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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 cat (Smilodon) of North America, which is not ancestral to any living species. However, a species may also become extinct through its gradual evolutionevolution,
concept that embodies the belief that existing animals and plants developed by a process of gradual, continuous change from previously existing forms. This theory, also known as descent with modification, constitutes organic evolution.
..... Click the link for more information. 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 horseshorse,
hoofed, herbivorous mammal now represented by a single extant genus, Equus. The term horse commonly refers only to the domestic Equus caballus and to the wild Przewalski's horse.
..... Click the link for more information. 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 pigeonpigeon,
common name for members of the large family Columbidae, land birds, cosmopolitan in temperate and tropical regions, characterized by stout bodies, short necks, small heads, and thick, heavy plumage.
..... Click the link for more information. and dodododo,
a flightless forest-dwelling bird of Mauritius, extinct since the late 17th cent. The dodo was closely related to the two species of solitaire bird, extinct flightless giants found on the other islands in the Mascarene Islands.
..... Click the link for more information. , and threatened many others (see endangered speciesendangered species,
any plant or animal species whose ability to survive and reproduce has been jeopardized by human activities. In 1999 the U.S. government, in accordance with the U.S.
..... Click the link for more information. ). See also mass extinctionmass extinction,
the extinction of a large percentage of the earth's species, opening ecological niches for other species to fill. There have been at least ten such events.
..... Click the link for more information. .
See M. V. Barrow, Jr., Nature's Ghosts: Confronting Extinction from the Age of Jefferson to the Age of Ecology (2009); E. Fuller, Lost Animals: Extinction and the Photographic Record (2014).
extinctionThe reduction in the amount of light or other radiation received from a celestial body as a result of absorption and scattering of the radiation by intervening dust grains in space (interstellar extinction) and in the Earth's atmosphere (atmospheric extinction). The extinction decreases with wavelength of the radiation and increases with the pathlength through the absorbing medium and with the density of the medium.
The starlight is also reddened since the extinction of blue light by dust is greater than that of red light. The reddening may be given in terms of the color excess, E ,
where (B –V) and (B –V )0 are the observed and intrinsic color indices of the star. Most stars are reddened by a few tenths of a magnitude although values of up to two magnitudes are not uncommon. Stars lying behind extremely dense matter might only be detectable at radio or infrared wavelengths. See also infrared sources.
(also extinctive inhibition), in physiology, a form of internal conditioned inhibition in accordance with Pavlovian theory.
The simplest form of extinction is the progressive weakening of external manifestations of the orienting reflex when the subject is repeatedly exposed to an extraneous stimulus. A more complex form of extinction is the gradual decrease in magnitude of a conditioned reflex in the absence of reinforcement by an unconditioned stimulus. The time required for any given degree of extinction, as well as its degree, depends on various factors, including the modality of the conditional signal, the type of unconditioned reflex (for example, the alimentary or defensive types), the type of registrable reaction (such as motor or secretory reactions), and the extent to which the conditioned reflex has become established. It is presumed that extinction is based on inhibitory activity in the conductive links by which signals are transmitted from the sensory (afferent) pathways to the effector (efferent) systems of the brain.