Some examples of a bottleneck effect are the American bison in the late 1800s, the Northern elephant seal in the 1890s and the greater prairie chicken during the 19th and 20th centuries. A bottleneck effect is when a disaster reduces a population to a small number and, in turn, its genetic makeup suffers.
When any given population's size quickly becomes very small, this is referred to as a bottleneck effect. Because the size is reduced so quickly, the genetic variation decreases as well. The Toba catastrophe is an example of a human bottleneck effect that occurred about 70,000 years ago. This was a supervolcanic eruption that resulted in a global volcanic winter that lasted anywhere from 6 to 10 years, and a cooling episode that possibly lasted for 1,000 years after that.
One type of bottleneck effect is called a founder effect. The founder effect is an instant event, and occurs when a small group of individuals leaves the larger population. In essence, this group is starting their own population, but, due to the small size of the break-out group, their genetic makeup is not likely to be similar to that of the larger group. Some examples of the founder effect are Amish communities and the blue people of Kentucky.