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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.
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 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.
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 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.
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 and dodododo,
a flightless forest-dwelling bird of Mauritius, extinct since the late 17th cent. The dodo was closely related to the Rodrigues solitaire, extinct flightless giant found on another island in the Mascarene Islands.
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, 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.
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). 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.
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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).


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.



(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 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.
(physical chemistry)
Decrease in frequency and elimination of a conditioned response if reinforcement of the response is withheld.


bald eagle
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]
great auk
hunters killed such large numbers, these birds became extinct in 1840s. [Ecology: Hammond, 290]
heath hen
human settlement of U.S. Atlantic Coast contributed to the extinction of these birds. [Ecology: Hammond, 290]
Last of the Mohicans, The
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]
passenger pigeon
hunted to extinction by 1914; vast numbers once darkened American skies during migratory flights. [Ecology: EB, VII: 786]
saber-toothed tiger
wild cat that died out about 12,000 years ago. [Ecology: Hammond, 290]
Last of the Barons, The
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]


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
References in periodicals archive ?
It rejects blaming species extinctions on birth rates in poor countries, instead identifying the poorest people in those countries as allies.
2006), such indices may not only be used for estimating the probability of species extinction, but also to infer the threat and decline in species that are considered to be still extant.
The study warned that temperature rises of 2degC would still lead to wide scale damage such as species extinction and melting of ice sheets.
The human race overwhelms Earth's balancing and support systems with chemicals, air pollution, species extinction, and a dozen other activities that destroy this planet's ecosystems.
Victorian-era civilization, it was believed, had the physical means, moral determination and intellectual capacity to alter the culture and religion of distant peoples, while the economic needs of the industrialized metropolises inevitably caused environmental change, species extinction and societal collapse far from the great centres of science and commerce.
Additionally, animal agriculture is at or near the top of the worldwide list for water usage, water pollution, topsoil loss, air pollution, tropical rainforest destruction, desertification, habitat loss and species extinction.
We are already witnessing what may be an even larger species extinction than occurred with the dinosaurs--but this time thanks to various human activities.
We, as a planet, will suffer species extinction and hundreds of millions of people will be at risk from drought, hunger and flooding.
In its latest assessment the IPCC said "if warming is not kept below 2C, which currently looks very unlikely to be achieved, substantial global impacts will occur such as species extinction and hundreds of millions of people at risk from drought, hunger and flooding.
DON BARGER, senior director of NPCA's Southeast regional office, quoted in the Knoxville News Sentinel on the need to recognize the impact of global warming on Great Smoky Mountains National Park and other parks, as documented in yet another government report anticipating severe weather and species extinction.
Religion and science have traditionally been at loggerheads over issues like these, but in January 2007 a group of scientists and evangelicals put aside their differences to battle environmental woes such as climate change and species extinction.
Dick, Human sexual reproduction without male genetic material, digital viruses, identity theft, robot firefighters and minesweepers weather control, pharmaceutical mood engineering, rapid species extinction.