Answer:
Genetic drift.
Explanation:
Genetic variation is the change in the genome of the organisms. The genetic variation might occur in the population due to the natural selection, mutation and genetic drift.
As the information given in the population, the variation is possible due to the genetic drift. This mainly occur due to the random sample of the small population and might cause the complete loss of the particular gene. The genetic variation might decreases in the organisms.
Thus, the answer is genetic drift.