is defined as the random and directionless change in the allele frequency simply as a result of chance from one generation to the next generation in a small population.

Let us understand it with the help of an example.

Suppose there are 9 beetles in a population. Out of 9, 3 are green beetles and the remaining 6 are brown beetles. At random a person passed nearby beetles and by mistake, he steps up on those beetle populations. Due to this casualty, 2 green beetles died. Now the remaining population of beetles consist of 1 green beetle and 6 brown beetles.

So overall this casualty is not preplanned. It happened randomly. It occurred by chance. The effects are also seen here because of the presence of a small population. If there is a large population, it may be difficult to examine the effect of such a casualty. If there are 100 beetles and only 4 are killed then obviously it will not affect the population. As the initial population is small in no. i.e. 9 so the killing of 2 beetles not going to affect the overall population, that is why we used to call it that the genetic drift is seen in a small population.

The genetic drift is divided into two categories-

Bottle-neck effect and Founder effect. Whenever genetic drift occurs, it may occur either by the bottleneck effect or by the founder effect.

Bottle-neck effect

By definition, the bottleneck effect occurs when there is a disaster that reduces a population, which rarely represents the actual genetic makeup of the initial population. This leaves smaller variation among the surviving individuals.

There is a bottle with a narrow neck. It has balls of different colours (white, yellow and blue). When we turn the bottle upside down we don’t know what colour ball will come out. So there is no pre-deciding factor. This is pure coincidence. It is decided randomly. The ball that is near the neck is more likely to come out of the bottle. So those balls which come out of the bottle represents a surviving population and the ball which are still i.e. yellow in the bottle represent the original population. If we compare balls with alleles, then the blue and white alleles represent the surviving population without telling about the original population. So in this way, the overall allele frequency changed.

It happens due to natural calamity. A natural disaster does not decide which population will be wiped out, it is completely a random process. During this process, it may be possible that the strongest person is wiped out or the weakest person may still survive. There is no pre-planned task.

This may occur due to environmental disasters, hunting of species or habitat destruction.

Founder effect

It is a loss of genetic variation that occurs when a new population is established by a small no. of individuals from the original population.

For example, there is a population of beetles composed of red and yellow beetles. Let’s say red beetles move to an Island and establish their population there. There is a change in gene frequency by chance. Due to this, the new population is completely different from the original population.

Properties of genetic drift

• Involve in the evolutionary process
• Random changes
• It has drastic effects on a small population
• Brings homogeneity
• This leads to the formation of new species
• It reduces genetic variation
• It acts on genotypic frequency
• It is driven by chance, not selection

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