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The entire course of evolution depends upon the origin of new populations having greater adaptive efficiency than their ancestors. The study of speciation or populational divergence is therefore crucial to an understanding of evolution. At one time, even at present, the process of speciation was thought to be synonymous with evolution. As most biologists now realize that evolution is not a simple process involving only the origin of divergence.

The factors for race and species formation are considerably different from those operating to produce sequential microevolution, and evolution above the species level also entails a different complex of processes. Evidences have accumulated to the point where it is no longer proper to speak of a single origin of species, since we know that species may originate in several very different ways.

Species was originally used as a Latin equivalent of ‘kind’. The precise meaning of species has been a subject of much discussion but now most of the biologists are agreed on what a species is: According to them a species is an assemblage of animals which do not differ from one another and which interbreed freely with one another to produce fertile offsprings.

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They usually inhibit geographical areas (niches) distinct from those inhibited by the nearly related species. According to Ross (1962) the species is the total of the individuals forming its population. Now it is regarded that a population, which is morphologically different is taken as a separate species.

According to May (1940), “species is a group of actually or potentially interbreeding natural population that is reproductively isolated from other such groups.” According to Dobzhansky (1950), “A species is the most inconclusive Mendelian population.” Merrell (1962) mentioned, “The species is a natural, biological unit, tied together by the bond of mating and sharing a common gene pool. For this reason it has objective reality.”

However, there is difficulty with biological species definition, it is limited to sexually reproducing species. May (1963) enumerative three important character of a species as follows:

(i) Species form reproductive community.

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(ii) It is an ecological unit as it interacts as a unit with other species which it shares the environment.

(iii)Species is a genetic unit consisting of large intercommunicating gene pool.

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