Q.Brinjal and potato belong to the same genus Solanum, but to two different species. What defines them as seperate species?

Imagine walking into a huge library where millions of books have simply been dumped into one enormous pile. Finding anything would be hopeless. What makes a library usable is that books are sorted onto shelves, into sections, by subject and author. Taxonomic classification does exactly this for the living world: it takes the astonishing variety of organisms around us and arranges them into an orderly system so we can study, name and talk about them without confusion.

At its heart, classification is the process of placing organisms into convenient groups based on features we can observe. When you say a word like "dog", "insect", "mammal" or "plant", a whole set of shared characteristics springs to mind at once — that mental grouping is the everyday version of what biologists do formally. Each such group is called a taxon (the plural is taxa), and the science of sorting organisms into taxa on the basis of their characters is called taxonomy.

The crucial idea is that these groups sit at different levels. "Plants" is a taxon and "wheat" is a taxon; "animals", "mammals" and "dogs" are all taxa too — yet a dog is a kind of mammal, and mammals are a kind of animal. So classification is not one single step but a ladder of steps, each step being a rank or category. The four processes that taxonomy rests on are characterisation (working out an organism's features), identification (knowing exactly which organism we are dealing with), classification (placing it into groups) and nomenclature (giving it a proper name). …