when an organism moves from one population to the other the previous population loses some genes and the new population gains some this is called genetic drift.
if genetic drift happens many times it is termed as gene flow.
In a small population (having less than hundred members) of a species change rather than natural selection may play an important role in determing in the compostion of the next generation In a small interbreeding population, there is a relatively high probility of heterozygous gene pair to become homozygous for one celeleur than other by chance. this may lead to the accumulation of certain disavantageous traits and the group with such traits may be eliminated. The tendency in small populations to become homozygous for one allele or other by chance rather than selection has been called genetic drift.