Genetic drift

From RationalWiki

Genetic drift is the tendency of populations of organisms to become more homozygous over time due to statistical effects.

Genetic drift occurs within all populations, although it is more pronounced in small, isolated populations. Traits held by a smaller number of individuals within the population tend to be removed from the population, while traits of the majority tend to become "set" among all members of the population.

Genetic drift counters the increase in genetic diversity caused by mutation.