The bottleneck effect occurs when there is a disaster of some sort that reduces a population to a small handful, which rarely represents the actual genetic makeup of the initial population. In essence, it is the sharp lowering of a population's gene pool because of an environmental or human-caused change.
The bottleneck effects leaves smaller genetic variation among the surviving individuals. It might turn out to be an advantage for the surviving individuals as the environment changes, but it also might not be. Having diversity of characteristics in a population's gene pool is almost always helpful when a disease or other calamity comes along. Earthquakes, drought, flood, fire and disease are all potential natural causes of the bottleneck effect. Humans can also cause bottlenecks with actions such as hunting or deforestation.
The population decline of the northern elephant seal is an example of the bottleneck effect. Hunting reduced their population size to as few as 20 individuals at the end of the 19th century, drastically reducing their genetic variation. Since then, their population rebounded to over 30,000, but their genes still carry the marks of their bottleneck.
The Toba catastrophe theory provides another example of a bottleneck. Evidence suggests that humans went through a bottleneck about 70,000 years ago during the eruption of the Toba supervolcano in Indonesia. The theory states that the volcano reduced the human population to perhaps 10,000 to 30,000 individuals, partly based on the low level of genetic variation in humans.