Hierarchical classification is a system of classifying organisms in different hierarchical levels. It includes the sequence of categories in a decreasing or increasing order from kingdom to species and vice versa. The domain is the highest rank in the hierarchy followed by division or kingdom, phylum, class, order, family, genus, and species. Species is the lowest rank in the hierarchy. The species is the basic unit of biological classification and a taxonomic rank. These are the large number of physically similar organisms which can exchange genetic information.