Genetic drift corresponds to a process of random change in the allele frequencies of a population.
Genetic drift is a stochastic process, making it impossible to predict the direction of change. This means that changes occur at random and not by adapting to the environment .
Fires, deforestation, floods and other types of changes in the environment can reduce the size of a population.
This can occur to the point that the surviving individuals do not represent a genetic sample of the primitive population. These drastic changes in the size of a population can change the frequency of an allele.
What are the consequences of genetic drift?
Genetic drift removes genetic variation . Since alleles are fixed or lost due to genetic changes, they can be neutral, harmful or advantageous.
Small populations are more sensitive to this process, occurring more quickly. In larger populations, it takes many generations to eliminate or fix an allele.
How does Genetic Drift occur?
Genetic drift can occur in two ways and at different times in a population’s evolutionary history.
The two forms are the founding effect and the bottleneck effect:
Founder Effect
This case of genetic drift occurs when a new population is founded by a few individuals. This is because the primitive population has been drastically reduced or because some individuals have migrated to another area.
In both cases, a new population is made up of few members of the original population. However, these few founders do not contain the total genetic variation of the original population. Thus, the new population has reduced genetic variation.
Example of a founding effect on the human species
We have as an example the religious communities in Germany that migrated to the United States. Because of their beliefs, community members remained isolated from the American population.
From the analysis of the allele frequency of the community members, significant differences were observed in relation to the North American population.
It is concluded that this population did not represent a representative sample of the original German population and its allele frequencies were different from the American population.
Bottleneck effect
The bottleneck effect is a drastic reduction in population size. It occurs when the population size is reduced by at least a generation. As a result of the bottleneck effect, genetic variation is reduced.
The bottleneck effect can be caused by natural disasters, predation, human hunting, loss of habitats, reduced migration, among others. These events can randomly eliminate many members of the population, regardless of their genotypes.
Survivors start a new population, most of the time, in the same area occupied by the original population. The main difference between the bottleneck effect and the founding effect is the existence of migrants in the founding effect.
Bottleneck Effect Example
An example of a bottleneck effect is the case of northern elephant seals. Intense hunting has reduced the population to a few dozen individuals.
Its population reached about 20 individuals at the end of the 19th century. However, its populations have exceeded 30,000 since then.
However, their genes still carry much less genetic variation compared to southern elephant seals, which suffered less from predatory hunting.
Genetic Drift and Natural Selection
Genetic drift, natural selection , mutation and migration are basic mechanisms of evolution.
Genetic drift alters the population’s allelic frequency at random. It does not work to produce adaptations.
In the process of natural selection, the individuals most adapted to a given ecological condition are selected. It does not act randomly.