A species is a group of organisms capable of interbreeding and producing fertile offspring of both genders, and separated from other such groups with which interbreeding does not characteristically occur: however, for asexual organisms, a distinct species may be considered a collection of organisms which have very similar DNA or physical characteristics. Certain species are further subdivided into subspecies.
History of the concept
The early Greeks and Romans had a well established set of taxonomic names for species of animals and plants, based upon the macroscopically observable characteristics of organisms, with Aristotle being the chief architect of this codification; even earlier, the Egyptians and Cretans developed basic symbols and names for species important in farming and culture. It was not until the year 1686 when English naturalist John Ray introduced the concept that species were distinguished by inevitably producing the same species, though considerable morphological variation was observed within a species. Carl Linnaeus (1707–1778) formalized the taxonomic rank of species, and developed the two part naming system of binomial nomenclature that survives to current times, with genus and species names in Latin form.
Estimation of species numbers
Since most of the planet's species are deemed to be undiscovered, it is exceedingly difficult even to estimate the total number of species on Earth. An 2011 innovative study estimated the total number of species to be about 8.7 million, with around 86 percent of which are presently undiscovered. The following represents a rough approximation of the number of species by taxonomic group, with ranges given for varying estimates of the species total numbers:
Total species: 7,000,000 to 100,000,000 (the lower number reflecting described species and the higher based upon estimates of Earth's species):
- Bacteria: 5,000,000 to 10,000,000
- Archaea: 20,000 (based upon only marine species) 
- Eukarya: 1,660,000
Of the described eukarya species 1,600,000 based on described species, including:
- 297,326 plants, including:
100,000 fungi (of an estimated total 1,500,000 other non-animals) including:
- 25,000 lichens,
- 16,000 mushrooms
- 30,000 red, brown and blue-green molds
- 17,000 conidial fungi
- 1,260,000 animals, including:
See Main Article: Endangered species
An endangered species is a biological taxon that is at risk of becoming extinct in a proximate time frame. The application of this term is typically assigned when a species or subspecies population is near its minimum viable population size or is threatened by current or expected environmental alteration. There are vast numbers of species currently in an endangered status, chiefly due to the intrusion of humans into heretofore natural environments and the subsequent destruction and fragmentation of habitats. Many governmental units apply the categorization of endangerment in order to create mechanisms of protection for the species involved. The International Union for Conservation of Nature (IUCN) asserts that over one third of all extant organisms are currently endangered based on the sample of species that have been evaluated through the year 2006; however, the IUCN classifies certain other species in this summary statistics that are considered at a somewhat lesser level of threat.
Ring species are a set of geographically neighboring populations that can interbreed with somewhat closely related populations, but for which there exist at least two terminus populations in the series which are too distantly related to interbreed. Often such non-breeding, but genetically connected populations, co-exist in the same region thus creating a ring. These sets of ring species provide evidence of evolution by illustrating the time progression as populations genetically diverge, and because they represent in living populations what normally happens over time between long deceased ancestor populations and extant populations.
- ^ John S.Wilkins. 2006. Species, Kinds, and Evolution. Reports of the United States National Center for Science Education
- ^C.Mora, C.P.Tittensor, S.Adl, A.G.B.Simpson & B.Worm. 2011. How Many Species Are There on Earth and in the Ocean? PLoS Biol 9(8): e1001127. doi:10.1371/journal.pbio.1001127 Academic Editor: Georgina M.Mace
- ^ M.L.Sogin, H.G Morrison, J.A.Huber et al. 2006. Microbial diversity in the deep sea and the underexplored "rare biosphere". Proc. Natl. Acad. Sci. U.S.A. volume 103, issue 32, pp 12115–20 The magnitude of fungal diversity: the 1•5 million species estimate revisited. Mycological Research, vol 105, pages 1422–1432
- ^ N.M.van Straalen and Dick Roelofs. 2006. An introductions to ecological genomics. 307 pages
- ^ David L. Hawksworth. 2001. 
- ^ Paul R. Ehrlich and Anne H. Ehrlich. 1981. Extinction: The causes and consequences of the disappearance of species. Random House, New York, New York, USA