It is a mechanism by which evolution develops over generations due to change in allele frequencies of a population that occurs by chance.
There are two types of genetic drift: The founder effect and the Population bottleneck
Founder effect:
This type of genetic drift occurs when some individuals of a population migrate to a new location and colonize there to form a new population.
The founder effect reduces the genetic variability of a population and the size of the population.
Population bottleneck:
The type of genetic drift which occurs due to catastrophic environmental events, hunting of particular species that are near to extinction, and destruction of habitat.
This reduces the size of the population very quickly and genetic variability also.