Genetic drift corresponds to a process of random change in the allelic frequencies of a population.
Genetic drift is a stochastic process, making it impossible to predict the direction of change. That means that changes occur by chance and not by adaptation to the environment.
Fires, deforestation, floods and other types of changes in the environment can reduce the size of a population.
This can happen to the point where 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. Because they are random changes, the alleles fixed or lost by genetic drift can be neutral, deleterious 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.
Also read about Genetic variability.
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 formed by a few members of the original population. However, these few founders do not contain the full genetic variation of the original population. Thus, the new population has reduced genetic variation.
Example of founding effect in the human species
We have as an example the religious communities from Germany that migrated to the United States. Because of their beliefs, community members have kept themselves isolated from the American population.
From the analysis of the allelic frequency of 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 allelic frequencies proved to be different from the American population.
bottleneck effect
The bottleneck effect is a drastic reduction in population size. Occurs when population size is reduced for at least one generation. As a result of the bottleneck effect, genetic variation is reduced.
The bottleneck effect can be caused by natural disasters, predation, human hunting, habitat loss, 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 founder effect is the existence of migrants in the founder effect.
Bottleneck Effect Example
An example of a bottleneck effect is the case of the northern elephant seals. Intense hunting reduced the population to a few dozen individuals.
Its population reached about 20 individuals at the end of the 19th century. However, their populations have surpassed 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
The genetic drift, natural selection, mutation and migration are basic mechanisms of evolution.
Genetic drift changes the allelic frequency of a population in a random way. It doesn't work to produce adaptations.
In the process of natural selection, the individuals best adapted to a certain ecological condition are selected. It doesn't act randomly.
