The term would be genetic drift.
Genetic drift, also known as allelic drift, is one of the basic mechanisms of evolution, in which random changes in alleles from generation to generation in a population occur.
Genetic Drift
Reason -
In a genetic drift the allele frequencies change randomly from one generation to other. It occur in population of all sizes but it has highest impact on the smallest population. The alleles are either lost or fixed up to 100% frequency in a genetic drift. It produces two effects a) bottleneck effect – where a population reduces due to genetic change introduced by a natural disaster and b) Founder Effect - when a small population group separates from the main population to develop into a separate colony
