In general, evolution is brought about by three major forces - mutation, natural selection, and genetic drift. Genetic drift is a random alteration in the frequency of allele (an alternative form of a gene), that results in a change in the composition of genes in the population.
The changes take place slowly and become noticeable over time. However, it is more rapid and significant, in smaller populations, since an allele represents a large fraction in the genetic pool. Due to it, phenotypes (physical appearance) and genotypes (genetic constitution) of individuals change over time, resulting into variations in population.
Understanding Genetic Drift
Even though genetic drift plays an important role in the process of evolution, the concept is not clear among many of the evolutionists. The main reason may be due the fact that both natural selection and drift act on the population are not mentioned in Darwin's Theory of Evolution.
However, unlike natural selection that is a non-random mechanism of evolution, it is random and happens by chance. In natural selection, individuals that are best adapted to the prevailing environment, survive, and pass on their genes to the next generation, those who can't survive, become extinct.
In case of genetic drift, genes of individuals are passed on to the next generation, irrespective of whether they can adapt to the environment or not. Drift results in changes of genes that are not adaptive. The causes are spontaneous mutation and incomplete breeding in the small population.
For example, if by chance, some of the individuals leave behind more descendants than other individuals, then the gene pool of the population in the next generation will be represented by the offspring of those who have survived.
Genetic drift is also caused due to separation of a smaller group from a larger population. In such a condition, there is a chance of biological evolution of a species (speciation).
If a small group gets isolated from the larger group, then the small group is drifted from the remaining population. Since it is random, there may be a situation when the gene pool of the smaller group becomes totally different from the larger one. Thus, prompting the birth of a new species.
According to population genetics, drift is due to random sampling effects. The drifted alleles have a lifetime; its frequency may go up or down over a period of time. However, the drift continues until it becomes fixed.
There are two consequences of this; frequency either becomes zero and the allele gets eliminated from the population, or the frequency may reach 1 and become fixed. Thus, as a generation progresses, the alleles of a particular population are fixed. Once an allele is fixed, all members of the population will have the allele.
Natural disasters such as earthquakes, volcanoes and floods also contribute to genetic drift. Many individuals get killed in such calamities resulting reduction in size of a specific population.
The ultimate outcome is that the genetic pool (alleles of all the genes) of the population is reduced and the surviving individuals are no longer proper representatives of the original population.
This situation is referred to as the bottleneck effect. In such cases, there are chances that many of the beneficial adaptations (genes) might get eliminated. Thus, the overall gene pool of the population before the catastrophe is different from that after the catastrophe.