There are 2 species of squirrels in a forest; Their alleles are brown (B allele) and white (b allele).
This is a small population of squirrels, in which the 2 phenotypes have remained prevalent in this environment.
After a disastrous earthquake, a bottleneck effect (when population size is reduced and then increased in population size) occurs, and the white phenotype is lost.
After multiple generations, the B allele frequencies have increased until the entire population has the brown phenotype.
Due to genetic drift (a random change in the allele frequencies in a population to the next generation due to random chance), the entire population of squirrels has changed from half white and half brown to all brown, causing less genetic diversity.