Taxonomic hierarchy is the system of arrangement of taxonomic categories in descending order. It was introduced by Linnaeus in 1751, who initially proposed five categories (class, order, genus, species and variety).
However, 'variety' was obliterated and four new categories were introduced. Thus, the seven obligate categories in descending order are kingdom, phylum or division, class, order, family, genus and species.
The placement of individuals or organisms in taxonomic categories is determined by their specific similar characters, dissimilarities, complexity and evolutionary relationships.
Species: It is the basic taxonomic category which consists of individuals that resemble more closely than individuals of other species. Members of a particular species have a distinct genetic makeup, can freely interbreed and are reproductively isolated from others. For example, potato (Solanum tuberosum) and mango (Mangifera indica).
Genus: A genus may have one to many species. As per the rules of binomial nomenclature, a species can be named only if it is assigned to a genus. All the species of a genus have a number of common features referred to as correlated characters.
A genus with a single species is known as monotypic, e.g., Homo is monotypic with a single species of Homo sapiens.
A genus with two or more species is known as polytypic. For example, the genus Solanum with S. tuberosum, S. melongina and S. lycopersicum.