Species, in biology, is the classification comprising related organisms that share common characteristics and are capable of interbreeding. This biological species concept is widely utilized in biology and related fields of study. The designation of species originates in taxonomy, where the species is that the fundamental unit of classification recognized by the International Commission of Zoological Nomenclature. Every species is assigned a typical two-part name of genus and species. The first part of a binomial is the genus to which the species belongs and the second part is called the specific name or the specific epithet. Organisms are grouped into species partly consistent with their morphological, or external, similarities, but the most important thing in classifying sexually reproducing organisms is that the organisms’ ability to successfully interbreed. Species identification is extremely important for the conservation of biodiversity, but achieving a reasonable estimate of the entire number of species on Earth has been elusive. Genes can be exchanged between species by horizontal gene transfer; new species can arise through hybridisation and polyploidy; and species may become extinct for a variety of reasons. A typological species is a group of organisms in which individuals conform to certain fixed properties, so that even pre-literate people often recognise the same taxon as do modern taxonomists. The clusters of variations or phenotypes within specimens would differentiate the species. A mate-recognition species is a group of sexually reproducing organisms that recognize one another as potential mates. A phylogenetic species is an evolutionarily divergent lineage, one that has maintained its hereditary integrity through time and space.