Answer:
Genetic drift
Step-by-step explanation:
Genetic drift is a change in genetic allele frequencies of a population. Bottleneck effect, caused by disasters such as in this case, causes genetic drift in a very rapid way hence reducing variability in a population. Reduced genetic variability in a small population is especially threatening because the chances of population surviving environmental changes get slimmer. This is because natural selection acts of genetic variability to get a population to adapt to changes in the environment.