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Biological classification L PengoSpeciesGenusFamilyOrderClassPhylumKingdomDomainLife

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The hierarchy of biological classification's major eight taxonomic ranks. A genus contains one or more species. Intermediate minor rankings are not shown.

In biology, a species is one of the basic units of biological classification and a taxonomic rank. 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.

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. In other cases common names do not: for example, "deer" refers to a family of 34 species, including Eld's Deer, Red Deer and Elk (Wapiti). 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. 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). For example, Pinus palustris (commonly known as the Longleaf Pine). The taxonomic ranks are life, domain, kingdom, phylum, class, order, family, genus, and species.

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. 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.

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.[1]

Binomial convention for naming species Edit

Main article: Binomial nomenclature


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. 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.


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:

  • The authors are confident that some individuals belong to a particular genus but are not sure to which exact species they belong. This is particularly common in paleontology.
  • The authors use "spp." as a short way of saying that something applies to many species within a genus, but do not wish to say that it applies to all species within that genus. If scientists mean that something applies to all species within a genus, they use the genus name without the specific epithet.

In books and articles that use the genus and species names are usually printed in italics. If using "sp." and "spp.," these should not be italicized.

Difficulty of defining "species" and identifying particular species Edit

Main article: Species problem
File:Phylloscopus trochiloides NAUMANN.jpg
The greenish warbler demonstrates the concept of a ring species.

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.

Most textbooks follow Ernst Mayr's definition of 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.[2]

Various parts of this definition are there to exclude some unusual or artificial matings:

  • Those which occur only in captivity (when the animal's normal mating partners may not be available) or as a result of deliberate human action.
  • Animals which may be physically and physiologically capable of mating but do not normally do so in the wild, for whatever reason.
  • Animals whose offspring are normally sterile. For example, mules and hinnies have rarely produced further offspring (only one documented case for hinnies, and seven for mules) when mated with a creature of the same type (a mule with a mule, or a hinny with a hinny).

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

  • By definition it applies only to organisms that reproduce sexually. So it does not work for asexually reproducing single-celled organisms and for the relatively few parthenogenetic multi-celled organisms. The term "phylotype" is often applied to such organisms.
  • Biologists frequently do not know whether two morphologically similar groups of organisms are "potentially" capable of interbreeding.
  • 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. Some hybrids, e.g., mules, hinnies, ligers and tigons, apparently cannot produce offspring when mated with one of their own kind (e.g. a mule with a mule), but sometimes do produce offspring when mated with members of one of the parent species (e.g. a liger with a lion). Usually in such hybrids the males are sterile, so one could improve the basic textbook definition by changing "... whose interbreeding produces fertile offspring" to "... whose interbreeding produces offspring in which both sexes are normally fertile".
  • In ring species, members of adjacent populations interbreed successfully but members of widely-separated populations do not.
  • In a few cases it may be physically impossible for animals that are members of the same species to mate. For example, a Great Dane and a Chihuahua are both dogs and therefore members of the same species, but cannot mate because of the great difference in size and weight (physical build).

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. 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.

Definitions of speciesEdit

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 modern theory of evolution depends on a fundamental redefinition of "species." Prior to Darwin, naturalists viewed species as ideal or general types, which could be exemplified by an ideal specimen bearing all the traits general to the species. Darwin's theories shifted attention from uniformity to variation and from the general to the particular. According to intellectual historian Louis Menand,


This shift results in a new approach to "species;" Darwin



Nevertheless, a kind of rule of thumb has been established, saying that species of Bacteria or Archaea with 16S rRNA gene sequences more similar than 97% to each other need to be checked by DNA-DNA Hybridization if they belong to the same species or not [5]. This concept has been updated recently, saying that the border of 97% was too low and can be risen up to 98.7% [6].



Consequently, any single, universal definition of "species" is necessarily arbitrary. Instead, biologists have proposed a range of definitions; which definition a biologists uses is a pragmatic choice, depending on the particularities of that biologist's research.

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. 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.
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.
Recognition species
based on shared reproductive systems, including mating behavior. The Recognition concept of species has been introduced by Hugh E. H. Paterson.
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[verification needed] 
A group of organisms that shares an ancestor; a lineage that maintains its integrity with respect to other lineages through both time and space. 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.
Ecological species
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).
Phenetic species
based on phenotypes.
Species that reproduce without meiosis or fertilization so that each generation is genetically identical to the previous generation. 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.
Evolutionarily Significant Unit (ESU) 
An evolutionarily significant unit is a population of organisms that is considered distinct for purposes of conservation. Often referred to as a species or a wildlife species, an ESU also has several possible definitions, which coincide with definitions of species.


Importance in biological classificationEdit

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

  • It is the standard binomial nomenclature (or trinomial nomenclature) by which scientists typically refer to organisms.
  • It is the highest taxonomic level which mostly cannot be made more or less inclusionary.


Implications of assignment of species statusEdit




The isolation species concept in more detailEdit


It has been suggested that this article or section be merged into Species problem. (Discuss)
A mule is the infertile offspring of a male donkey and a female horse.

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. 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.


The isolation questionEdit

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 questionEdit

Obviously when defining a species, the geographic circumstances become meaningful only if the population 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".






Historical development of the species conceptEdit

Carolus Linnaeus (cleaned up version)
Linnaeus believed in the fixity of species.
PhloxBotAdded by PhloxBot

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 and other pre-Darwinian scientists took the species to be distinct and unchanging, with an "essence", like the chemical elements. 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. 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. 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). 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. Recent interest in inheritance of acquired characteristics centers around epigenetic processes, e.g. methylation, that do not affect DNA sequences, but instead alter expression in an inheritable manner. 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. 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. 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 traits will tend to 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. 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.) Although Gregor Mendel's paper on genetics was published in 1866, its significance was not recognized. 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.



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. 118). However, most if not all taxonomists would strongly disagree. 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. 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.

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. 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). 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. DNA-DNA hybridization results have led to misleading conclusions, the Pomarine Skua - Great Skua phenomenon being a famous example. 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. 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). 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.

See alsoEdit

Notes and referencesEdit

  1. Just How Many Species Are There, Anyway?, 2003-05-26,, retrieved on 2008-01-15 
  2. de Queiroz K (2005). Ernst Mayr and the modern concept of species. Proc. Natl. Acad. Sci. U.S.A. 102 Suppl 1: 6600–7.
  3. David I. Williamson, "The Origins of Larvae". Kluwer (2003) ISBN 1-4020-1514-3
  4. Louis Menand (2001) The Metaphysical Club New York: Farrar, Straus and Giroux 123-124
  5. Stackebrandt E., Goebel B.M. (1994) Taxonomic note: a place for DNA-DNA reassociation and 16S rRNA sequence analysis in the present species definition in bacteriology. Int. J. Syst. Bacteriol. 44:846-849
  6. Stackebrandt E., Ebers J. (2006) Taxonomic parameters revisited: tarnished gold standards. Microbiol. Today 33:152-155

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