Changes in the genetic composition of a population due to unrepresentative random sampling of individuals.
Populations are not static: they are always changing and adapting to the environment where they live.
When a population is isolated, there is no way of enriching its gene pool since there is no exchange of individuals with other populations.
Besides, if the population consists of few individuals reproducing, their offspring will have a tiny random sample of the gene pool of the original population.
How it works:
In small, isolated populations, some alleles may be lost after several generations due to sampling errors.
Even if the original population has considerable genetic variation, the composition of genes will change after several generations: the population will inevitably undergo genetic drift.
This effect happens for the same reason that when we toss a coin ten times, although we know that the probability of getting heads is 0.5, it may happen that we get seven heads and three tails, instead of the expected 5 and 5. We will have to toss the coin countless times to get the same number of heads and tails.