Extinction may be defined as the termination of a lineage without is-sue or abrupt disappearance of specific groups of organisms without leaving descendents. Extinctions are of two types namely true extinctions and pseudo extinction. In true extinctions a particular lineage totally disappears without any progeny or evolutionary descendents.
Pseudo extinction may also be called as phyletic extinction or phyletic transformation In this type a group may disappear leaving descen-dents with evolutionary modifications. In horse evolution while the earliest ancestor Eohippus became extinct its desecendent survived to produce the modern 'Equus'. Extinctions can happen for the taxonomic groups such as a family or genus. Thus the extinction of dinosaurs as a group is a case of true extinction. Similar extinctions happened to trilobites.
The process of extinction is not always predetermined. It happens due to abrupt changes in evironmental conditions or other biological factors. Through the use of computer models evolutionists can now examine the probalility of extinction for large as well as small groups of organims. From the fossil record it becomes evident that extinctions have occured at regular intervals of time.
Fossil record reveals a few patterns for extinction. Major groups of herbivorous vertebrates are more susceptible than the carnivorous vertebrates for extinction. Larger organisms easily became extinct. VanValen(1973) recorded a constancy in the rate of extinction in a number of groups. He explained this using 'Mac Arthor's law'. According to this law as every new adaptation encourages the survival of a possessor it also decreases a fitness of other related species of that area.
Even though extinctions are regular events in the history of earth they are caused due to specific reasons (1) A mass extinction may be due to dras-tic changes in the environmental conditions . (2) Any adaptive advance in one species decreases the fitness of all other species. Thus according to Red Queen's hypothesis you have to keep running pretty fast, just in order to stay in the same place. (3) Over specialistion to a specific situation may cause extinction.(ex. Antlers.) (4) The spread of an epidemic disease without any control can cause extinction. (5) An increase in the popu lation strength of herbivorus animals can cause rapid food shortage and cause extinction for several inter-related groups. (6) A sudden cosmic radiation can cause the death of large organisms. (7) A dust storm formed due to falling of a meteorite is commonly mentioned as a cause for the disappearance of dinosaurs.
In the recorded history of earth, extinctions of major groups of organisms were due to natural causes. By the end of permian period of the Paleozoic Era, nearly 60% of the varieties then existed, became extinct. Similar large scale extinctions have been observed by the end of Mesozoic era and during Cenozoic time.
However the extintion of animals and plants during our time is mostly due to human interference. Thus the cause for the modern extinctions is invariably human acitivities. The realisation of a such a cause lead to starting of several international voluntary agencies to monitor and control extinctions. 'The red-data book brought out regularly by W. W. F (World Wide Fund for nature, Formerly IUCN - International union for the conservation of nature and natural resources) provides a list of animals and plants that are endan-gered or have become extinct.
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