A species is extinct when the last existing member dies. Extinction therefore becomes a certainty when there are no surviving individuals that can reproduce and create a new generation. A species may become functionally extinct when only a handful of individuals survive, which cannot reproduce due to poor health, age, sparse distribution over a large range, a lack of individuals of both sexes (in sexually reproducing species), or other reasons.
Pinpointing the extinction (or pseudoextinction) of a species requires a clear definition of that species. If it is to be declared extinct, the species in question must be uniquely distinguishable from any ancestor or daughter species, and from any other closely related species. Extinction of a species (or replacement by a daughter species) plays a key role in the punctuated equilibrium hypothesis of Stephen Jay Gould and Niles Eldredge.
Skeleton of various extinct dinosaurs
; some other dinosaur lineages still flourish in the form of birds
In ecology, extinction is often used informally to refer to local extinction, in which a species ceases to exist in the chosen area of study, but may still exist elsewhere. This phenomenon is also known as extirpation. Local extinctions may be followed by a replacement of the species taken from other locations; wolf reintroduction is an example of this. Species which are not extinct are termed extant. Those that are extant but threatened by extinction are referred to as threatened or endangered species.
Currently an important aspect of extinction is human attempts to preserve critically endangered species. These are reflected by the creation of the conservation status "extinct in the wild" (EW). Species listed under this status by the International Union for Conservation of Nature (IUCN) are not known to have any living specimens in the wild, and are maintained only in zoos or other artificial environments. Some of these species are functionally extinct, as they are no longer part of their natural habitat and it is unlikely the species will ever be restored to the wild. When possible, modern zoological institutions try to maintain a viable population for species preservation and possible future reintroduction to the wild, through use of carefully planned breeding programs.
The extinction of one species' wild population can have knock-on effects, causing further extinctions. These are also called "chains of extinction". This is especially common with extinction of keystone species.
A 2018 study indicated that the 6th mass extinction started in the Late Pleistocene could take up to 5 to 7 million years to restore 2.5 billion years of unique mammal diversity to what it was before the human era.
Extinction of a parent species where daughter species or subspecies are still extant is called pseudoextinction or phyletic extinction. Effectively, the old taxon vanishes, transformed (anagenesis) into a successor, or split into more than one (cladogenesis).
Pseudoextinction is difficult to demonstrate unless one has a strong chain of evidence linking a living species to members of a pre-existing species. For example, it is sometimes claimed that the extinct Hyracotherium, which was an early horse that shares a common ancestor with the modern horse, is pseudoextinct, rather than extinct, because there are several extant species of Equus, including zebra and donkey. However, as fossil species typically leave no genetic material behind, one cannot say whether Hyracotherium evolved into more modern horse species or merely evolved from a common ancestor with modern horses. Pseudoextinction is much easier to demonstrate for larger taxonomic groups.
The coelacanth, a fish related to lungfish and tetrapods, was considered to have been extinct since the end of the Cretaceous Period until 1938 when a specimen was found, off the Chalumna River (now Tyolomnqa) on the east coast of South Africa. Museum curator Marjorie Courtenay-Latimer discovered the fish among the catch of a local angler, Captain Hendrick Goosen, on December 23, 1938. A local chemistry professor, JLB Smith, confirmed the fish's importance with a famous cable: "MOST IMPORTANT PRESERVE SKELETON AND GILLS = FISH DESCRIBED".
Far more recent possible or presumed extinctions of species which may turn out still to exist include the thylacine, or Tasmanian tiger (Thylacinus cynocephalus), the last known example of which died in Hobart Zoo in Tasmania in 1936; the Japanese wolf (Canis lupus hodophilax), last sighted over 100 years ago; the ivory-billed woodpecker (Campephilus principalis), last sighted for certain in 1944; and the slender-billed curlew (Numenius tenuirostris), not seen since 2007.