The word "extinction" can refer to several different phenomena. Most of the world's extinctions have been true extinctions. When a species completely dies out and leaves no descendants, it is extinct.

A few have been pseudoextinctions, when the original or ancestral species has become transformed by evolution into another species.

All species living today, including ourselves, evolved from another species.True extinctions and pseudoextinctions are both a type of global extinction.

Global extinction is the complete elimination of a particular species everywhere in the world. Many endemic species have a limited georgraphic range, such as a single island. No matter how small that area is, their disappearance from it is a global extinction if the species is not found anywhere else.

A local extinction is the extirpation of a species from a portion of its geographic range. Local extinctions mean the loss of the egenetic diversity represented by that population and the removal of that species' contribution to the local ecosystem.

Because members of the species still exist in other locations, local extinctions can be reversed if the original causes are addressed, and the species can recolonize or can be reintroduced into the area.

Unfortunately, local extinction is often the precursor to global extinction.

Extinction is not limited to application at the species level. Extinctions in the ancient past frequently are described in terms of whole groups of related species, such as a genus or a family. The farther back in time, the more difficult it is to distinguish different individual species from one another on the basis of fossils, and sometimes scientists can only tell when all the members of a genus or a family disappear.

In contrast, it is often useful categorize extinctions in the recent past by distinctions that are finer than the species level, such as subspecies and populations.

Another important type of extinction is extinction in the wild. Members of a species may exist in captive breeding programs in zoos, but if there are no individuals living in their natural habitat, that species has become extinct in the wild.

Similarly, a species may be effectively extinct if members of the species are still alive, but the species has no chance of reproducing.

These cases include those in which all the remaining individuals are of a single sex.

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