Species

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For other uses, see Species (disambiguation). Species_sentence_0

In biology, a species is the basic unit of classification and a taxonomic rank of an organism, as well as a unit of biodiversity. Species_sentence_1

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. Species_sentence_2

Other ways of defining species include their karyotype, DNA sequence, morphology, behaviour or ecological niche. Species_sentence_3

In addition, paleontologists use the concept of the chronospecies since fossil reproduction cannot be examined. Species_sentence_4

The total number of species is estimated to be between 8 and 8.7 million. Species_sentence_5

However the vast majority of them are not studied or documented and it may take over 1000 years to fully catalogue them all. Species_sentence_6

All species (except viruses) are given a two-part name, a "binomial". Species_sentence_7

The first part of a binomial is the genus to which the species belongs. Species_sentence_8

The second part is called the specific name or the specific epithet (in botanical nomenclature, also sometimes in zoological nomenclature). Species_sentence_9

For example, Boa constrictor is one of four species of the genus Boa, with constrictor being the species’ epithet. Species_sentence_10

While the definitions given above may seem adequate at first glance, when looked at more closely they represent problematic species concepts. Species_sentence_11

For example, the boundaries between closely related species become unclear with hybridisation, in a species complex of hundreds of similar microspecies, and in a ring species. Species_sentence_12

Also, among organisms that reproduce only asexually, the concept of a reproductive species breaks down, and each clone is potentially a microspecies. Species_sentence_13

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 for studying life on Earth, regardless of the theoretical difficulties. Species_sentence_14

If species were fixed and clearly distinct from one another, there would be no problem, but evolutionary processes cause species to change continually, and to grade into one another. Species_sentence_15

Species were seen from the time of Aristotle until the 18th century as fixed categories that could be arranged in a hierarchy, the great chain of being. Species_sentence_16

In the 19th century, biologists grasped that species could evolve given sufficient time. Species_sentence_17

Charles Darwin's 1859 book On the Origin of Species explained how species could arise by natural selection. Species_sentence_18

That understanding was greatly extended in the 20th century through genetics and population ecology. Species_sentence_19

Genetic variability arises from mutations and recombination, while organisms themselves are mobile, leading to geographical isolation and genetic drift with varying selection pressures. Species_sentence_20

Genes can sometimes be exchanged between species by horizontal gene transfer; new species can arise rapidly through hybridisation and polyploidy; and species may become extinct for a variety of reasons. Species_sentence_21

Viruses are a special case, driven by a balance of mutation and selection, and can be treated as quasispecies. Species_sentence_22

Definition Species_section_0

Biologists and taxonomists have made many attempts to define species, beginning from morphology and moving towards genetics. Species_sentence_23

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. Species_sentence_24

Ernst Mayr emphasised reproductive isolation, but this, like other species concepts, is hard or even impossible to test. Species_sentence_25

Later biologists have tried to refine Mayr's definition with the recognition and cohesion concepts, among others. Species_sentence_26

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. Species_sentence_27

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. Species_sentence_28

Typological or morphological species Species_section_1

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. Species_sentence_29

The clusters of variations or phenotypes within specimens (such as longer or shorter tails) would differentiate the species. Species_sentence_30

This method was used as a "classical" method of determining species, such as with Linnaeus early in evolutionary theory. Species_sentence_31

However, different phenotypes are not necessarily different species (e.g. a four-winged Drosophila born to a two-winged mother is not a different species). Species_sentence_32

Species named in this manner are called morphospecies. Species_sentence_33

In the 1970s, Robert R. Sokal, Theodore J. Crovello and Peter Sneath proposed a variation on this, a phenetic species, defined as a set of organisms with a similar phenotype to each other, but a different phenotype from other sets of organisms. Species_sentence_34

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. Species_sentence_35

Recognition and cohesion species Species_section_2

A mate-recognition species is a group of sexually reproducing organisms that recognize one another as potential mates. Species_sentence_36

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 hybridize successfully, they are still distinct cohesion species if the amount of hybridization is insufficient to completely mix their respective gene pools. Species_sentence_37

A further development of the recognition concept is provided by the biosemiotic concept of species. Species_sentence_38

Genetic similarity and barcode species Species_section_3

In microbiology, genes can move freely even between distantly related bacteria, possibly extending to the whole bacterial domain. Species_sentence_39

As a rule of thumb, microbiologists have assumed that kinds 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 or not. Species_sentence_40

This concept was narrowed in 2006 to a similarity of 98.7%. Species_sentence_41

DNA-DNA hybridisation is outdated, and results have sometimes led to misleading conclusions about species, as with the pomarine and great skua. Species_sentence_42

Modern approaches compare sequence similarity using computational methods. Species_sentence_43

DNA barcoding has been proposed as a way to distinguish species suitable even for non-specialists to use. Species_sentence_44

The so-called barcode is a region of mitochondrial DNA within the gene for cytochrome c oxidase. Species_sentence_45

A database, Barcode of Life Data Systems (BOLD) contains DNA barcode sequences from over 190,000 species. Species_sentence_46

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. Species_sentence_47

Genetic introgression mediated by endosymbionts and other vectors can further make barcodes ineffective in the identification of species. Species_sentence_48

Phylogenetic, cladistic, or evolutionary species Species_section_4

A phylogenetic or cladistic species is an evolutionarily divergent lineage, one that has maintained its hereditary integrity through time and space. Species_sentence_49

A cladistic species is the smallest group of populations that can be distinguished by a unique set of morphological or genetic traits. Species_sentence_50

Molecular markers may be used to determine genetic similarities in the nuclear or mitochondrial DNA of various species. Species_sentence_51

For example, in a study done on fungi, studying the nucleotide characters using cladistic species produced the most accurate results in recognising the numerous fungi species of all the concepts studied. Species_sentence_52

Versions of the phylogenetic species concept may emphasize monophyly or diagnosability. Species_sentence_53

The concept may lead to splitting of existing species, for example in Bovidae, by recognising old subspecies as species, despite the fact that there are no reproductive barriers, and populations may inter-grade morphologically. Species_sentence_54

Others have called this approach taxonomic inflation, diluting the species concept and making taxonomy unstable. Species_sentence_55

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. Species_sentence_56

Unlike the biological species concept, a cladistic species does not rely on reproductive isolation, so it is independent of processes that are integral in other concepts. Species_sentence_57

It works for asexual lineages. Species_sentence_58

However, it does not work in every situation, and may require more than one polymorphic locus to give an accurate result. Species_sentence_59

An evolutionary species, suggested by 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". Species_sentence_60

This differs from the biological species concept in embodying persistence over time. Species_sentence_61

Wiley and Mayden state that they see the evolutionary species concept as "identical" to Willi Hennig's species-as-lineages concept, and assert 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). Species_sentence_62

They further suggest that the concept works for both asexual and sexually-reproducing species. Species_sentence_63

Ecological species Species_section_5

An ecological species is a set of organisms adapted to a particular set of resources, called a niche, in the environment. Species_sentence_64

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. Species_sentence_65

Genetic species Species_section_6

A genetic species as defined by Robert Baker and Robert Bradley is a set of genetically isolated interbreeding populations. Species_sentence_66

This is similar to Mayr's Biological Species Concept, but stresses genetic rather than reproductive isolation. Species_sentence_67

In the 21st century, a genetic species can be established by comparing DNA sequences, but other methods were available earlier, such as comparing karyotypes (sets of chromosomes) and allozymes (enzyme variants). Species_sentence_68

Evolutionarily significant unit Species_section_7

An evolutionarily significant unit (ESU) or "wildlife species" is a population of organisms considered distinct for purposes of conservation. Species_sentence_69

Chronospecies Species_section_8

Main article: Chronospecies Species_sentence_70

In palaeontology, with only comparative anatomy (morphology) from fossils as evidence, the concept of a chronospecies can be applied. Species_sentence_71

During anagenesis (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. Species_sentence_72

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. Species_sentence_73

Viral quasispecies Species_section_9

Main article: Viral quasispecies Species_sentence_74

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. Species_sentence_75

All of these factors make conventional species concepts largely inapplicable. Species_sentence_76

A viral quasispecies is a group of genotypes related by similar mutations, competing within a highly mutagenic environment, and hence governed by a mutation–selection balance. Species_sentence_77

It is predicted that a viral quasispecies at a low but evolutionarily neutral and highly connected (that is, flat) region in the fitness landscape 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". Species_sentence_78

There is no suggestion that a viral quasispecies resembles a traditional biological species. Species_sentence_79

Taxonomy and naming Species_section_10

Common and scientific names Species_section_11

The commonly used names for kinds of organisms are often ambiguous: "cat" could mean the domestic cat, Felis catus, or the cat family, Felidae. Species_sentence_80

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. Species_sentence_81

In contrast, the scientific names of species are chosen to be unique and universal; they are in two parts used together: the genus as in Puma, and the specific epithet as in concolor. Species_sentence_82

Species description Species_section_12

Main article: Species description Species_sentence_83

A species is given a taxonomic name when a type specimen is described formally, in a publication that assigns it a unique scientific name. Species_sentence_84

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 botany) or an available name (in zoology) when the paper is accepted for publication. Species_sentence_85

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. Species_sentence_86

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". Species_sentence_87

Abbreviations Species_section_13

Books and articles sometimes intentionally do not identify species fully and use 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.). Species_sentence_88

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. Species_sentence_89

Authors may also use "spp." as a short way of saying that something applies to many species within a genus, but not to all. Species_sentence_90

If scientists mean that something applies to all species within a genus, they use the genus name without the specific name or epithet. Species_sentence_91

The names of genera and species are usually printed in italics. Species_sentence_92

However, abbreviations such as "sp." should not be italicised. Species_sentence_93

When a species' identity is not clear, a specialist may use "cf." Species_sentence_94

before the epithet to indicate that confirmation is required. Species_sentence_95

The abbreviations "nr." Species_sentence_96

(near) or "aff." Species_sentence_97

(affine) may be used when the identity is unclear but when the species appears to be similar to the species mentioned after. Species_sentence_98

Identification codes Species_section_14

With the rise of online databases, codes have been devised to provide identifiers for species that are already defined, including: Species_sentence_99

Species_unordered_list_0

Lumping and splitting Species_section_15

Main article: Lumpers and splitters Species_sentence_100

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. Species_sentence_101

As further information comes to hand, the hypothesis may be confirmed or refuted. Species_sentence_102

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. Species_sentence_103

When two named species are discovered to be of the same species, the older species name is given priority and usually retained, and the newer name considered as a junior synonym, a process called synonymisation. Species_sentence_104

Dividing a taxon into multiple, often new, taxa is called splitting. Species_sentence_105

Taxonomists are often referred to as "lumpers" or "splitters" by their colleagues, depending on their personal approach to recognising differences or commonalities between organisms. Species_sentence_106

Broad and narrow senses Species_section_16

Main article: Sensu Species_sentence_107

The nomenclatural codes that guide the naming of species, including the ICZN for animals and the ICN for plants, do not make rules for defining the boundaries of the species. Species_sentence_108

Research can change the boundaries, also known as circumscription, based on new evidence. Species_sentence_109

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. Species_sentence_110

Other abbreviations such as "auct." Species_sentence_111

("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. Species_sentence_112

Mayr's biological species concept Species_section_17

Main article: Species problem § Mayr's Biological Species Concept Species_sentence_113

Most modern textbooks make use of Ernst Mayr's 1942 definition, known as the Biological Species Concept as a basis for further discussion on the definition of species. Species_sentence_114

It is also called a reproductive or isolation concept. Species_sentence_115

This defines a species as Species_sentence_116

It has been argued that this definition is a natural consequence of the effect of sexual reproduction on the dynamics of natural selection. Species_sentence_117

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. Species_sentence_118

The species problem Species_section_18

Main article: Species problem Species_sentence_119

It is difficult to define a species in a way that applies to all organisms. Species_sentence_120

The debate about species delimitation is called the species problem. Species_sentence_121

The problem was recognized even in 1859, when Darwin wrote in On the Origin of Species: Species_sentence_122

When Mayr's concept breaks down Species_section_19

A simple textbook definition, following Mayr's concept, works well for most multi-celled organisms, but breaks down in several situations: Species_sentence_123

Species_unordered_list_1

  • When organisms reproduce asexually, as in single-celled organisms such as bacteria and other prokaryotes, and parthenogenetic or apomictic multi-celled organisms. The term quasispecies is sometimes used for rapidly mutating entities like viruses.Species_item_1_4
  • 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, as breeding experiments are not possible.Species_item_1_5
  • When hybridisation permits substantial gene flow between species.Species_item_1_6
  • In ring species, when members of adjacent populations in a widely continuous distribution range interbreed successfully but members of more distant populations do not.Species_item_1_7

Species identification is made difficult by discordance between molecular and morphological investigations; these can be categorized as two types: (i) one morphology, multiple lineages (e.g. morphological convergence, cryptic species) and (ii) one lineage, multiple morphologies (e.g. phenotypic plasticity, multiple life-cycle stages). Species_sentence_124

In addition, horizontal gene transfer (HGT) makes it difficult to define a species. Species_sentence_125

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. Species_sentence_126

There is strong evidence of HGT between very dissimilar groups of prokaryotes, and at least occasionally between dissimilar groups of eukaryotes, including some crustaceans and echinoderms. Species_sentence_127

The evolutionary biologist James Mallet concludes that Species_sentence_128

Aggregates of microspecies Species_section_20

Main article: Species complex Species_sentence_129

The species concept is further weakened by the existence of microspecies, groups of organisms, including many plants, with very little genetic variability, usually forming species aggregates. Species_sentence_130

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, apomixis and polyploidy, making gene flow between populations difficult to determine, and their taxonomy debatable. Species_sentence_131

Species complexes occur in insects such as Heliconius butterflies, vertebrates such as Hypsiboas treefrogs, and fungi such as the fly agaric. Species_sentence_132

Species_unordered_list_2

  • Species_item_2_8
  • Species_item_2_9
  • Species_item_2_10

Hybridisation Species_section_21

Main article: Hybrid (biology) Species_sentence_133

Natural hybridisation presents a challenge to the concept of a reproductively isolated species, as fertile hybrids permit gene flow between two populations. Species_sentence_134

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. Species_sentence_135

Species_unordered_list_3

  • Hybridisation of carrion and hooded crows permits gene flow between 'species'Species_item_3_11
  • Species_item_3_12
  • Species_item_3_13
  • Species_item_3_14
  • Species_item_3_15

Ring species Species_section_22

Main article: Ring species Species_sentence_136

A 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. Species_sentence_137

Such non-breeding, though genetically connected, "end" populations may co-exist in the same region thus closing the ring. Species_sentence_138

Ring species thus present a difficulty for any species concept that relies on reproductive isolation. Species_sentence_139

However, ring species are at best rare. Species_sentence_140

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. Species_sentence_141

Species_unordered_list_4

  • Species_item_4_16
  • Species_item_4_17
  • Species_item_4_18
  • Species_item_4_19

Change Species_section_23

Species are subject to change, whether by evolving into new species, exchanging genes with other species, merging with other species or by becoming extinct. Species_sentence_142

Speciation Species_section_24

Main article: Speciation Species_sentence_143

The evolutionary process by which biological populations evolve to become distinct or reproductively isolated as species is called speciation. Species_sentence_144

Charles Darwin was the first to describe the role of natural selection in speciation in his 1859 book The Origin of Species. Species_sentence_145

Speciation depends on a measure of reproductive isolation, a reduced gene flow. Species_sentence_146

This occurs most easily in allopatric speciation, where populations are separated geographically and can diverge gradually as mutations accumulate. Species_sentence_147

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. Species_sentence_148

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. Species_sentence_149

Exchange of genes between species Species_section_25

Main article: Horizontal gene transfer Species_sentence_150

Horizontal gene transfer between organisms of different species, either through hybridisation, antigenic shift, or reassortment, is sometimes an important source of genetic variation. Species_sentence_151

Viruses can transfer genes between species. Species_sentence_152

Bacteria can exchange plasmids with bacteria of other species, including some apparently distantly related ones in different phylogenetic domains, making analysis of their relationships difficult, and weakening the concept of a bacterial species. Species_sentence_153

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. Species_sentence_154

Bacteria may thus form species, analogous to Mayr's biological species concept, consisting of asexually reproducing populations that exchange genes by homologous recombination. Species_sentence_155

Extinction Species_section_26

Further information: Extinction and Extinction event Species_sentence_156

A species is extinct when the last individual of that species dies, but it may be functionally extinct well before that moment. Species_sentence_157

It is estimated that over 99 percent of all species that ever lived on Earth, some five billion species, are now extinct. Species_sentence_158

Some of these were in mass extinctions such as those at the ends of the Ordovician, Devonian, Permian, Triassic and Cretaceous periods. Species_sentence_159

Mass extinctions had a variety of causes including volcanic activity, 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. Species_sentence_160

Another form of extinction is through the assimilation of one species by another through hybridization. Species_sentence_161

The resulting single species has been termed as a "compilospecies". Species_sentence_162

Practical implications Species_section_27

Biologists and conservationists need to categorise and identify organisms in the course of their work. Species_sentence_163

Difficulty assigning organisms reliably to a species constitutes a threat to the validity of research results, for example making measurements of how abundant a species is in an ecosystem moot. Species_sentence_164

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. Species_sentence_165

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. Species_sentence_166

Conservation laws in many countries make special provisions to prevent species from going extinct. Species_sentence_167

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. Species_sentence_168

One of the classic cases in North America is that of the protected northern spotted owl which hybridizes with the unprotected California spotted owl and the barred owl; this has led to legal debates. Species_sentence_169

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. Species_sentence_170

It has been argued, too, that since species are not comparable, counting them is not a valid measure of biodiversity; alternative measures of phylogenetic biodiversity have been proposed. Species_sentence_171

History Species_section_28

Classical forms Species_section_29

Main article: Aristotle's biology Species_sentence_172

In his biology, Aristotle used the term γένος (génos) to mean a kind, such as a bird or fish, and εἶδος (eidos) to mean a specific form within a kind, such as (within the birds) the crane, eagle, crow, or sparrow. Species_sentence_173

These terms were translated into Latin as "genus" and "species", though they do not correspond to the Linnean terms thus named; today the birds are a class, the cranes are a family, and the crows a genus. Species_sentence_174

A kind was distinguished by its attributes; for instance, a bird has feathers, a beak, wings, a hard-shelled egg, and warm blood. Species_sentence_175

A form was distinguished by being shared by all its members, the young inheriting any variations they might have from their parents. Species_sentence_176

Aristotle believed all kinds and forms to be distinct and unchanging. Species_sentence_177

His approach remained influential until the Renaissance. Species_sentence_178

Fixed species Species_section_30

Main article: Great chain of being Species_sentence_179

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. Species_sentence_180

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). Species_sentence_181

John Ray, an English naturalist, was the first to attempt a biological definition of species in 1686, as follows: Species_sentence_182

In the 18th century, the Swedish scientist Carl Linnaeus classified organisms according to shared physical characteristics, and not simply based upon differences. Species_sentence_183

He established the idea of a taxonomic hierarchy of classification based upon observable characteristics and intended to reflect natural relationships. Species_sentence_184

At the time, however, it was still widely believed that there was no organic connection between species, no matter how similar they appeared. Species_sentence_185

This view was influenced by European scholarly and religious education, which held that the categories of life are dictated by God, forming an Aristotelian hierarchy, the scala naturae or great chain of being. Species_sentence_186

However, whether or not it was supposed to be fixed, the scala (a ladder) inherently implied the possibility of climbing. Species_sentence_187

Mutability Species_section_31

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. Species_sentence_188

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. Species_sentence_189

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. Species_sentence_190

In 1859, Charles Darwin and Alfred Russel Wallace provided a compelling account of evolution and the formation of new species. Species_sentence_191

Darwin argued that it was populations that evolved, not individuals, by natural selection from naturally occurring variation among individuals. Species_sentence_192

This required a new definition of species. Species_sentence_193

Darwin concluded that species are what they appear to be: ideas, provisionally useful for naming groups of interacting individuals, writing: Species_sentence_194

See also Species_section_32

Credits to the contents of this page go to the authors of the corresponding Wikipedia page: en.wikipedia.org/wiki/Species.