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In
biology Biology is the natural science that studies life and living organisms, including their anatomy, physical structure, Biochemistry, chemical processes, Molecular biology, molecular interactions, Physiology, physiological mechanisms, Development ...

biology
, a species is the basic unit of
classification Classification is a process related to categorization Categorization is the human ability and activity of recognizing shared features or similarities between the elements of the experience of the world (such as Object (philosophy), objects, eve ...
and a
taxonomic rank In biological classification In biology, taxonomy () is the scientific study of naming, defining (Circumscription (taxonomy), circumscribing) and classifying groups of biological organisms based on shared characteristics. Organisms are gro ...
of an
organism In biology Biology is the natural science that studies life and living organisms, including their anatomy, physical structure, Biochemistry, chemical processes, Molecular biology, molecular interactions, Physiology, physiological me ...

organism
, as well as a unit of
biodiversity Biodiversity is the biological variety and of . Biodiversity is a measure of variation at the , , and level. Terrestrial biodiversity is usually greater near the , which is the result of the warm and high . Biodiversity is not distributed ev ...

biodiversity
. A species is often defined as the largest group of
organism In biology Biology is the natural science that studies life and living organisms, including their anatomy, physical structure, Biochemistry, chemical processes, Molecular biology, molecular interactions, Physiology, physiological me ...

organism
s in which any two individuals of the appropriate
sex Sex is either of two divisions, typically male Male (♂) is the sex of an organism that produces the gamete known as sperm. A male gamete can fuse with a larger female gamete, or ovum, in the process of fertilization. A male cannot sexual r ...
es or mating types can
produce Produce is a generalized term for many farm-produced crops, including fruits and vegetables (grains, oats, etc. are also sometimes considered ''produce''). More specifically, the term ''produce'' often implies that the products are wikt:fresh, fr ...

produce
fertile Fertility is the quality of being able to produce children. As a measure, the fertility rate is the average number of children that a woman has in her lifetime and is quantified demographically. Fertility is most commonly considered when there ...
offspring In biology, offspring are the young creation of living organisms, produced either by a Asexual reproduction, single organism or, in the case of sexual reproduction, two organisms. Collective offspring may be known as a brood or progeny in a more ...

offspring
, typically by
sexual reproduction Sexual reproduction is a type of reproduction Reproduction (or procreation or breeding) is the biological process Biological processes are those processes that are vital for an organism In biology, an organism (from Ancient Greek, ...
. Other ways of defining species include their
karyotype A karyotype is a preparation of the complete set of metaphase Metaphase () is a stage of mitosis In cell biology Cell biology (also cellular biology or cytology) is a branch of biology Biology is the natural science that studies ...

karyotype
,
DNA Deoxyribonucleic acid (; DNA) is a molecule File:Pentacene on Ni(111) STM.jpg, A scanning tunneling microscopy image of pentacene molecules, which consist of linear chains of five carbon rings. A molecule is an electrically neutral gro ...

DNA
sequence,
morphology Morphology, from the Greek and meaning "study of shape", may refer to: Disciplines * Morphology (archaeology), study of the shapes or forms of artifacts * Morphology (astronomy), study of the shape of astronomical objects such as nebulae, galaxies ...
, behaviour or
ecological niche In ecology Ecology (from el, οἶκος, "house" and el, -λογία, label=none, "study of") is the study of the relationships between living organisms, including humans, and their physical environment. Ecology considers organisms ...

ecological niche
. In addition,
paleontologist Paleontology (), also spelled palaeontology or palæontology, is the scientific study of life that existed prior to, and sometimes including, the start of the Holocene The Holocene ( ) is the current Geologic time scale, geological epoch. ...
s use the concept of the
chronospecies A chronospecies is a species In biology, a species is the basic unit of biological classification, 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 organ ...

chronospecies
since
fossil A fossil (from Classical Latin Classical Latin is the form of Latin, Latin language recognized as a Literary language, literary standard language, standard by writers of the late Roman Republic and early Roman Empire. It was used from 75 ...

fossil
reproduction cannot be examined. The most recent rigorous estimate for the total number of species of
eukaryote Eukaryotes () are organism In biology, an organism () is any organic, life, living system that functions as an individual entity. All organisms are composed of cells (cell theory). Organisms are classified by taxonomy (biology), tax ...

eukaryote
s is between 8 and 8.7 million. However, only about 14% of these had been described by 2011. All species (except
virus A virus is a submicroscopic infectious agent In biology Biology is the natural science that studies life and living organisms, including their anatomy, physical structure, Biochemistry, chemical processes, Molecular biology, molecu ...

virus
es) are given a two-part name, a "binomial". The first part of a binomial is the
genus Genus /ˈdʒiː.nəs/ (plural genera /ˈdʒen.ər.ə/) is a taxonomic rank In biological classification In biology, taxonomy () is the scientific study of naming, defining (Circumscription (taxonomy), circumscribing) and classifying gr ...
to which the species belongs. The second part is called the
specific nameSpecific name may refer to: * in Database management systems, a system-assigned name that is unique within a particular database In Taxonomy (biology), taxonomy, either of these two meanings, each with its own set of rules: * Specific name (botany), ...
or the
specific epithet In taxonomy, binomial nomenclature ("two-term naming system"), also called nomenclature ("two-name naming system") or binary nomenclature, is a formal system of naming species In biology, a species is the basic unit of biological classi ...
(in
botanical nomenclature Botanical nomenclature is the formal, scientific naming of plants. It is related to, but distinct from taxonomy Taxonomy (general) is the practice and science of classification of things or concepts, including the principles that underlie such ...
, also sometimes in
zoological nomenclature The International Code of Zoological Nomenclature (ICZN) is a widely accepted Convention (norm), convention in zoology that rules the formal scientific name, scientific naming of organisms treated as animals. It is also informally known as the IC ...
). For example, '''' is one of four species of the genus ''
Boa Kwon Bo-ah (; born November 5, 1986), known professionally as BoA, is a South Korean singer, songwriter, dancer, record producer and actress. One of the most successful and influential Korean entertainers, she has been dubbed the "Queen of K-pop ...
'', with ''constrictor'' being the species's epithet. While the definitions given above may seem adequate at first glance, when looked at more closely they represent problematic
species concept The species problem is the set of questions that arises when biologist A biologist is a professional who has specialized knowledge in the field of biology, understanding the underlying mechanisms that govern the functioning of biological system ...
s. For example, the boundaries between closely related species become unclear with hybridisation, in a
species complex In biology, a species complex is a group of closely related organism In biology, an organism () is any organic, life, living system that functions as an individual entity. All organisms are composed of cells (cell theory). Organisms a ...
of hundreds of similar
microspecies In biology, a species complex is a group of closely related organism In biology, an organism (from Ancient Greek, Greek: ὀργανισμός, ''organismos'') is any individual contiguous system that embodies the Life#Biology, properties ...
, and in a
ring species In biology Biology is the natural science that studies life and living organisms, including their anatomy, physical structure, Biochemistry, chemical processes, Molecular biology, molecular interactions, Physiology, physiological mechanisms, ...

ring species
. Also, among organisms that reproduce only asexually, the concept of a reproductive species breaks down, and each clone is potentially a
microspecies In biology, a species complex is a group of closely related organism In biology, an organism (from Ancient Greek, Greek: ὀργανισμός, ''organismos'') is any individual contiguous system that embodies the Life#Biology, properties ...
. Although none of these are entirely satisfactory definitions, and while the concept of species may not be a perfect model of life, it is still an incredibly useful tool to scientists and
conservationists The conservation movement, also known as nature conservation, is a political, environmental, and social movement that seeks to manage and protect natural resource , Malaysia is an example of undisturbed natural resource. Waterfalls provide sp ...
for studying life on Earth, regardless of the theoretical difficulties. If species were fixed and clearly distinct from one another, there would be no problem, but
evolution Evolution is change in the heritable Heredity, also called inheritance or biological inheritance, is the passing on of Phenotypic trait, traits from parents to their offspring; either through asexual reproduction or sexual reproduction, ...

evolution
ary processes cause species to change continually, and to grade into one another. Species were seen from the time of
Aristotle Aristotle (; grc-gre, Ἀριστοτέλης ''Aristotélēs'', ; 384–322 BC) was a Greek philosopher A philosopher is someone who practices philosophy Philosophy (from , ) is the study of general and fundamental quest ...

Aristotle
until the 18th century as fixed categories that could be arranged in a hierarchy, the
great chain of being The Great Chain of Being is a hierarchical structure of all matter and life, thought by medieval Christianity Christianity is an Abrahamic religions, Abrahamic Monotheism, monotheistic religion based on the Life of Jesus in the New Testament, ...
. In the 19th century, biologists grasped that species could evolve given sufficient time.
Charles Darwin Charles Robert Darwin (; ; 12 February 1809 – 19 April 1882) was an English natural history#Before 1900, naturalist, geologist and biologist, best known for his contributions to the science of evolution. His proposition that all sp ...

Charles Darwin
's 1859 book ''
On the Origin of Species ''On the Origin of Species'' (or, more completely, ''On the Origin of Species by Means of Natural Selection, or the Preservation of Favoured Races in the Struggle for Life''),The book's full original title was ''On the Origin of Species by Me ...
'' explained how by
natural selection Natural selection is the differential survival and reproduction of individuals due to differences in phenotype right , Here the relation between genotype and phenotype is illustrated, using a Punnett square, for the character of peta ...
. That understanding was greatly extended in the 20th century through
genetics Genetics is a branch of biology Biology is the natural science that studies life and living organisms, including their anatomy, physical structure, Biochemistry, chemical processes, Molecular biology, molecular interactions, Physiology, ...

genetics
and population
ecology Ecology (from el, οἶκος, "house" and el, -λογία, label=none, "study of") is the study of the relationships between living organisms, including humans, and their physical environment. Ecology considers at the individual, , , , an ...
. Genetic variability arises from
mutation In biology Biology is the natural science that studies life and living organisms, including their anatomy, physical structure, Biochemistry, chemical processes, Molecular biology, molecular interactions, Physiology, physiological mechan ...
s and recombination, while organisms themselves are mobile, leading to geographical isolation and
genetic drift Genetic drift (allelic drift or the Sewall Wright effect) is the change in the frequency of an existing gene In biology, a gene (from ''genos'' "...Wilhelm Johannsen coined the word gene to describe the Mendelian_inheritance#History, M ...

genetic drift
with varying selection pressures. Genes can sometimes be exchanged between species by
horizontal gene transfer Horizontal gene transfer (HGT) or lateral gene transfer (LGT) is the movement of genetic material between unicellular A unicellular organism, also known as a single-celled organism, is an organism In biology, an organism (from Ancient G ...
; new species can arise rapidly through hybridisation and
polyploid Polyploidy is a condition in which the biological cell, cells of an organism have more than two paired (Homologous chromosome, homologous) sets of chromosomes. Most species whose cells have Cell nucleus, nuclei (eukaryotes) are diploid, meaning t ...
y; and species may for a variety of reasons.
Viruses A virus is a submicroscopic infectious agent In biology Biology is the natural science that studies life and living organisms, including their anatomy, physical structure, Biochemistry, chemical processes, Molecular biology, molecu ...
are a special case, driven by a
balance of mutation and selection Balance may refer to: Common meanings * Balance (ability) in biomechanics * Balance (accounting) * Balance or weighing scale Arts and entertainment Film * Balance (1983 film), ''Balance'' (1983 film), a Bulgarian film * Balance (1989 film), ''Bal ...
, and can be treated as
quasispeciesThe quasispecies model is a description of the process of the Darwinian evolution Evolution is change in the Heredity, heritable Phenotypic trait, characteristics of biological populations over successive generations. These characteristics are ...
.


Definition

Biologists and taxonomists have made many attempts to define species, beginning from
morphology Morphology, from the Greek and meaning "study of shape", may refer to: Disciplines * Morphology (archaeology), study of the shapes or forms of artifacts * Morphology (astronomy), study of the shape of astronomical objects such as nebulae, galaxies ...
and moving towards
genetics Genetics is a branch of biology Biology is the natural science that studies life and living organisms, including their anatomy, physical structure, Biochemistry, chemical processes, Molecular biology, molecular interactions, Physiology, ...

genetics
. Early taxonomists such as Linnaeus had no option but to describe what they saw: this was later formalised as the typological or morphological species concept.
Ernst Mayr Ernst Walter Mayr (; 5 July 1904 – 3 February 2005) was one of the 20th century's leading evolutionary biologists File:Francesco Redi.jpg, Francesco Redi, the founder of biology, is recognized to be one of the greatest biologists of all ti ...
emphasised reproductive isolation, but this, like other species concepts, is hard or even impossible to test. Later biologists have tried to refine Mayr's definition with the recognition and cohesion concepts, among others. Many of the concepts are quite similar or overlap, so they are not easy to count: the biologist R. L. Mayden recorded about 24 concepts, and the philosopher of science John Wilkins counted 26. Wilkins further grouped the species concepts into seven basic kinds of concepts: (1) agamospecies for asexual organisms (2) biospecies for reproductively isolated sexual organisms (3) ecospecies based on ecological niches (4) evolutionary species based on lineage (5) genetic species based on gene pool (6) morphospecies based on form or phenotype and (7) taxonomic species, a species as determined by a taxonomist.


Typological or morphological species

A typological species is a group of organisms in which individuals conform to certain fixed properties (a type), so that even pre-literate people often recognise the same taxon as do modern taxonomists. The clusters of variations or phenotypes within specimens (such as longer or shorter tails) would differentiate the species. This method was used as a "classical" method of determining species, such as with Linnaeus early in evolutionary theory. However, different phenotypes are not necessarily different species (e.g. a four-winged ''
Drosophila ''Drosophila'' () is a genus of fly, flies, belonging to the family (biology), family Drosophilidae, whose members are often called "small fruit flies" or (less frequently) pomace flies, vinegar flies, or wine flies, a reference to the character ...

Drosophila
'' born to a two-winged mother is not a different species). Species named in this manner are called ''morphospecies''. In the 1970s,
Robert R. Sokal Robert Reuven Sokal (January 13, 1926 in Vienna en, Viennese , iso_code = AT-9 , registration_plate = Vehicle registration plates of Austria, W , postal_code_type = Postal code , posta ...
, Theodore J. Crovello and
Peter Sneath Peter Henry Andrews Sneath FRS FRS may also refer to: Government and politics * Facility Registry System, a centrally managed Environmental Protection Agency database that identifies places of environmental interest in the United States * Family ...
proposed a variation on the morphological species concept, a
phenetic In biology Biology is the natural science that studies life and living organisms, including their anatomy, physical structure, Biochemistry, chemical processes, Molecular biology, molecular interactions, Physiology, physiological mechanism ...
species, defined as a set of organisms with a similar
phenotype right , Here the relation between genotype and phenotype is illustrated, using a Punnett square, for the character of petal color in pea plants. The letters B and b represent gene In biology, a gene (from ''genos'' "...Wilhelm Johannsen ...

phenotype
to each other, but a different phenotype from other sets of organisms. 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.


Recognition and cohesion species

A mate-recognition species is a group of sexually reproducing organisms that recognise one another as potential mates. Expanding on this to allow for post-mating isolation, a cohesion species is the most inclusive population of individuals having the potential for phenotypic cohesion through intrinsic cohesion mechanisms; no matter whether populations can hybridise successfully, they are still distinct cohesion species if the amount of hybridisation is insufficient to completely mix their respective
gene pool The gene pool is the set of all genes, or genetic information, in any population, usually of a particular species. Description A large gene pool indicates extensive genetic diversity, which is associated with robust populations that can surviv ...
s. A further development of the recognition concept is provided by the biosemiotic concept of species.


Genetic similarity and barcode species

In
microbiology Microbiology (from Greek#REDIRECT Greek Greek may refer to: Greece Anything of, from, or related to Greece Greece ( el, Ελλάδα, , ), officially the Hellenic Republic, is a country located in Southeast Europe. Its population is appro ...

microbiology
, genes can move freely even between distantly related bacteria, possibly extending to the whole bacterial domain. As a rule of thumb, microbiologists have assumed that kinds of
Bacteria Bacteria (; common noun bacteria, singular bacterium) are ubiquitous, mostly free-living organisms often consisting of one Cell (biology), biological cell. They constitute a large domain (biology), domain of prokaryotic microorganisms. Typ ...

Bacteria
or
Archaea Archaea ( ; singular archaeon ) constitute a domain Domain may refer to: Mathematics *Domain of a function, the set of input values for which the (total) function is defined **Domain of definition of a partial function **Natural domain of a pa ...

Archaea
with
16S ribosomal RNA 16 S ribosomal RNA (or 16 S rRNA Ribosomal ribonucleic acid (rRNA) is a type of which is the primary component of s, essential to all cells. rRNA is a which carries out in ribosomes. Ribosomal RNA is transcribed from (rDNA) and then ...
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 or not. (See This concept was narrowed in 2006 to a similarity of 98.7%. DNA-DNA hybridisation is outdated, and results have sometimes led to misleading conclusions about species, as with the pomarine and
great skua The great skua (''Stercorarius skua'') is a large seabird Seabirds (also known as marine birds) are bird Birds are a group of warm-blooded vertebrates constituting the class (biology), class Aves , characterised by feathers, toothle ...
. Modern approaches compare sequence similarity using computational methods.
DNA barcoding DNA barcoding is a method of species identification using a short section of DNA from a specific gene or genes. The premise of DNA barcoding is that, by comparison with a reference library of such DNA sections (also called "DNA sequence, sequenc ...
has been proposed as a way to distinguish species suitable even for non-specialists to use. One of the barcodes is a region of mitochondrial DNA within the gene for
cytochrome c oxidase The enzyme Enzymes () are proteins that act as biological catalysts (biocatalysts). Catalysts accelerate chemical reactions. The molecules upon which enzymes may act are called substrate (chemistry), substrates, and the enzyme converts the su ...
. A database,
Barcode of Life Data SystemsThe Barcode of Life Data System (commonly known as BOLD or BOLDSystems) is a web platform specifically devoted to DNA barcoding. It is a cloud-based data storage and analysis platform developed at the Centre for Biodiversity Genomics in Canada. It co ...
(BOLD), contains DNA barcode sequences from over 190,000 species. 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. Genetic introgression mediated by endosymbionts and other vectors can further make barcodes ineffective in the identification of species.


Phylogenetic or cladistic species

A phylogenetic or
cladistic Cladistics (; ) is an approach to Taxonomy (biology), biological classification in which organisms are categorized in groups ("clades") based on hypotheses of most recent common ancestry. The evidence for hypothesized relationships is typically ...

cladistic
species is "the smallest aggregation of populations (sexual) or lineages (asexual) diagnosable by a unique combination of character states in comparable individuals (semaphoronts)". 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. Molecular markers may be used to determine diagnostic genetic differences in the nuclear or
mitochondrial DNA Mitochondrial DNA (mtDNA or mDNA) is the DNA Deoxyribonucleic acid (; DNA) is a molecule File:Pentacene on Ni(111) STM.jpg, A scanning tunneling microscopy image of pentacene molecules, which consist of linear chains of five car ...

mitochondrial DNA
of various species. For example, in a study done on
fungi A fungus (plural The plural (sometimes abbreviated An abbreviation (from Latin ''brevis'', meaning ''short'') is a shortened form of a word or phrase, by any method. It may consist of a group of letters, or words taken from the full ...

fungi
, studying the nucleotide characters using cladistic species produced the most accurate results in recognising the numerous fungi species of all the concepts studied. Versions of the phylogenetic species concept that emphasise monophyly or diagnosability may lead to splitting of existing species, for example in
Bovidae The Bovidae comprise the biological family Family ( la, familia, plural ') is one of the eight major hierarchical taxonomic ranks in Linnaean taxonomy; it is classified between order (biology), order and genus. A family may be divided into su ...
, by recognising old subspecies as species, despite the fact that there are no reproductive barriers, and populations may intergrade morphologically. Others have called this approach
taxonomic inflation''Taxonomic inflation'' is a pejorative term for what is perceived to be an excessive increase in the number of recognised taxa in a given context, due not to the discovery of new taxa but rather to putatively arbitrary changes to how taxa are deline ...
, diluting the species concept and making taxonomy unstable. 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 An endangered species is a species In biology, a species is the basic unit of biological classification, classification and a taxonomic rank of an organism, as well as a unit of biodiversity. A species is often defined as the largest group ...

endangered
in the
IUCN The International Union for Conservation of Nature (IUCN; officially International Union for Conservation of Nature and Natural Resources) is an international organization ''International Organization'' is a quarterly peer-reviewed academic journ ...
red list The International Union for Conservation of Nature (IUCN) Red List of Threatened Species (also known as the IUCN Red List or Red Data Book), founded in 1964, is the world's most comprehensive inventory of the global Conservation movement, conserv ...
and can attract conservation legislation and funding. Unlike the biological species concept, a cladistic species does not rely on reproductive isolation – its criteria are independent of processes that are integral in other concepts. Therefore, it applies to asexual lineages. 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.


Evolutionary species

An evolutionary species, suggested by
George Gaylord Simpson George Gaylord Simpson (June 16, 1902 – October 6, 1984) was a United States, US paleontologist. Simpson was perhaps the most influential paleontologist of the twentieth century, and a major participant in the Modern synthesis (20th century), mo ...

George Gaylord Simpson
in 1951, 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". This differs from the biological species concept in embodying persistence over time. Wiley and Mayden stated that they see the evolutionary species concept as "identical" to
Willi Hennig Emil Hans Willi Hennig (20 April 1913 – 5 November 1976) was a German German(s) may refer to: Common uses * of or related to Germany * Germans, Germanic ethnic group, citizens of Germany or people of German ancestry * For citizens of German ...

Willi Hennig
's species-as-lineages concept, and asserted that the biological species concept, "the several versions" of the phylogenetic species concept, and the idea that species are of the same kind as higher taxa are not suitable for biodiversity studies (with the intention of estimating the number of species accurately). They further suggested that the concept works for both asexual and sexually-reproducing species. A version of the concept is Kevin de Queiroz's "General Lineage Concept of Species".


Ecological species

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.


Genetic species

A genetic species as defined by Robert Baker and Robert Bradley is a set of genetically isolated interbreeding populations. This is similar to Mayr's Biological Species Concept, but stresses genetic rather than reproductive isolation. In the 21st century, a genetic species can be established by comparing DNA sequences, but other methods were available earlier, such as comparing
karyotype A karyotype is a preparation of the complete set of metaphase Metaphase () is a stage of mitosis In cell biology Cell biology (also cellular biology or cytology) is a branch of biology Biology is the natural science that studies ...

karyotype
s (sets of
chromosome A chromosome is a long DNA molecule with part or all of the genome, genetic material of an organism. Most eukaryotic chromosomes include packaging proteins called histones which, aided by Chaperone (protein), chaperone proteins, bind to and ...

chromosome
s) and
allozyme Alloenzymes (or also called allozymes) are variant forms of an enzyme Enzymes () are proteins that act as biological catalysts (biocatalysts). Catalysts accelerate chemical reactions. The molecules upon which enzymes may act are called substr ...
s (
enzyme Enzymes () are proteins that act as biological catalysts (biocatalysts). Catalysts accelerate chemical reactions. The molecules upon which enzymes may act are called substrate (chemistry), substrates, and the enzyme converts the substrates in ...

enzyme
variants).


Evolutionarily significant unit

An
evolutionarily significant unitAn evolutionarily significant unit (ESU) is a population of organism In biology, an organism (from Ancient Greek, Greek: ὀργανισμός, ''organismos'') is any individual contiguous system that embodies the Life#Biology, properties of ...
(ESU) or "wildlife species" is a population of organisms considered distinct for purposes of conservation.


Chronospecies

In
palaeontology Paleontology (), also spelled palaeontology or palæontology, is the scientific study of life that existed prior to, and sometimes including, the start of the Holocene epoch (geology), epoch (roughly 11,700 years before present). It includes th ...
, with only
comparative anatomy Comparative anatomy is the study of similarities and differences in the anatomy Anatomy (Greek ''anatomē'', 'dissection') is the branch of biology concerned with the study of the structure of organism In biology, an organism (from Anc ...
(morphology) from
fossil A fossil (from Classical Latin Classical Latin is the form of Latin, Latin language recognized as a Literary language, literary standard language, standard by writers of the late Roman Republic and early Roman Empire. It was used from 75 ...

fossil
s as evidence, the concept of a
chronospecies A chronospecies is a species In biology, a species is the basic unit of biological classification, 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 organ ...

chronospecies
can be applied. During
anagenesis Anagenesis is the gradual evolution Evolution is change in the Heredity, heritable Phenotypic trait, characteristics of biological populations over successive generations. These characteristics are the Gene expression, expressions of genes th ...
(evolution, not necessarily involving branching), 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, palaeontologists assess how much change is required for a morphologically distinct form to be considered a different species from its ancestors.


Viral quasispecies

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. All of these factors make conventional species concepts largely inapplicable. A viral
quasispeciesThe quasispecies model is a description of the process of the Darwinian evolution Evolution is change in the Heredity, heritable Phenotypic trait, characteristics of biological populations over successive generations. These characteristics are ...
is a group of genotypes related by similar mutations, competing within a highly
mutagenic In genetics, a mutagen is a physical or chemical agent that permanently changes genetic material, usually DNA, in an organism In biology, an organism (from Ancient Greek, Greek: ὀργανισμός, ''organismos'') is any individual cont ...
environment, and hence governed by a
mutation–selection balance Mutation–selection balance is an equilibrium in the number of deleterious alleles in a population that occurs when the rate at which deleterious alleles are created by mutation equals the rate at which deleterious alleles are eliminated by Natural ...
. It is predicted that a viral quasispecies at a low but evolutionarily neutral and highly connected (that is, flat) region in the
fitness landscape In evolutionary biology Evolutionary biology is the subfield of biology that studies the evolution, evolutionary processes (natural selection, common descent, speciation) that produced the Biodiversity, diversity of life on Earth. In the 1930s ...
will outcompete a quasispecies located at a higher but narrower fitness peak in which the surrounding mutants are unfit, "the quasispecies effect" or the "survival of the flattest". There is no suggestion that a viral quasispecies resembles a traditional biological species.


Taxonomy and naming


Common and scientific names

The commonly used names for kinds of organisms are often ambiguous: "cat" could mean the domestic cat, ''
Felis catus The cat (''Felis catus'') is a domestic Domestic may refer to: In the home * Anything relating to the human home A home, or domicile, is a space used as a permanent or semi-permanent residence for an individual, group or famil ...

Felis catus
'', or the cat family,
Felidae Felidae () is a Family (biology), family of mammals in the Order (biology), order Carnivora, colloquially referred to as cats, and constitutes a clade. A member of this family is also called a felid (). The term "cat" refers both to felids in ge ...
. 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 The cougar (''Puma concolor'') is a large cat of the subfamily Felinae The Felinae are a subfamily of the family Felidae. This subfamily comprises the small cats having a bony hyoid, because of which they are able to purr but not roa ...

Puma concolor
'' in various parts of America, while "panther" may also mean the
jaguar The jaguar (''Panthera onca'') is a large cat The cat (''Felis catus'') is a domestic species In biology, a species is the basic unit of biological classification, classification and a taxonomic rank of an organism, as well as ...

jaguar
(''Panthera onca'') of Latin America or the
leopard The leopard (''Panthera pardus'') is one of the five extant in the ', a member of the cat , . It occurs in a wide range in , in some parts of and , , and on the to and . It is listed as on the because leopard populations are threatened ...

leopard
(''Panthera pardus'') of Africa and Asia. In contrast, the scientific names of species are chosen to be unique and universal; they are in two parts used together: the
genus Genus /ˈdʒiː.nəs/ (plural genera /ˈdʒen.ər.ə/) is a taxonomic rank In biological classification In biology, taxonomy () is the scientific study of naming, defining (Circumscription (taxonomy), circumscribing) and classifying gr ...
as in ''Puma'', and the
specific epithet In taxonomy, binomial nomenclature ("two-term naming system"), also called nomenclature ("two-name naming system") or binary nomenclature, is a formal system of naming species In biology, a species is the basic unit of biological classi ...
as in ''concolor''.


Species description

A species is given a
taxonomic Taxonomy (general) is the practice and science of classification of things or concepts, including the principles that underlie such classification. The term may also refer to a specific classification scheme. Originally used only about biological ...
name when a
type specimen In biology, a type is a particular specimen (or in some cases a group of specimens) of an organism In biology Biology is the natural science that studies life and living organisms, including their anatomy, physical structure, Bi ...
is described formally, in a publication that assigns it a unique scientific name. The description typically provides means for identifying the new species, differentiating it from other previously described and related or confusable species and provides a
validly published name In botanical nomenclature, a validly published name is a name that meets the requirements in the '' International Code of Nomenclature for algae, fungi, and plants'' for valid publication. Valid publication of a name represents the minimum require ...
(in botany) or an
available nameIn zoological Zoology ()The pronunciation of zoology as is typically regarded as nonstandard, though it is not uncommon. is the branch of biology Biology is the natural science that studies life and living organisms, including their anatom ...
(in zoology) when the paper is accepted for publication. 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.One example of an abstract of an article naming a new species can be found at 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".


Abbreviations

Books and articles sometimes intentionally do not identify species fully, using the abbreviation "sp." in the singular or "spp." (standing for ''species pluralis'', the Latin for multiple species) in the plural in place of the specific name or epithet (e.g. ''Canis'' sp.). This commonly occurs when authors are confident that some individuals belong to a particular genus but are not sure to which exact species they belong, as is common in paleontology. Authors may also use "spp." as a short way of saying that something applies to many species within a genus, but not to all. If scientists mean that something applies to all species within a genus, they use the genus name without the specific name or epithet. The names of genera and species are usually printed in italics. However, abbreviations such as "sp." should not be italicised. When a species's identity is not clear, a specialist may use "cf." before the epithet to indicate that confirmation is required. The abbreviations "nr." (near) or "aff." (affine) may be used when the identity is unclear but when the species appears to be similar to the species mentioned after.


Identification codes

With the rise of online databases, codes have been devised to provide identifiers for species that are already defined, including: * National Center for Biotechnology Information (NCBI) employs a numeric 'taxid' or ''Taxonomy identifier'', a "stable unique identifier", e.g., the taxid of ''Homo sapiens'' is 9606. * Kyoto Encyclopedia of Genes and Genomes (KEGG) employs a three- or four-letter code for a limited number of organisms; in this code, for example, ''H. sapiens'' is simply ''hsa''. * UniProt employs an "organism mnemonic" of not more than five alphanumeric characters, e.g., ''HUMAN'' for ''H. sapiens''. * Integrated Taxonomic Information System (ITIS) provides a unique number for each species. The LSID for ''Homo sapiens'' is urn:lsid:catalogueoflife.org:taxon:4da6736d-d35f-11e6-9d3f-bc764e092680:col20170225.


Lumping and splitting

The naming of a particular species, including which genus (and higher taxa) it is placed in, is a ''hypothesis'' about the evolutionary relationships and distinguishability of that group of organisms. As further information comes to hand, the hypothesis may be corroborated or refuted. Sometimes, especially in the past when communication was more difficult, taxonomists working in isolation have given two distinct names to individual organisms later identified as the same species. When two species names are discovered to apply to the same species, the older species name is given Principle of Priority, priority and usually retained, and the newer name considered as a junior synonym, a process called ''synonymy (taxonomy), synonymy''. Dividing a taxon into multiple, often new, taxa is called ''splitting''. Taxonomists are often referred to as "lumpers" or "splitters" by their colleagues, depending on their personal approach to recognising differences or commonalities between organisms. 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.


Broad and narrow senses

The Nomenclature codes, nomenclatural codes that guide the naming of species, including the International Code of Zoological Nomenclature, ICZN for animals and the International Code of Nomenclature for algae, fungi, and plants, ICN for plants, do not make rules for defining the boundaries of the species. Research can change the boundaries, also known as circumscription, based on new evidence. 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. Other abbreviations such as "auct." ("author"), and qualifiers such as "non" ("not") may be used to further clarify the sense in which the specified authors delineated or described the species.


Mayr's biological species concept

Most modern textbooks make use of
Ernst Mayr Ernst Walter Mayr (; 5 July 1904 – 3 February 2005) was one of the 20th century's leading evolutionary biologists File:Francesco Redi.jpg, Francesco Redi, the founder of biology, is recognized to be one of the greatest biologists of all ti ...
's 1942 definition, known as the Biological Species Concept as a basis for further discussion on the definition of species. It is also called a reproductive or isolation concept. This defines a species as It has been argued that this definition is a natural consequence of the effect of sexual reproduction on the dynamics of natural selection. Mayr's use of the adjective "potentially" has been a point of debate; some interpretations exclude unusual or artificial matings that occur only in captivity, or that involve animals capable of mating but that do not normally do so in the wild.


The species problem

It is difficult to define a species in a way that applies to all organisms. The debate about species delimitation is called the Species concept, species problem. The problem was recognised even in 1859, when Darwin wrote in ''
On the Origin of Species ''On the Origin of Species'' (or, more completely, ''On the Origin of Species by Means of Natural Selection, or the Preservation of Favoured Races in the Struggle for Life''),The book's full original title was ''On the Origin of Species by Me ...
'':


When Mayr's concept breaks down

A simple textbook definition, following Mayr's concept, works well for most multi-celled organisms, but breaks down in several situations: * When organisms Asexual reproduction, reproduce asexually, as in single-celled organisms such as bacteria and other prokaryotes, and parthenogenetic or apomixis, apomictic multi-celled organisms. Barcoding and phylogenetics are commonly used in these cases. The term quasispecies is sometimes used for rapidly mutating entities like viruses. * When scientists do not know whether two morphologically similar groups of organisms are capable of interbreeding; this is the case with all extinct life-forms in
palaeontology Paleontology (), also spelled palaeontology or palæontology, is the scientific study of life that existed prior to, and sometimes including, the start of the Holocene epoch (geology), epoch (roughly 11,700 years before present). It includes th ...
, as breeding experiments are not possible. * When hybridisation permits substantial gene flow between species. * In
ring species In biology Biology is the natural science that studies life and living organisms, including their anatomy, physical structure, Biochemistry, chemical processes, Molecular biology, molecular interactions, Physiology, physiological mechanisms, ...

ring species
, when members of adjacent populations in a widely continuous distribution range interbreed successfully but members of more distant populations do not. Species identification is made difficult by discordance between molecular and morphological investigations; these can be categorised as two types: (i) one morphology, multiple lineages (e.g. convergent evolution, morphological convergence, cryptic species) and (ii) one lineage, multiple morphologies (e.g. phenotypic plasticity, multiple biological life cycle, life-cycle stages). In addition,
horizontal gene transfer Horizontal gene transfer (HGT) or lateral gene transfer (LGT) is the movement of genetic material between unicellular A unicellular organism, also known as a single-celled organism, is an organism In biology, an organism (from Ancient G ...
(HGT) makes it difficult to define a species. All species definitions assume that an organism acquires its genes from one or two parents very like the "daughter" organism, but that is not what happens in HGT. There is strong evidence of HGT between very dissimilar groups of prokaryotes, and at least occasionally between dissimilar groups of
eukaryote Eukaryotes () are organism In biology, an organism () is any organic, life, living system that functions as an individual entity. All organisms are composed of cells (cell theory). Organisms are classified by taxonomy (biology), tax ...

eukaryote
s, including some crustaceans and echinoderms. The evolutionary biologist James Mallet concludes that


Aggregates of microspecies

The species concept is further weakened by the existence of
microspecies In biology, a species complex is a group of closely related organism In biology, an organism (from Ancient Greek, Greek: ὀργανισμός, ''organismos'') is any individual contiguous system that embodies the Life#Biology, properties ...
, groups of organisms, including many plants, with very little genetic variability, usually forming Species complex, species aggregates. For example, the dandelion ''Taraxacum officinale'' and the blackberry ''Rubus fruticosus'' are aggregates with many microspecies—perhaps 400 in the case of the blackberry and over 200 in the dandelion, complicated by Hybridisation (biology), hybridisation, apomixis and polyploidy, making gene flow between populations difficult to determine, and their taxonomy debatable. Species complexes occur in insects such as ''Heliconius'' butterflies, vertebrates such as ''Hypsiboas'' treefrogs, and fungi such as the fly agaric. File:Ripe, ripening, and green blackberries.jpg, Blackberries belong to any of hundreds of microspecies of the ''Rubus fruticosus'' species aggregate. File:Heliconius mimicry.png, The butterfly genus ''Heliconius'' contains many similar species. File:Systematics-of-treefrogs-of-the-Hypsiboas-calcaratus-and-Hypsiboas-fasciatus-species-complex-(Anura-ZooKeys-370-001-g009.jpg, The ''Hypsiboas calcaratus''–''Hypsiboas fasciatus, fasciatus'' species complex contains at least six species of treefrog.


Hybridisation

Natural Hybrid (biology), hybridisation presents a challenge to the concept of a reproductively isolated species, as fertile hybrids permit gene flow between two populations. For example, the carrion crow ''Corvus corone'' and the hooded crow ''Corvus cornix'' appear and are classified as separate species, yet they hybridise freely where their geographical ranges overlap. File:Krähe_65(loz) (cropped).JPG, Carrion crow File:20151221 hybrid Corvus corone × Corvus cornix.jpg, Hybrid with dark belly, dark gray nape File:Corvus cornix (Scops).jpg, Hybrid (biology), Hybrid with dark belly File:Wrona_Siwa.jpg, Hooded crow


Ring species

A
ring species In biology Biology is the natural science that studies life and living organisms, including their anatomy, physical structure, Biochemistry, chemical processes, Molecular biology, molecular interactions, Physiology, physiological mechanisms, ...

ring species
is a connected series of neighbouring populations, each of which can sexually interbreed with adjacent related populations, but for which there exist at least two "end" populations in the series, which are too distantly related to interbreed, though there is a potential gene flow between each "linked" population. Such non-breeding, though genetically connected, "end" populations may Sympatric, co-exist in the same region thus closing the ring. Ring species thus present a difficulty for any species concept that relies on reproductive isolation. However, ring species are at best rare. Proposed examples include the herring gull-lesser black-backed gull complex around the North pole, the ''Ensatina eschscholtzii'' group of 19 populations of salamanders in America, and the greenish warbler in Asia, but many so-called ring species have turned out to be the result of misclassification leading to questions on whether there really are any ring species. File:Ring species seagull.svg, Seven "species" of ''Larus'' gulls interbreed in a ring around the Arctic. File:PT05 ubt.jpeg, Opposite ends of the ring: a herring gull (''Larus argentatus'') (front) and a lesser black-backed gull (''Larus fuscus'') in Norway File:Greenish Warbler Sikkim India 11.05.2014.jpg, A greenish warbler, ''Phylloscopus trochiloides'' File:Greenish warbler ring.svg, Presumed
evolution Evolution is change in the heritable Heredity, also called inheritance or biological inheritance, is the passing on of Phenotypic trait, traits from parents to their offspring; either through asexual reproduction or sexual reproduction, ...

evolution
of five "species" of greenish warblers around the Himalayas


Change

Species are subject to change, whether by evolving into new species, exchanging genes with other species, merging with other species or by becoming extinct.


Speciation

The
evolution Evolution is change in the heritable Heredity, also called inheritance or biological inheritance, is the passing on of Phenotypic trait, traits from parents to their offspring; either through asexual reproduction or sexual reproduction, ...

evolution
ary process by which biological populations evolve to become distinct or reproductively isolated as species is called speciation.
Charles Darwin Charles Robert Darwin (; ; 12 February 1809 – 19 April 1882) was an English natural history#Before 1900, naturalist, geologist and biologist, best known for his contributions to the science of evolution. His proposition that all sp ...

Charles Darwin
was the first to describe the role of
natural selection Natural selection is the differential survival and reproduction of individuals due to differences in phenotype right , Here the relation between genotype and phenotype is illustrated, using a Punnett square, for the character of peta ...
in speciation in his 1859 book ''The Origin of Species''. Speciation depends on a measure of reproductive isolation, a reduced gene flow. This occurs most easily in allopatric speciation, where populations are separated geographically and can diverge gradually as mutations accumulate. Reproductive isolation is threatened by hybridisation, but this can be selected against once a pair of populations have incompatible alleles of the same gene, as described in the Bateson–Dobzhansky–Muller model. A different mechanism, phyletic speciation, involves one lineage gradually changing over time into a new and distinct form, without increasing the number of resultant species.


Exchange of genes between species

Horizontal gene transfer between organisms of different species, either through Hybrid (biology), hybridisation, antigenic shift, or reassortment, is sometimes an important source of genetic variation. Viruses can transfer genes between species. Bacteria can exchange plasmids with bacteria of other species, including some apparently distantly related ones in different phylogenetic Three-domain system, domains, making analysis of their relationships difficult, and weakening the concept of a bacterial species. Louis-Marie Bobay and Howard Ochman suggest, based on analysis of the genomes of many types of bacteria, that they can often be grouped "into communities that regularly swap genes", in much the same way that plants and animals can be grouped into reproductively isolated breeding populations. Bacteria may thus form species, analogous to Mayr's biological species concept, consisting of asexually reproducing populations that exchange genes by homologous recombination.


Extinction

A species is extinct when the endling, last individual of that species dies, but it may be functional extinction, functionally extinct well before that moment. It is estimated that over 99 percent of all species that ever lived on Earth, some five billion species, are now extinct. Some of these were in Extinction event, mass extinctions such as those at the ends of the Ordovician, Devonian, Permian, Triassic and Cretaceous periods. Mass extinctions had a variety of causes including volcano, volcanic activity, climate change (general concept), climate change, and changes in oceanic and atmospheric chemistry, and they in turn had major effects on Earth's ecology, atmosphere, land surface and waters. Another form of extinction is through the assimilation of one species by another through hybridization. The resulting single species has been termed as a "compilospecies".


Practical implications

Biologists and
conservationists The conservation movement, also known as nature conservation, is a political, environmental, and social movement that seeks to manage and protect natural resource , Malaysia is an example of undisturbed natural resource. Waterfalls provide sp ...
need to categorise and identify organisms in the course of their work. Difficulty assigning organisms reliably to a species constitutes a threat to the external validity, validity of research results, for example making measurements of how abundant a species is in an ecosystem moot. Surveys using a phylogenetic species concept reported 48% more species and accordingly smaller populations and ranges than those using nonphylogenetic concepts; this was termed "taxonomic inflation", which could cause a false appearance of change to the number of endangered species and consequent political and practical difficulties. Some observers claim that there is an inherent conflict between the desire to understand the processes of speciation and the need to identify and to categorise. Conservation laws in many countries make special provisions to prevent species from going extinct. Hybridization zones between two species, one that is protected and one that is not, have sometimes led to conflicts between lawmakers, land owners and conservationists. One of the classic cases in North America is that of the protected northern spotted owl which hybridises with the unprotected California spotted owl and the barred owl; this has led to legal debates. It has been argued that the species problem is created by the varied uses of the concept of species, and that the solution is to abandon it and all other taxonomic ranks, and use unranked monophyletic groups instead. It has been argued, too, that since species are not comparable, counting them is not a valid measure of
biodiversity Biodiversity is the biological variety and of . Biodiversity is a measure of variation at the , , and level. Terrestrial biodiversity is usually greater near the , which is the result of the warm and high . Biodiversity is not distributed ev ...

biodiversity
; alternative measures of phylogenetic biodiversity have been proposed.


History


Classical forms

In Aristotle's biology, his biology,
Aristotle Aristotle (; grc-gre, Ἀριστοτέλης ''Aristotélēs'', ; 384–322 BC) was a Greek philosopher A philosopher is someone who practices philosophy Philosophy (from , ) is the study of general and fundamental quest ...

Aristotle
used the term γένος (génos) to mean a kind, such as a bird or fish, and εἶδος (eidos) to mean a specific Hylomorphism, form within a kind, such as (within the birds) the crane (bird), crane, eagle, crow, or Old World sparrow, sparrow. These terms were translated into Latin as "genus" and "species", though they do not correspond to the Linnean taxonomy, Linnean terms thus named; today the birds are a class (taxonomy), class, the cranes are a family (taxonomy), family, and the crows a
genus Genus /ˈdʒiː.nəs/ (plural genera /ˈdʒen.ər.ə/) is a taxonomic rank In biological classification In biology, taxonomy () is the scientific study of naming, defining (Circumscription (taxonomy), circumscribing) and classifying gr ...
. A kind was distinguished by its Property (philosophy), attributes; for instance, a bird has feathers, a beak, wings, a hard-shelled egg, and warm blood. A form was distinguished by being shared by all its members, the young inheriting any variations they might have from their parents. Aristotle believed all kinds and forms to be distinct and unchanging. His approach remained influential until the Renaissance.


Fixed species

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. Many of these early delineation schemes would now be considered whimsical: schemes included consanguinity based on colour (all plants with yellow flowers) or behaviour (snakes, scorpions and certain biting ants). John Ray, an English naturalist, was the first to attempt a biological definition of species in 1686, as follows: In the 18th century, the Swedish scientist Carl Linnaeus classified organisms according to shared physical characteristics, and not simply based upon differences. He established the idea of a Taxonomy (biology), taxonomic hierarchy of classification based upon observable characteristics and intended to reflect natural relationships. At the time, however, it was still widely believed that there was no organic connection between species, no matter how similar they appeared. This view was influenced by European scholarly and religious education, which held that the categories of life are dictated by God, forming an Aristotelianism, Aristotelian hierarchy, the Great chain of being#The Great Chain in natural science, ''scala naturae'' or great chain of being. However, whether or not it was supposed to be fixed, the ''scala'' (a ladder) inherently implied the possibility of climbing.


Mutability

In viewing evidence of hybridisation, Linnaeus recognised that species were not fixed and could change; he did not consider that new species could emerge and maintained a view of divinely fixed species that may alter through processes of hybridisation or acclimatisation. By the 19th century, naturalists understood that species could change form over time, and that the history of the planet provided enough time for major changes. Jean-Baptiste Lamarck, in his 1809 ''Zoological Philosophy'', described the transmutation of species, proposing that a species could change over time, in a radical departure from Aristotelian thinking. In 1859,
Charles Darwin Charles Robert Darwin (; ; 12 February 1809 – 19 April 1882) was an English natural history#Before 1900, naturalist, geologist and biologist, best known for his contributions to the science of evolution. His proposition that all sp ...

Charles Darwin
and Alfred Russel Wallace provided a compelling account of
evolution Evolution is change in the heritable Heredity, also called inheritance or biological inheritance, is the passing on of Phenotypic trait, traits from parents to their offspring; either through asexual reproduction or sexual reproduction, ...

evolution
and the formation of new species. Darwin argued that it was populations that evolved, not individuals, by
natural selection Natural selection is the differential survival and reproduction of individuals due to differences in phenotype right , Here the relation between genotype and phenotype is illustrated, using a Punnett square, for the character of peta ...
from naturally occurring variation among individuals. This required a new definition of species. Darwin concluded that species are what they appear to be: ideas, provisionally useful for naming groups of interacting individuals, writing:


See also

* Cline (population genetics), Cline * Encyclopedia of Life * Endangered species * Global biodiversity * Lists of animal species * Systematics * :Lists of animal species * :Lists of plant species


References


Sources

* * *


External links


Barcoding of species

Catalogue of Life

European Species Names in Linnaean, Czech, English, German and French

Stanford Encyclopedia of Philosophy entry: ''Species''

VisualTaxa

Wikispecies
– The free species directory that anyone can edit from the Wikimedia Foundation {{Authority control Species, Species Biological concepts Biology terminology Botanical nomenclature, Species Plant taxonomy, 1rank24 Zoological nomenclature, rank24 Bacterial nomenclature