Species
: species) is a population of organisms in which any two individuals of the appropriate sexes or mating types can produce fertile offspring, typically by sexual reproduction.This obliges taxonomists to decide, for example, when enough change has occurred to declare that a lineage should be divided into multiple chronospecies, or when populations have diverged to have enough distinct character states to be described as cladistic species.[15] It differs from the morphological species concept in including a numerical measure of distance or similarity to cluster entities based on multivariate comparisons of a reasonably large number of phenotypic traits.As a rule of thumb, microbiologists have assumed that members of Bacteria or Archaea with 16S ribosomal RNA gene sequences more similar than 97% to each other need to be checked by DNA–DNA hybridisation to decide if they belong to the same species.[22] The average nucleotide identity (ANI) method quantifies genetic distance between entire genomes, using regions of about 10,000 base pairs.[27][28] However, scientists such as Rob DeSalle have expressed concern that classical taxonomy and DNA barcoding, which they consider a misnomer, need to be reconciled, as they delimit species differently.A phylogenetic or cladistic species is "the smallest aggregation of populations (sexual) or lineages (asexual) diagnosable by a unique combination of character states in comparable individuals (semaphoronts)".[31] The empirical basis – observed character states – provides the evidence to support hypotheses about evolutionarily divergent lineages that have maintained their hereditary integrity through time and space.[41] Yet others defend this approach, considering "taxonomic inflation" pejorative and labelling the opposing view as "taxonomic conservatism"; claiming it is politically expedient to split species and recognise smaller populations at the species level, because this means they can more easily be included as endangered in the IUCN red list and can attract conservation legislation and funding.[36][37] However, it does not always provide clear cut and intuitively satisfying boundaries between taxa, and may require multiple sources of evidence, such as more than one polymorphic locus, to give plausible results.During anagenesis (evolution, not necessarily involving branching), some palaeontologists seek to identify a sequence of species, each one derived from the phyletically extinct one before through continuous, slow and more or less uniform change.In such a time sequence, some palaeontologists assess how much change is required for a morphologically distinct form to be considered a different species from its ancestors.[52][53][54][55] Viruses have enormous populations, are doubtfully living since they consist of little more than a string of DNA or RNA in a protein coat, and mutate rapidly.[88] The evolutionary biologist James Mallet concludes that there is no easy way to tell whether related geographic or temporal forms belong to the same or different species.One is forced to admit that Darwin's insight is correct: any local reality or integrity of species is greatly reduced over large geographic ranges and time periods.[92] Other biologists have gone further and argued that we should abandon species entirely, and refer to the "Least Inclusive Taxonomic Units" (LITUs),[96] a view that would be coherent with current evolutionary theory.[104] Natural hybridisation presents a challenge to the concept of a reproductively isolated species, as fertile hybrids permit gene flow between two populations.Another problem with common names is that they often vary from place to place, so that puma, cougar, catamount, panther, painter and mountain lion all mean Puma concolor in various parts of America, while "panther" may also mean the jaguar (Panthera onca) of Latin America or the leopard (Panthera pardus) of Africa and Asia.In contrast, the scientific names of species are chosen to be unique and universal (except for some inter-code homonyms); they are in two parts used together: the genus as in Puma, and the specific epithet as in concolor.The type material is usually held in a permanent repository, often the research collection of a major museum or university, that allows independent verification and the means to compare specimens.[117][118][119] Describers of new species are asked to choose names that, in the words of the International Code of Zoological Nomenclature, are "appropriate, compact, euphonious, memorable, and do not cause offence".[126][127][121] The circumscription of taxa, considered a taxonomic decision at the discretion of cognizant specialists, is not governed by the Codes of Zoological or Botanical Nomenclature, in contrast to the PhyloCode, and contrary to what is done in several other fields, in which the definitions of technical terms, like geochronological units and geopolitical entities, are explicitly delimited.Species may then need to be distinguished by the boundary definitions used, and in such cases the names may be qualified with sensu stricto ("in the narrow sense") to denote usage in the exact meaning given by an author such as the person who named the species, while the antonym sensu lato ("in the broad sense") denotes a wider usage, for instance including other subspecies.[133] The evolutionary process by which biological populations of sexually-reproducing organisms evolve to become distinct or reproductively isolated as species is called speciation.[131] A different mechanism, phyletic speciation, involves one lineage gradually changing over time into a new and distinct form (a chronospecies), without increasing the number of resultant species.[137] Horizontal gene transfer between organisms of different species, either through hybridisation, antigenic shift, or reassortment, is sometimes an important source of genetic variation.[154] When observers in the Early Modern period began to develop systems of organization for living things, they placed each kind of animal or plant into a context.[155]In the 18th century, the Swedish scientist Carl Linnaeus classified organisms according to shared physical characteristics, and not simply based upon differences.[156] Like many contemporary systematists,[157][158][159] he established the idea of a taxonomic hierarchy of classification based upon observable characteristics and intended to reflect natural relationships.This view was influenced by European scholarly and religious education, which held that the taxa had been created by God, forming an Aristotelian hierarchy, the scala naturae or great chain of being.
Species (disambiguation)organismsmating typesproducefertileoffspringsexual reproductionclassificationtaxonomic rankbiodiversitykaryotypemorphologyecological nichepalaeontologistschronospeciesfossileukaryotesvirusestwo-part namespecific namespecific epithetbotanical nomenclaturezoological nomenclatureBoa constrictorhybridisationspecies complexmicrospeciesring speciesasexuallyconservationistsevolutionarytaxonomistscladisticAristotlegreat chain of beingCharles DarwinOn the Origin of Speciesspecies could arisenatural selectiongreatly extended in the 20th centurygeneticsecologymutationsrecombinationgeographical isolationgenetic driftselection pressureshorizontal gene transferpolyploidybecome extinctbalance of mutation and selectionquasispeciesErnst MayragamospeciesecospeciesEurasian blue titsDrosophilaRobert R. SokalPeter Sneathpheneticphenotypegene poolscytochrome c oxidaseenzymeBarcode of Life Data SystemsmicrobiologyBacteriaArchaea16S ribosomal RNADNA–DNA hybridisationaverage nucleotide identitygenetic distancegenomesbase pairsPseudomonas avellanaeDNA barcodingBarcode of Life Data SystemGenetic introgressionendosymbiontsmitochondrial DNABovidaetaxonomic inflationendangeredred listGeorge Gaylord SimpsonWilli HennigKevin de Queirozkaryotypeschromosomesallozymesevolutionarily significant unitpalaeontologycomparative anatomyhistologyfossilsanagenesisphyletically extinctViral quasispeciesmutagenicmutation–selection balanceevolutionarily neutralfitness landscapeInternational Committee on Taxonomy of VirusesInoceramusmulti-celled organismsreproduce asexuallysingle-celled organismsprokaryotesparthenogeneticapomicticphylogeneticswillow warblerchiffchaffmorphological convergencecryptic speciesphenotypic plasticitylife-cyclecrustaceansechinodermsJames MalletBrent MishlercichlidteleostsEast African Great LakesCharles Tate ReganPhilip Kitcher