Final answer:
The bottleneck effect occurs when a population gets smaller due to a natural disaster, resulting in a change in the genetic structure of the population.
Step-by-step explanation:
The bottleneck effect occurs when a population suddenly gets much smaller, usually due to a natural disaster. This results in a large portion of the genome being wiped out, and the surviving individuals contribute to the genetic structure of the entire population. It is possible for a species to undergo a population bottleneck without a significant genetic bottleneck if the surviving individuals have a similar genetic makeup to the pre-disaster population.