Change/error in the gene pool of a population is called genetic drift.
When a change or an error occurs in a said gene pool, it is called gene drift. It occurs due to an event strictly based on chance. A particular genetic trait is either lost or spreads in the said population. "Population bottlenecks and the founder effect" are the two types of gene drifts.
Bottlenecks effect: Drastic degradation in the size of a population due to a large-scale environmental hazard.
Example: Floods
Founder effect: Formation of separate colonies by a few individuals of the said population. It results in a decrease in population as well as genetic variation.
Example: Colonization of certain individuals carrying a contagious virus.
