The random changes in gene frequencies in a population occurring by chance alone rather than by natural selection are called genetic drift. Although genetic drift occurs in all populations, its affects are most marked in very small isolated population. Two important examples of genetic drift are founder effect and bottleneck effect.
Founder effect is an important example of genetic drift in human population. It is noted when a small group of persons called founders, leave their homes to find a new settlement, the population in a new settlement may have different genotype frequencies from that of parent population.