Bottleneck Effect: The bottleneck effect occurs when the population size is reduced drastically due to natural calamities such as earthquakes, floods, etc. The surviving species form the new population. The allele frequency of the new population will be completely different from the original allele frequency.
Founder Effect: The founder effect occurs when a small population dissociates from a larger population and forms a new colony. The allele frequencies of both the populations are different.
The concept of bottleneck effect and the founder effect are similar. The only difference is in the mechanism.