A bottleneck effect is an ecological phenomenon in which the population of a species is drastically reduced to the point where the species is still able to carry on, but the genetic diversity of the species is severely limited. This type of event only occurs when members of the population are killed at random, and their death has nothing to do with genetic flaws or inability to adapt.
There are a very specific set of events that can cause a bottleneck effect, because bottlenecks can only be caused by factors that kill members of the population indiscriminately. If a plague sweeps through a population and kills individuals who have a certain genetic makeup more than others, it cannot be considered a bottleneck situation because it is simply natural selection. Bottlenecks usually occur after earthquakes, tsunamis or overhunting, because these events kill indifferently.
Bottlenecks are harmful to populations because they leave only a few members of the species left to reproduce. This means much of the gene pool is lost and the species must be rebuilt from the genetic makeup of only a few individuals. This lack of genetic diversity occasionally makes populations more susceptible to genetic conditions or diseases.
A classic example of a bottleneck is the elephant seal population, which was hunted almost to extinction, explains a University of California website. The species managed to rebuild its population from only 20 members, but scientists have compared the hunted population to another population which was not hunted to the same extent, and found the hunted population had less genetic diversity.