Extinction is the dying out or the extermination of a species. Extinction occurs when a species is diminished for a particular reason, including:

• Environmental factors (habitat fragmentation, climate change, natural disaster, overexploitation of a species), and

• Evolutionary changes in the member of a species (genetic inbreeding, poor reproduction, decline in population numbers)

The moment of extinction is usually considered to be the death of the last individual of the species, however the capacity to breed and recover may have very well been lost prior to this point, and so defining when extinction occurs in a species can be difficult.

Mass Extinction Extinctions usually occur quite regularly. However, there have been periods where extinction rates are extremely high, which is known as a mass extinction. There have been five mass extinctions which were recorded on the fossil record, these being:

• Ordovician-silurian Extinction: 440 mya

• Devonian Extinction: 365 mya

• Permian-triassic Extinction: 250 mya

• Triassic-jurassic Extinction: 210 mya

• Cretaceous-tertiary Extinction: 65 mya

The most severe mass extinction was the Permian extinction, which eliminated 95% of all marine species and about 70% of all land species.