Hello! Your answer is the bottleneck effect.
Speaking more broadly, genetic drift is when a population becomes smaller, and there are fewer genotypes in the gene pool. There are two types of genetic drift: the bottleneck effect and the founder effect.
The bottleneck effect is when an environmental change / human activity causes many members of the population to die, which means that the gene pool is now represented by just a few organisms. This is correct in this scenario, as an environmental change caused the gene pool to be reduced.
The founder effect is when a new colony is established elsewhere, so only alleles in the smaller founder group will be reproduced.
Hope this helps!