Evolution
Bottleneck events: when a large population is suddenly and drastically reduced in size by some event, the genetic variety will be very much reduced. Infections and extreme climate events are frequent causes. Occasionally, invasions by more competitive species can be devastating. ♦ In late 1800s, hunting reduced the Northern elephant seal to only about 20 individuals. Although the population has rebounded, its genetic variability is much less than that of the Southern elephant seal.♦ Cheetahs have very little variation. We think the species was reduced to a small number at some recent time. Because it lacks genetic variation, it is in danger of infectious diseases.