Initially the Species was considered as a group of organisms showing similar or specific characters. The Species is considered as the most important taxon. A Species represents a natural unit. All other taxa remain arbitrary and are subjected to revision. A Species is considered a reality. It is the fundamental unit in taxonomy. Evolution basically operates at the Species level only. However modern workers have identified three main concepts regarding Species.
(i) Typological Species concept – This concept has its beginning from the essentialism concept of Aristotle. According to this concept a Species is recognised by its essential characters expressed in morphology.
(ii) Nominalistic Species concept – According to this concept Species are man made ideas. Nature produces individuals and not Species. Thus a Spe-cies is considered as a mental concept.
(iii) Biological Species concept – According to this concept, “Species are groups of interbreeding natural populations that are reproductively isolated from other such groups”. This concept is mostly accepted by present day taxonomists.