A natural disaster wipes out most of a population. The allele frequencies of the surviving population may not be the same as the allele frequencies of the original population. This form of genetic drift is called

Bottleneck effect

Explanation:

In bottle neck effect, the amount of genetic variation of a population reduces in lieu to its inability to adapt to new selection pressure such as climatic change, natural disaster etc. thereby reducing the size of the population by at least one generation.

For example -

The genetic variation of Northern elephant seals is caused by the human inflicted bottleneck i. e excessive hunting