The word ‘species’ literally means outward or visible form. It comprises groups of actually or potentially interbreeding natural populations which are reproductively isolated from other such groups (Mayr, 1940). The Linnaean concept of species as relatively constant unit with most of the variations occurring among them is different from Darwin’s theory of evolution by gradual change, which states that the variations between species must be generated from variation within species. The historical review of the species and their varieties continued to the end of 19th century and it was during this period Mendel's concept of heredity made its appearance. Later, as the integration of Mendelian genetics and Darwin's evolutionary theory, often termed as the 'neo-Darwinism' was making its impact on biology, the significance of genetic variations within species invited wide attention. Studies on intraspecific (genetic) variations were initiated and further elaborated by many workers. During 1920s, Vavilov based on his epoch making observations on genetic diversity of cultivated plants and their wild relatives described what are known today as the geographical centers of genetic diversity/origin mostly in the tropical belt. Though these earlier workers described vast wealth of previously unknown genetic variations, little attention was paid to the necessity to preserve these reservoirs of genetic diversity or 'natural gene pools' (Dodds, 1991).
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