Extinction

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.

Mass extinctions in earlier eras

There have been periodic mass extinctions, in which many species have disappeared in a relatively short period of geological time.

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.

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Are we in the middle of a mass extinction?

The rate of species extinction, now and in the past, is highly controversial. Some believe that human influences are causing a current mass extinction of species; others believe this thesis to be false, and the result of poor reasoning based on faulty data. Part of the difficulty in resolving this question is that we do not have a thorough count of the number of living species; estimates vary by an order of magnitude