What causes genetic drift?
When a very small group of living organisms suddenly get isolated from the original population (may be due to outward migration/due to natural calamity) [genetic drift] (https://socratic.org/biology/evolution/genetic-drift-1) takes place.
A sudden random change in allele frequency within a gene pool is called genetic drift.
Genetic drift obviously occurs in a small, isolated population and it takes place entirely by chance. It was first described by biologist Sewall Wright.
Genetic drift reduces variability, hence frequency of some alleles suddenly increase. This phenomenon provides an immense evolutionary potential.