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For other uses, see Species (disambiguation).
Species Genus Family Order Class Phylum Kingdom Domain Life
The various levels of the scientific classification system.
The hierarchy of biological classification's major eight taxonomic ranks. Taxonomic rank ( rank, category, taxonomic category is an abstract term used in the Scientific classification, or Taxonomy, of organisms A genus contains one or more species. A genus (plural genera from Γένος Latin genus "descent family type gender" is a low-level Taxonomic Intermediate minor rankings are not shown.

In biology, a species is one of the basic units of biological classification and a taxonomic rank. Foundations of modern biology There are five unifying principles Taxonomic rank ( rank, category, taxonomic category is an abstract term used in the Scientific classification, or Taxonomy, of organisms A species is often defined as a group of organisms capable of interbreeding and producing fertile offspring. While in many cases this definition is adequate, more precise or differing measures are often used, such as based on similarity of DNA or morphology. Presence of specific locally adapted traits may further subdivide species into subspecies. In Zoology, as in other branches of Biology, subspecies is the Taxonomic rank immediately subordinate to a Species.

The commonly used names for plant and animal taxa sometimes correspond to species: for example, "lion," "walrus," and "Camphor tree" – each refers to a species. The lion ( Panthera leo) is a member of the family Felidae and one of four Big cats in the Genus Panthera. The walrus ( Odobenus rosmarus) is a large flippered Marine mammal with a discontinuous circumpolar distribution in the Arctic Ocean and Cinnamomum camphora (commonly known as Camphor tree, Camphorwood or camphor laurel) is a large Evergreen Tree that In other cases common names do not: for example, "deer" refers to a family of 34 species, including Eld's Deer, Red Deer and Wapiti (Elk). A deer is a Ruminant Mammal belonging to the family Cervidae. In Biological classification, family ( Latin The Eld's Deer ( Cervus eldii) also called the Thamin or Brow-antlered Deer, is a Deer indigenous to Southeast Asia. The Red Deer ( Cervus elaphus) is one of the largest Deer species The last two species were once considered a single species, illustrating how species boundaries may change with increased scientific knowledge.

Each species is placed within a single genus. A genus (plural genera from Γένος Latin genus "descent family type gender" is a low-level Taxonomic This is a hypothesis that the species is more closely related to other species within its genus than to species of other genera. All species are given a binomial name consisting of the generic name and specific name (or specific epithet). In zoological nomenclature a specific name or specific epithet is the second part (second name in the name of a Species (a Binomen) For example, Pinus palustris (commonly known as the Longleaf Pine). The 'Longleaf Pine' ( Pinus palustris) is a Pine native to the southeast United States, found along the coastal plain from eastern Texas to southeast

A usable definition of the word "species" and reliable methods of identifying particular species are essential for stating and testing biological theories and for measuring biodiversity. Biodiversity is the variation of Life forms within a given Ecosystem, Biome or for the entire Earth. Traditionally, multiple examples of a proposed species must be studied for unifying characters before it can be regarded as a species. Extinct species known only from fossils are generally difficult to give precise taxonomic rankings to. A species which has been described scientifically can be referred to by its binomial names.

Nevertheless, as Charles Darwin remarked,

I look at the term species as one arbitrarily given for the sake of convenience to a set of individuals closely resembling each other . Charles Robert Darwin (February 12 1809 &ndash April 19 1882 was an English naturalist, who realised and demonstrated that all Species of life . . . it does not essentially differ from the term variety, which is given to less distinct and more fluxtuating forms. The term variety, again in comparison with mere individual difference, is also applied arbitrarily, and for mere convenience sake. [1]

Because of the difficulties with both defining and tallying the total numbers of different species in the world, it is estimated that there are anywhere between 2 and 100 million different species. [2]

Contents

Binomial convention for naming species

Main article: Binomial nomenclature

In scientific classification, a species is assigned a two-part name, treated as Latin, although roots from any language can be used as well as names of locales or individuals. Latin ( lingua Latīna, laˈtiːna is an Italic language, historically spoken in Latium and Ancient Rome. The genus is listed first (with its leading letter capitalized), followed by a second term: for example, gray wolves belong to the species Canis lupus, coyotes to Canis latrans, golden jackals to Canis aureus, etc. A genus (plural genera from Γένος Latin genus "descent family type gender" is a low-level Taxonomic , and all of those belong to the genus Canis (which also contains many other species). Canis is a Genus containing 7 to 10 extant species and many extinct species including wolves, Coyotes, and Jackals. The name of the species is the whole binomial, not just the second term (which may be called specific name for animals). In zoological nomenclature a specific name or specific epithet is the second part (second name in the name of a Species (a Binomen)

The binomial naming convention that is used, later formalized in the biological codes of nomenclature, was first used by Leonhart Fuchs and introduced as the standard by Carolus Linnaeus in his 1758 classical work Systema Naturae 10th edition. Taxonomy is the practice and science of classification The word comes from the Greek, taxis (meaning 'order' 'arrangement' and, nomos Leonhart Fuchs ( 17 January 1501 &ndash 10 May 1566) sometimes spelled Leonhard Fuchs, was a German Physician Carl Linnaeus (Latinized as Carolus Linnaeus, also known after his ennoblement as, May 23 new style (13 May old style 1707 who laid the foundations for As a result, it is sometimes called the "binomial nomenclature. " At that time, the chief biological theory was that species represented independent acts of creation by God and were therefore considered objectively real and immutable. God is the principal or sole Deity in Religions and other belief systems that worship one deity.

Abbreviation

Books and articles sometimes intentionally do not identify species fully and use the abbreviation "sp. " in the singular or "spp. " in the plural in place of the specific epithet: for example, Canis sp. This commonly occurs in the following types of situation:

In books and articles that use the genus and species names are usually printed in italics. In Typography, italic type /ɪˈtælɪk/ or /aɪˈtælɪk/ refers to cursive Typefaces based on a stylized form of calligraphic Handwriting. If using "sp. " and "spp. ," these should not be italicized.

Difficulty of defining "species" and identifying particular species

Main article: Species problem
The greenish warbler demonstrates the concept of a ring species.
The greenish warbler demonstrates the concept of a ring species. The species problem is a mixture of difficult related questions that often come up when Biologists identify Species and when they define the word "species" The Greenish Warbler, Phylloscopus trochiloides, is a widespread Leaf-warbler throughout its breeding range in northeast Europe and northern In Biology, a ring species is a connected series of neighboring populations that can interbreed with relatively closely related populations but for which there exist at least

It is surprisingly difficult to define the word "species" in a way that applies to all naturally occurring organisms, and the debate among biologists about how to define "species" and how to identify actual species is called the species problem. The species problem is a mixture of difficult related questions that often come up when Biologists identify Species and when they define the word "species"

Most textbooks define a species as all the individual organisms of a natural population that generally interbreed at maturity in the wild and whose interbreeding produces fertile offspring. Various parts of this definition are there to exclude some unusual or artificial matings:

Living organisms

The typical textbook definition (above) works well for most multi-celled organisms, but there are several types of situations in which it breaks down:

Horizontal gene transfer makes it even more difficult to define the word "species". There is strong evidence of horizontal gene transfer between very dissimilar groups of procaryotes, and possibly between dissimilar groups of single-celled eucaryotes; and Williamson[3] argues that there is evidence for it in some crustaceans and echinoderms. The prokaryotes (proʊˈkærioʊts singular prokaryote /proʊˈkæriət/ are a group of Organisms that lack a Cell nucleus (= karyon or any other Animals Plants fungi, and Protists are eukaryotes (juːˈkærɪɒt or -oʊt Organisms whose cells are organized into complex Structure of crustaceans As Arthropods crustaceans have a stiff Exoskeleton, which must be shed to allow the animal to grow ( Ecdysis or molting Echinoderms (Phylum Echinodermata) are a phylum of marine Animals (including Sea stars) All definitions of the word "species" assume that an organism gets all its genes from one or two parents which are very like that organism, but horizontal gene transfer makes that assumption false.

Extinct organisms

Main article: Extinction

Many extinct organisms are known only from fossils, which generally only preserve hard features. In Biology and Ecology, extinction is the cessation of existence of a Species or group of taxa. FOSSIL is a standard protocol for allowing serial communication for Telecommunications programs under the DOS Operating system. Fossils have not (so far) shown us what bred with what, and cannot tell us whether any resulting offspring would have been fertile. So paleontologists generally use either the morphological or the evolutionary definition of species (see below). Palaeontology redirects here For the Scientific journal, see Palaeontology (journal.

Paleontologists also have to cope with another difficulty: one species may gradually evolve into one or more others after a few million years; the original type of organism and the final one are so different that one could not regard the ancestors and the descendants as members of the same species if they existed at the same time; but the intermediate types are so similar to the next and previous types that one cannot say exactly where species A changed into species B. eVolution is the third Album by eLDee, it was due to be released in 2008 Paleontologists devised the concept of chronospecies to describe the simplest case, where at the end of the process there is only one descendant type of organism and there are no longer any individuals of the ancestral type. A chronospecies is a Species which changes physically morphologically, genetically, and/or behaviorally over time on an evolutionary scale such But even this refinement does not work in cases where several descendant types are alive at the same time or where the ancestral type and at least one descendant type are alive at the same time - and both of these situations are common in the evolution of life on Earth. Human evolution may offer a striking example: some geneticists have suggested that for about 1 million years there was some interbreeding between the early ancestors of humans and the early ancestors of chimpanzees (James Mallet and other MIT and Harvard scientists, as quoted in the news magazine This Week, June 9, 2006). Human evolution, or anthropogenesis, is the part of biological Evolution concerning the emergence of Homo sapiens as a distinct Species Genetics (from Ancient Greek grc-Latn genetikos, “genitive” and that from grc-Latn genesis, “origin” a discipline of Biology, is Chimpanzee (often shortened to chimp) is the common name for the two extant Species of Apes in the Genus Pan. Events 53 - Roman Emperor Nero marries Claudia Octavia 62 - Claudia Octavia commits Year 2006 ( MMVI) was a Common year starting on Sunday of the Gregorian calendar.

Definitions of species

See also: Species problem

The question of how best to define "species" is one that has occupied biologists for centuries, and the debate itself has become known as the species problem. The species problem is a mixture of difficult related questions that often come up when Biologists identify Species and when they define the word "species" One definition that is widely used is that a species is a group of actually or potentially interbreeding populations that are reproductively isolated from other such groups. [4]

The definition of a species given above is derived from the behavioral biologist Ernst Mayr, and is somewhat unrealistic. A definition is a statement of the meaning of a Word or Phrase. Since it assumes sexual reproduction, it leaves the term undefined for a large class of organisms that reproduce asexually. The Evolution of sexual reproduction is a major puzzle The first Fossilized evidence of sexually reproducing Organisms is from Eukaryotes of the Stenian Biologists frequently do not know whether two morphologically similar groups of organisms are "potentially" capable of interbreeding. Further, there is considerable variation in the degree to which hybridization may succeed under natural and experimental conditions, or even in the degree to which some organisms use sexual reproduction between individuals to breed. Consequently, several lines of thought in the definition of species exist:

Typological species 
A group of organisms in which individuals are members of the species if they sufficiently conform to certain fixed properties. The clusters of variations or phenotypes within specimens (i. e. longer and 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, we now know that different phenotypes do not always constitute different species (e. g. : a 4-winged Drosophila born to a 2-winged mother is not a different species). Species named in this manner are called morphospecies.
Morphological species 
A population or group of populations that differs morphologically from other populations. For example, we can distinguish between a chicken and a duck because they have different shaped bills and the duck has webbed feet. The chicken ( Gallus gallus, sometimes G gallus domesticus) is a domesticated Fowl which is traditionally believed to have descended from For duck as a food see Duck (food; for other meanings see Duck (disambiguation. Species have been defined in this way since well before the beginning of recorded history. This species concept is much criticised because more recent genetic data reveal that genetically distinct populations may look very similar and, contrarily, large morphological differences sometimes exist between very closely-related populations. Nonetheless, most species known have been described solely from morphology. The term morphology in Biology refers to the outward appearance ( Shape, Structure, Colour, Pattern) of an Organism
Biological / Isolation species 
A set of actually or potentially interbreeding populations. This is generally a useful formulation for scientists working with living examples of the higher taxa like mammals, fish, and birds, but meaningless for organisms that do not reproduce sexually. It does not distinguish between the theoretical possibility of interbreeding and the actual likelihood of gene flow between populations and is thus impractical in instances of allopatric (geographically isolated) populations. The results of breeding experiments done in artificial conditions may or may not reflect what would happen if the same organisms encountered each other in the wild, making it difficult to gauge whether or not the results of such experiments are meaningful in reference to natural populations.
Biological / reproductive species 
Two organisms that are able to reproduce naturally to produce fertile offspring. Organisms that can reproduce but almost always make infertile hybrids, such as a mule or hinny, are not considered to be the same species. In its common modern meaning a mule is the offspring of a male Donkey and a female Horse, which is classified as a kind of F1 hybrid. The word "hinny" is also a term of endearment used in North East England, equivalent to "honey" see Geordie.
Mate-recognition species 
A group of organisms that are known to recognize one another as potential mates. Like the isolation species concept above, it applies only to organisms that reproduce sexually. Unlike the isolation species concept, it focuses specifically on pre-mating reproductive isolation.
Phylogenetic (Cladistic)/ Evolutionary / Darwinian species 
A group of organisms that shares an ancestor; a lineage that maintains its integrity with respect to other lineages through both time and space. Cladistics is the hierarchical classification of Species based on evolutionary ancestry At some point in the progress of such a group, members may diverge from one another: when such a divergence becomes sufficiently clear, the two populations are regarded as separate species. Subspecies as such are not recognized under this approach; either a population is a phylogenetic species or it is not taxonomically distinguishable. In Zoology, as in other branches of Biology, subspecies is the Taxonomic rank immediately subordinate to a Species.
Ecological species
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 recognize as species because the ecological and evolutionary processes controlling how resources are divided up tend to produce those clusters
Genetic species 
based on similarity of DNA of individuals or populations. Techniques to compare similarity of DNA include DNA-DNA hybridization, and genetic fingerprinting (or DNA barcoding). DNA-DNA hybridization generally refers to a Molecular biology technique that measures the degree of genetic similarity between pools of DNA sequences DNA barcoding is a taxonomic method that uses a short genetic marker in an organism's mitochondrial DNA to identify it as belonging to a particular Species
Phenetic species
based on phenotypes
Recognition species
based on shared reproductive systems, including mating behavior
Microspecies 
Species that reproduce without meiosis or fertilization so that each generation is genetically identical to the previous generation. A phenotype is any observable characteristic of an Organism, such as its morphology, Development, biochemical or physiological properties In Biology or life science meiosis (pronounced my-oh-sis is a process of reductional division in which the number of chromosomes per cell is cut in half See also apomixis.
Cohesion species 
Most inclusive population of individuals having the potential for phenotypic cohesion through intrinsic cohesion mechanisms. This is an expansion of the mate-recognition species concept to allow for post-mating isolation 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. In Population genetics, a gene pool is the complete set of unique Alleles in a Species or Population.
Evolutionarily Significant Unit (ESU) 
An evolutionarily significant unit is a population of organisms that is considered distinct for purposes of conservation. An Evolutionarily Significant Unit (ESU (often lowercased where used without abbreviation as evolutionarily significant unit) is a population of Organisms that is Often referred to as a species or a wildlife species, an ESU also has several possible definitions, which coincide with definitions of species.

In practice, these definitions often coincide, and the differences between them are more a matter of emphasis than of outright contradiction. Nevertheless, no species concept yet proposed is entirely objective, or can be applied in all cases without resorting to judgment. Given the complexity of life, some have argued that such an objective definition is in all likelihood impossible, and biologists should settle for the most practical definition. For most vertebrates, this is the biological species concept (BSC), and to a lesser extent (or for different purposes) the phylogenetic species concept (PSC). Vertebrates are members of the Subphylum Vertebrata, Chordates with backbones or spinal columns The grouping sometimes includes Many BSC subspecies are considered species under the PSC; the difference between the BSC and the PSC can be summed up insofar as that the BSC defines a species as a consequence of manifest evolutionary history, while the PSC defines a species as a consequence of manifest evolutionary potential. In Zoology, as in other branches of Biology, subspecies is the Taxonomic rank immediately subordinate to a Species. Thus, a PSC species is "made" as soon as an evolutionary lineage has started to separate, while a BSC species starts to exist only when the lineage separation is complete. Accordingly, there can be considerable conflict between alternative classifications based upon the PSC versus BSC, as they differ completely in their treatment of taxa that would be considered subspecies under the latter model (e. g. , the numerous subspecies of honey bees). This article refers collectively to all true honey bees for the "common" domesticated honey bee see European honey bee Honey bees

Importance in biological classification

The idea of species has a long history. It is one of the most important levels of classification, for several reasons:

After thousands of years of use, the concept remains central to biology and a host of related fields, and yet also remains at times ill-defined.

Implications of assignment of species status

The naming of a particular species should be regarded as a hypothesis about the evolutionary relationships and distinguishability of that group of organisms. As further information comes to hand, the hypothesis may be confirmed 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 named species are discovered to be of the same species, the older species name is usually retained, and the newer species name dropped, a process called synonymization, or convivially, as lumping. Dividing a taxon into multiple, often new, taxons is called splitting. Taxonomists are often referred to as "lumpers" or "splitters" by their colleagues, depending on their personal approach to recognizing differences or commonalities between organisms (see lumpers and splitters). Lumping and splitting refers to a well known problem in any discipline which has to place individual examples into rigorously defined categories

Traditionally, researchers relied on observations of anatomical differences, and on observations of whether different populations were able to interbreed successfully, to distinguish species; both anatomy and breeding behavior are still important to assigning species status. As a result of the revolutionary (and still ongoing) advance in microbiological research techniques, including DNA analysis, in the last few decades, a great deal of additional knowledge about the differences and similarities between species has become available. Many populations which were formerly regarded as separate species are now considered to be a single taxon, and many formerly grouped populations have been split. A taxon (plural taxa) or taxonomic unit, is a name designating an organism or a group of Organisms In Biological nomenclature according to Any taxonomic level (species, genus, family, etc. ) can be synonymized or split, and at higher taxonomic levels, these revisions have been still more profound.

From a taxonomical point of view, groups within a species can be defined as being of a taxon hierarchically lower than a species. Taxonomy is the practice and science of classification The word comes from the Greek, taxis (meaning 'order' 'arrangement' and, nomos In zoology only the subspecies is used, while in botany the variety, subvariety, and form are used as well. Zoology (from Greek ζῷον, zoon, "animal" + λόγος, " Logos " "knowledge" is the branch of In Zoology, as in other branches of Biology, subspecies is the Taxonomic rank immediately subordinate to a Species. Botany, plant science(s, phytology, or plant biology is a branch of Biology and is the scientific study of plant Life In Botanical nomenclature or biological nomenclature variety is a low-level Taxonomic rank below that of species and signifies members of different populations can In botanical nomenclature a subvariety ( subvarietas) is a Taxonomic rank below that of variety ( varietas) but above that of form This article is about "form" in botany For the use in zoology see Form (zoology. In conservation biology, the concept of evolutionary significant units (ESU) is used, which may be define either species or smaller distinct population segments. "Conservation Biology" redirects here For the Scientific journal, see Conservation Biology (journal. An Evolutionarily Significant Unit (ESU (often lowercased where used without abbreviation as evolutionarily significant unit) is a population of Organisms that is

The isolation species concept in more detail

A mule is the infertile offspring of a male donkey and a female horse.
A mule is the infertile offspring of a male donkey and a female horse. The donkey or ass, Equus asinus, is a member of the Equidae or horse family and an odd-toed ungulate. The horse ( Equus caballus) is a hoofed ( Ungulate) Mammal, one of eight living species of the family Equidae.

In general, for large, complex, organisms that reproduce sexually (such as mammals and birds), one of several variations on the isolation or biological species concept is employed. Mammals ( class Mammalia) are a class of Vertebrate Animals characterized by the presence of Sweat glands, including sweat glands Birds ( class Aves) are bipedal endothermic ( Warm-blooded) Vertebrate animals that lay eggs. Often, the distinction between different species, even quite closely related ones, is simple. Horses (Equus caballus) and donkeys (Equus asinus) are easily told apart even without study or training, and yet are so closely related that they can interbreed after a fashion. Because the result, a mule or hinny, is not fertile, they are clearly separate species. In its common modern meaning a mule is the offspring of a male Donkey and a female Horse, which is classified as a kind of F1 hybrid. The word "hinny" is also a term of endearment used in North East England, equivalent to "honey" see Geordie.

But many cases are more difficult to decide. This is where the isolation species concept diverges from the evolutionary species concept. Both agree that a species is a lineage that maintains its integrity over time, that is diagnosably different from other lineages (else we could not recognise it), is reproductively isolated (else the lineage would merge into others, given the chance to do so), and has a working intra-species recognition system (without which it could not continue). Intra-species recognition is recognition by a member of a Species of another member of the same species In practice, both also agree that a species must have its own independent evolutionary history—otherwise the characteristics just mentioned would not apply. The species concepts differ in that the evolutionary species concept does not make predictions about the future of the population: it simply records that which is already known. In contrast, the isolation species concept refuses to assign the rank of species to populations that, in the best judgement of the researcher, would recombine with other populations if given the chance to do so.

The isolation question

There are, essentially, two questions to resolve. First, is the proposed species consistently and reliably distinguishable from other species? Second, is it likely to remain so in the future? To take the second question first, there are several broad geographic possibilities.

The difference question

Obviously, when defining a species, the geographic circumstances become meaningful only if the populations groups in question are clearly different: if they are not consistently and reliably distinguishable from one another, then we have no grounds for believing that they might be different species. The key question in this context, is "how different is different?" and the answer is usually "it all depends".

In theory, it would be possible to recognise even the tiniest of differences as sufficient to delineate a separate species, provided only that the difference is clear and consistent (and that other criteria are met). There is no universal rule to state the smallest allowable difference between two species, but in general, very trivial differences are ignored on the twin grounds of simple practicality, and genetic similarity: if two population groups are so close that the distinction between them rests on an obscure and microscopic difference in morphology, or a single base substitution in a DNA sequence, then a demonstration of restricted gene flow between the populations will probably be difficult in any case. Deoxyribonucleic acid ( DNA) is a Nucleic acid that contains the genetic instructions used in the development and functioning of all known

More typically, one or other of the following requirements must be met:

Sometimes it is not possible to isolate a single difference between species, and several factors must be taken in combination. This is often the case with plants in particular. In eucalypts, for example, Corymbia ficifolia cannot be reliably distinguished from its close relative Corymbia calophylla by any single measure (and sometimes individual trees cannot be definitely assigned to either species), but populations of Corymbia can be clearly told apart by comparing the colour of flowers, bark, and buds, number of flowers for a given size of tree, and the shape of the leaves and fruit. Eucalypts are woody plants belonging to three closely related genera Eucalyptus, Corymbia and Angophora. Corymbia ficifolia or the Red Flowering Gum (previously known as Eucalyptus ficifolia) is one of the most commonly planted ornamental Trees

When using a combination of characteristics to distinguish between populations, it is necessary to use a reasonably small number of factors (if more than a handful are needed, the genetic difference between the populations is likely to be insignificant and is unlikely to endure into the future), and to choose factors that are functionally independent (height and weight, for example, should usually be considered as one factor, not two).

Historical development of the species concept

Linnaeus believed in the fixity of species.
Linnaeus believed in the fixity of species.

In the earliest works of science, a species was simply an individual organism that represented a group of similar or nearly identical organisms. No other relationships beyond that group were implied. Aristotle used the words genus and species to mean generic and specific categories. Aristotle (Greek Aristotélēs) (384 BC – 322 BC was a Greek philosopher a student of Plato and teacher of Alexander the Great. Aristotle and other pre-Darwinian scientists took the species to be distinct and unchanging, with an "essence", like the chemical elements. In Philosophy, essence is the attribute or set of attributes that make an object or substance what it fundamentally is and which it has by necessity A chemical element is a type of Atom that is distinguished by its Atomic number; that is by the number of Protons in its nucleus. When early observers began to develop systems of organization for living things, they began to place formerly isolated species into a context. Many of these early delineation schemes would now be considered whimsical and these included consanguinity based on color (all plants with yellow flowers) or behavior (snakes, scorpions and certain biting ants).

In the 18th century Carolus Linnaeus classified organisms according to differences in the form of reproductive apparatus. Carl Linnaeus (Latinized as Carolus Linnaeus, also known after his ennoblement as, May 23 new style (13 May old style 1707 who laid the foundations for Although his system of classification sorts organisms according to degrees of similarity, it made no claims about the relationship between similar species. At that time, it was still widely believed that there was no organic connection between species, no matter how similar they appeared. This approach also suggested a type of idealism: the notion that each species existed as an "ideal form". Although there are always differences (although sometimes minute) between individual organisms, Linnaeus considered such variation problematic. He strove to identify individual organisms that were exemplary of the species, and considered other non-exemplary organisms to be deviant and imperfect.

By the 19th century most 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, offered one of the first logical arguments against creationism. Jean-Baptiste Pierre Antoine de Monet Chevalier de Lamarck ( August 1, 1744 &ndash December 18, 1829) was a French Soldier "Creationism" can also refer to Creation myths in general or to a concept about the origin of the soul. The new emphasis was on determining how a species could change over time. Lamarck suggested that an organism could pass on an acquired trait to its offspring, i. e. , the giraffe's long neck was attributed to generations of giraffes stretching to reach the leaves of higher treetops (this well-known and simplistic example, however, does not do justice to the breadth and subtlety of Lamarck's ideas). The giraffe ( Giraffa camelopardalis) is an African Even-toed ungulate Mammal, the tallest of all land-living Animal Species With the acceptance of the natural selection idea of Charles Darwin in the 1860s, however, Lamarck's view of goal-oriented evolution, also known as a teleological process, was eclipsed. Charles Robert Darwin (February 12 1809 &ndash April 19 1882 was an English naturalist, who realised and demonstrated that all Species of life Teleology ( Greek: telos: end purpose is the philosophical study of design and Purpose. Recent interest in inheritance of acquired characteristics centers around epigenetic processes, e. In Biology, the term epigenetics refers to changes in Gene expression caused by mechanisms other than changes in the underlying DNA sequence g. methylation, that do not affect DNA sequences, but instead alter expression in an inheritable manner. Methylation is a term used in the chemical sciences to denote the attachment or substitution of a methyl group on various substrates. Thus, neo-lamarckism, as it is sometimes termed, is not a challenge to the theory of evolution by natural selection.

Charles Darwin and Alfred Wallace provided what scientists now consider as the most powerful and compelling theory of evolution. Alfred Russel Wallace OM, FRS (8 January 1823 &ndash 7 November 1913 was an British naturalist, Explorer, Geographer eVolution is the third Album by eLDee, it was due to be released in 2008 Darwin argued that it was populations that evolved, not individuals. His argument relied on a radical shift in perspective from that of Linnaeus: rather than defining species in ideal terms (and searching for an ideal representative and rejecting deviations), Darwin considered variation among individuals to be natural. He further argued that variation, far from being problematic, actually provides the explanation for the existence of distinct species.

Darwin's work drew on Thomas Malthus' insight that the rate of growth of a biological population will always outpace the rate of growth of the resources in the environment, such as the food supply. Thomas Robert Malthus FRS (13 February 1766 – 23 December 1834 was an English political economist and demographer who expressed views As a result, Darwin argued, not all the members of a population will be able to survive and reproduce. Those that did will, on average, be the ones possessing variations—however slight—that make them slightly better adapted to the environment. If these variable traits are heritable, then the offspring of the survivors will also possess them. Thus, over many generations, adaptive variations will accumulate in the population, while counter-adaptive will be eliminated.

It should be emphasized that whether a variation is adaptive or non-adaptive depends on the environment: different environments favor different traits. Since the environment effectively selects which organisms live to reproduce, it is the environment (the "fight for existence") that selects the traits to be passed on. This is the theory of evolution by natural selection. Natural selection is the process by which favorable Heritable traits become more common in successive Generations of a Population of In this model, the length of a giraffe's neck would be explained by positing that proto-giraffes with longer necks would have had a significant reproductive advantage to those with shorter necks. Over many generations, the entire population would be a species of long-necked animals.

In 1859, when Darwin published his theory of natural selection, the mechanism behind the inheritance of individual traits was unknown. Although Darwin made some speculations on how traits are inherited (pangenesis), his theory relies only on the fact that inheritable traits exist, and are variable (which makes his accomplishment even more remarkable. Pangenesis was Charles Darwin 's hypothetical mechanism for Heredity. ) Although Gregor Mendel's paper on genetics was published in 1866, its significance was not recognized. Gregor Johann Mendel ( July 20, 1822 &ndash January 6, 1884) was Genetics (from Ancient Greek grc-Latn genetikos, “genitive” and that from grc-Latn genesis, “origin” a discipline of Biology, is It was not until 1900 that his work was rediscovered by Hugo de Vries, Carl Correns and Erich von Tschermak, who realised that the "inheritable traits" in Darwin's theory are genes. Hugo Marie de Vries ( Feb 16 1848, Haarlem - May 21 1935, Lunteren) was a Dutch Botanist and one of Carl Erich Correns ( September 10, 1864 - February 14, 1933) was a German Botanist and Geneticist, who is notable Erich von Tschermak-Seysenegg ( November 15 1871 &ndash October 11 1962) was an Austrian Agronomist. History See also History of genetics The existence of genes was first suggested by Gregor Mendel (1822-1884 who in the 1860s studied inheritance

The theory of the evolution of species through natural selection has two important implications for discussions of species -- consequences that fundamentally challenge the assumptions behind Linnaeus' taxonomy. Taxonomy is the practice and science of classification The word comes from the Greek, taxis (meaning 'order' 'arrangement' and, nomos First, it suggests that species are not just similar, they may actually be related. Some students of Darwin argue that all species are descended from a common ancestor. Second, it supposes that "species" are not homogeneous, fixed, permanent things; members of a species are all different, and over time species change. This suggests that species do not have any clear boundaries but are rather momentary statistical effects of constantly changing gene-frequencies. One may still use Linnaeus' taxonomy to identify individual plants and animals, but one can no longer think of species as independent and immutable.

The rise of a new species from a parental line is called speciation. Speciation is the Evolutionary process by which new biological Species arise There is no clear line demarcating the ancestral species from the descendant species.

Although the current scientific understanding of species suggests that there is no rigorous and comprehensive way to distinguish between different species in all cases, biologists continue to seek concrete ways to operationalize the idea. One of the most popular biological definitions of species is in terms of reproductive isolation; if two creatures cannot reproduce to produce fertile offspring, then they are in different species. This definition captures a number of intuitive species boundaries, but it remains imperfect. It has nothing to say about species that reproduce asexually, for example, and it is very difficult to apply to extinct species. Moreover, boundaries between species are often fuzzy: there are examples where members of one population can produce fertile offspring with a second population, and members of the second population can produce fertile offspring with members of a third population, but members of the first and third population cannot produces fertile offspring. Consequently, some people reject this definition of a species.

Richard Dawkins defines two organisms as conspecific if and only if they have the same number of chromosomes and, for each chromosome, both organisms have the same number of nucleotides (The Blind Watchmaker, p. Clinton Richard Dawkins, FRS, FRSL (born 26 March 1941 is a British ethologist, evolutionary biologist, and Popular science A chromosome is an organized structure of DNA and Protein that is found in cells. Nucleotides are Organic compounds that consist of three joined structures a nitrogenous base a Sugar, and a Phosphate group The Blind Watchmaker is a 1986 book by Richard Dawkins in which he presents an explanation of and argument for the theory of Evolution by means of 118). However, most if not all taxonomists would strongly disagree. Taxonomy is the practice and science of classification The word comes from the Greek, taxis (meaning 'order' 'arrangement' and, nomos For example, in many amphibians, most notably in New Zealand's Leiopelma frogs, the genome consists of "core" chromosomes which are mostly invariable and accessory chromosomes, of which exist a number of possible combinations. Prehistoric amphibian Amphibians (class Amphibia such as Frogs Toads Salamanders Newts Gymnophiona, Sirens and Even though the chromosome numbers are highly variable between populations, these can interbreed successfully and form a single evolutionary unit. In plants, polyploidy is extremely commonplace with few restrictions on interbreeding; as individuals with an odd number of chromosome sets are usually sterile, depending on the actual number of chromosome sets present, this results in the odd situation where some individuals of the same evolutionary unit can interbreed with certain others and some cannot, with all populations being eventually linked as to form a common gene pool. Polyploidy occurs in cells and Organisms when there are more than two homologous sets of Chromosomes.

The classification of species has been profoundly affected by technological advances that have allowed researchers to determine relatedness based on molecular markers, starting with the comparatively crude blood plasma precipitation assays in the mid-20th century to Charles Sibley's ground-breaking DNA-DNA hybridisation studies in the 1970s leading to DNA sequencing techniques. Blood plasma is the Liquid component of Blood, in which the Blood cells are suspended Charles Gald Sibley ( August 7, 1917 – April 12, 1998) was an American Ornithologist and Molecular biologist DNA-DNA hybridization generally refers to a Molecular biology technique that measures the degree of genetic similarity between pools of DNA sequences The results of these techniques caused revolutionary changes in the higher taxonomic categories (such as phyla and classes), resulting in the reordering of many branches of the phylogenetic tree (see also: molecular phylogeny). A phylum ( Plural: phyla) is a Taxonomic rank between Kingdom and above Class. A class is the Taxonomic rank in the Biological classification of organisms in Biology below phylum and above order. A phylogenetic tree, also called an evolutionary tree, is a tree showing the Evolutionary relationships among various biological Species or other For taxonomic categories below genera, the results have been mixed so far; the pace of evolutionary change on the molecular level is rather slow, yielding clear differences only after considerable periods of reproductive separation. A genus (plural genera from Γένος Latin genus "descent family type gender" is a low-level Taxonomic DNA-DNA hybridization results have led to misleading conclusions, the Pomarine Skua - Great Skua phenomenon being a famous example. The Pomarine Skua, Stercorarius pomarinus, known as Pomarine Jaeger in North America, is a Seabird in the Skua family Stercorariidae The Great Skua, Stercorarius skua, is a large Seabird in the Skua family Stercorariidae Turtles have been determined to evolve with just one-eighth of the speed of other reptiles on the molecular level, and the rate of molecular evolution in albatrosses is half of what is found in the rather closely related storm-petrels. Turtles are Reptiles of the Order Testudines (all living turtles belong to the Crown group Chelonia) most of Albatrosses, of the biological family Diomedeidae, are large Seabirds allied to the procellariids, Storm-petrels and Diving-petrels The storm-petrels are Seabirds in the family Hydrobatidae, part of the order Procellariiformes. The hybridization technique is now obsolete and is replaced by more reliable computational approaches for sequence comparison. Molecular taxonomy is not directly based on the evolutionary processes, but rather on the overall change brought upon by these processes. The processes that lead to the generation and maintenance of variation such as mutation, crossover and selection are not uniform (see also molecular clock). The molecular clock (based on the molecular clock hypothesis ( MCH) is a technique in Molecular evolution to relate the divergence time of two Species DNA is only extremely rarely a direct target of natural selection rather than changes in the DNA sequence enduring over generations being a result of the latter; for example, silent transition-transversion combinations would alter the melting point of the DNA sequence, but not the sequence of the encoded proteins and thus are a possible example where, for example in microorganisms, a mutation confers a change in fitness all by itself. Not to be confused with the evolutionary concept of a Transitional fossil. In Molecular biology, transversion refers to the substitution of a Purine for a Pyrimidine or vice versa The melting point of a solid is the temperature range at which it changes state from solid to Liquid.

See also

External links

Notes and references

  1. ^ Charles Darwin 1988 (1859) On the Origin of Species in The Works of Charles Darwin edited by Paul H. Barrett and R. B. Freeman. Cambridge: Cambridge University Press vol. 15 page 39
  2. ^ Just How Many Species Are There, Anyway?, 2003-05-26, <http://www.sciencedaily.com/releases/2003/05/030526103731.htm>. Retrieved on 15 January 2008 
  3. ^ David I. Williamson, "The Origins of Larvae". Kluwer (2003) ISBN 1-4020-1514-3
  4. ^ de Queiroz K (2005). "Ernst Mayr and the modern concept of species". Proc. Natl. Acad. Sci. U. S. A. 102 Suppl 1: 6600–7. doi:10.1073/pnas.0502030102. A digital object identifier ( DOI) is a permanent identifier given to an Electronic document. PMID 15851674.  

Dictionary

species

-noun

  1. A group of plants or animals having similar appearance.
  2. (biology, taxonomy) A rank in the classification of organisms, below genus and above subspecies; a taxon at that rank
  3. (mineralogy) A mineral with a unique chemical formula whose crystals belong to a unique crystallographic system.
  4. (chemistry) A specific known or unknown element, molecule, radical or ion; used commonly when the entity's status as an element, molecule etc. is unknown.
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