The arrangement of organism into these ranking orders or taxonomic categories in an ascending series ranging from the lowest to the highest category is called taxonomic hierarchy. Taxon can be used at the level of variety, species, genus, family or any other higher category. The species is the fundamental category of the taxonomic hierarchy. More information is usually available about a family and it is comparatively easy to recognize a family than the lower categories like genus or species. Ecospecies is correlated with taxonomic species whereas ecotype is correlated with the taxonomic subspecies. Species concepts that emphasize the product of evolution include taxonomic, phylogenetic and genealogical species concepts. The biological species concept has been challenged with the increasing evidences of the occurrence of interspecific and intergeneric natural hybrids. Evolutionary Species is thus explained as a group of organisms having common evolutionary fate.