Extinction

A species is extinct when the last individual has died. Extinction is permanent: the species' unique combination of alleles is lost for ever. A species is extinct in the wild when it survives only in captivity or cultivation.Extinction is a natural part of evolution. Species die out when they cannot adapt quickly enough to changes in their environment or to new competitors, predators or diseases. More than 99% of all species that have ever lived are now extinct. The normal, slow rate at which this happens is called the background extinction rate.A mass extinction is a period when a large proportion of species (usually defined as over 75%) die out in a geologically short time. There have been five, including the end-Permian extinction about 252 million years ago and the end-Cretaceous extinction about 66 million years ago, which wiped out the non-bird dinosaurs after an asteroid impact.Current extinction rates are estimated to be tens to hundreds of times higher than the background rate, which has led many biologists to describe the present as a sixth mass extinction, driven by humans.Once a population becomes very small it can enter an extinction vortex: low numbers lead to inbreeding and loss of genetic diversity, which reduce fitness and reproduction, so numbers fall further. Species most at risk are those that are large, breed slowly, have a small range, are specialised feeders or live on islands.It can be hard to be sure a species is extinct, so it is usually only declared after thorough surveys have failed to find it over many years.