extinction, in biology, disappearance of species of living organisms. Extinction usually occurs as a result of changed conditions to which the species is not suited. If no member of the affected species survives and reproduces, the entire line dies out, leaving no descendants. This was the case with the saber-toothed tiger ( Smilodon ) of North America, which is not ancestral to any living species. However, a species may also become extinct through its gradual evolution into a new species, as a result of natural selection for characteristics suited for new conditions. An example of the latter situation is the evolution of horses from the eophippus ( Hyracotherium ) to Miohippus to Merychippus to the present-day Equus. There has been an unbroken line of descent, yet horses of the earlier types no longer exist. Human activities, such as overhunting a species or destroying its habitat, have caused the extinction of some species, such as the passenger pigeon and dodo, and threatened many others (see endangered species). See also mass extinction.
See M. V. Barrow, Jr., Nature's Ghosts: Confronting Extinction from the Age of Jefferson to the Age of Ecology (2009).