Extinction is a name we give to the disappearance of an animal group, such as species, from the evolutionary record. Extinction is considered to be the death of the last individual of that species. Extinction is not an unusual event, as species are created by speciation, and disappear through extinction. Extinction is a natural phenomenon, it is estimated that 99.9% of all species that have ever lived are now extinct. Nearly all animal and plant species that have lived on earth are now extinct, and extinction appears to be the ultimate fate of all species. These extinctions have happened continuously throughout the history of life, although the rate of extinction spikes in occasional mass extinction events. There are at least two ways in which a species may become extinct. First, the species may develop a way of life such that a change in the environment would prevent its persistence. This is the negative role of environmental selection in evolution.
Second, one species may become extinct when it is transformed into another. A species may be a segment of a continuous, progressive evolutionary lineage. The species of one time period in which this lineage exists is the ancestor of the succeeding species in the next time period. The ancestral species becomes extinct through the processes by which it is transformed into its descendants. The early Pleistocene hominids, the australopithecines are extinct, yet it is likely that some direct descendants of australopithecine genetic material exist in modern Homo sapiens.
The Cretaceous–Tertiary extinction event, during which the non-avian dinosaurs went extinct, is the most well-known, but the earlier Permian–Triassic extinction event was even more severe, with approximately 96 percent of species driven to extinction. The Holocene extinction event is an ongoing mass extinction associated with humanity’s expansion across the globe over the past few thousand years. Present-day extinction rates are 100–1000 times greater than the background rate, and up to 30 percent of species may be extinct by the mid 21st century. Human activities are now the primary cause of the ongoing extinction event; global warming may further accelerate it in the future. The role of extinction in evolution is not very well understood and may depend on which type of extinction is considered. The causes of the continuous “low-level” extinction events, which form the majority of extinctions, may be the result of competition between species for limited resources (competitive exclusion). If one species can out-compete another, this could produce species selection, with the fitter species surviving and the other species is driven to extinction. The intermittent mass extinctions are also important, but instead of acting as a selective force, they drastically reduce diversity in a non-specific manner and promote bursts of rapid evolution and speciation in survivors.
Pseudoextinction
Extinction of a parent species where daughter species or subspecies are still alive is also called pseudoextinction. Many of prehistoric extinct species have evolved into new species; for example the extinct Eohippus (an ancient horse like animal) was the ancestor of several extant species including the horse, the zebra and the donkey. The Eohippus itself is no more, but its descendants live on. It is therefore said to be pseudoextinct