Species is an entity composed of organisms which maintains its identity from other such entities through time and over space, and which has its own independent evolutionary fate and historical tendencies. A biological species is the basic unit of classification and a taxonomic rank of an organism, as well as a unit of biodiversity. A species is often defined as the largest group of organisms in which any two individuals of the appropriate sexes or mating types can produce fertile offspring, typically by sexual reproduction. Organisms from different species, although they belong under the same genus, generally cannot interbreed as their offspring would likely be infertile. The species of a particular group of organisms would, therefore, pertain to those that can reproduce and sire fertile offspring of the same kind. Thus, they would have the same set of DNA, similar physical and morphological attributes, and demonstrate communal behavior. An ecological species is a set of organisms adapted to a particular set of resources, called a niche, in the environment. According to this concept, populations form the discrete phenetic clusters that we recognise as species because the ecological and evolutionary processes controlling how resources are divided up tend to produce those clusters.
Certain species can still be further subdivided into subspecies, varieties, formae etc. The genus is the generic name whereas the species is the specific name in a binomial nomenclature. For example, Allium cepa (commonly known as onion). The Allium is the generic name whereas the cepa is the specific name.
