In biology and ecology, extinction is the disappearance of a species or group of species. The moment of extinction is generally considered to be the death of the last individual of that species.
In species which reproduce sexually, extinction of a species is generally inevitable when there is only one individual of that species left, or only individuals of a single sex.
Extinction is not an unusual event in geological time -- species are created by speciation, and disappear through extinction.
There have been periodic mass extinctions, in which many species have disappeared in a relatively short period of geological time. These are covered in more detail in the article on extinction events.
The most recent of these, the K-T extinction at the end of the Cretaceous period, is best known for having wiped out the dinosaurs.
Some also believe that we are currently in a period of mass extinction right now, the Holocene extinction event. While there is no room to doubt that human activity has increased the rate of species extinction worldwide, however, the exact extent of anthropogenic extinction remains controversal.
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