Genetic drift resulting from a disaster that drastically reduces population size is called

Bottle neck effect

Explanation:

Genetic drift occurs in a small population which signifies a change in the allele frequency as a result of random sampling. Bottleneck effect is one of the extreme case of genetic drift which occurs due to a natural calamity or natural disaster.

In bottleneck affect the population size of a species is reduced drastically because most of the individuals in their population are killed in a natural disaster like flood, volcanic eruption, etc.  

Due to mass killing, some beneficial alleles also get lost with those organisms reducing the gene pool of the population that survived the natural disaster. So the correct answer is the bottleneck effect.

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