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 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 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 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); E. Fuller, Lost Animals: Extinction and the Photographic Record (2014).
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extinction The 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 ,
E = (B –V) – (B –V )0
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.
Collins Dictionary of Astronomy © Market House Books Ltd, 2006
The following article is from The Great Soviet Encyclopedia (1979). It might be outdated or ideologically biased.
(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.
The Great Soviet Encyclopedia, 3rd Edition (1970-1979). © 2010 The Gale Group, Inc. All rights reserved.
The reduction in the apparent brightness of a celestial object due to absorption and scattering of its light by the atmosphere and by interstellar dust; it is greater at low altitudes.
The worldwide death and disappearance of a specific organism or group of organisms.
The drying up of lake by either water loss or destruction of the lake basin.
Phenomenon in which plane polarized light is almost completely absorbed by a polarizer whose axis is perpendicular to the plane of polarization.
Decrease in frequency and elimination of a conditioned response if reinforcement of the response is withheld.
McGraw-Hill Dictionary of Scientific & Technical Terms, 6E, Copyright © 2003 by The McGraw-Hill Companies, Inc.
once on verge of extinction, this bird is now protected; still an endangered species. [Ecology: Hammond, 290]
dinosaurs died out, unable to adapt to environmental change. [Ecology: Hammond, 290]
large, flightless bird exterminated on Mauritius. [Ecology: Wallechinsky, 131]
hunters killed such large numbers, these birds became extinct in 1840s. [Ecology: Hammond, 290]
Last of the Mohicans, The
human settlement of U.S. Atlantic Coast contributed to the extinction of these birds. [Ecology: Hammond, 290]
novel foreseeing the extinction of various Indian tribes. [Am. Lit.: The Last of the Mohicans]
similar to the elephant, the mastodon is now extinct. [Ecology: Hammond, 290]
large ostrichlike bird, hunted chiefly for its food; it died out in 1914. [Ecology: Hammond, 290]
hunted to extinction by 1914; vast numbers once darkened American skies during migratory flights. [Ecology: EB, VII: 786]
Last of the Barons, The
wild cat that died out about 12,000 years ago. [Ecology: Hammond, 290]
portrays England’s brilliant aristocracy as dying breed (1470s). [Br. Lit.: The Last of the Barons, Magill I, 492–494]
many species in danger of extinction, owing to massive hunting. [Ecology: Hammond, 290]
Allusions—Cultural, Literary, Biblical, and Historical: A Thematic Dictionary. Copyright 2008 The Gale Group, Inc. All rights reserved.
1. Physics reduction of the intensity of radiation as a result of absorption or scattering by matter
2. Astronomy the dimming of light from a celestial body as it passes through an absorbing or scattering medium, such as the earth's atmosphere or interstellar dust
3. Psychol a process in which the frequency or intensity of a learned response is decreased as a result of reinforcement being withdrawn
Collins Discovery Encyclopedia, 1st edition © HarperCollins Publishers 2005