In biology, a species (abbreviated sp., with the plural form species abbreviated spp.) is the basic unit of biological classification and a taxonomic rank. A species is often defined as the largest group of organisms in which two individuals can produce fertile offspring, typically by sexual reproduction. While this definition is often adequate, looked at more closely it is often problematic. For example, in a species complex, the boundaries between closely related species become unclear or disappear altogether. Other ways of defining species include similarity of DNA, morphology, or ecological niche. The presence of locally adaptive traits may further subdivide species into infraspecific taxa such as subspecies.
Species are grouped into genera (singular: genus) by taxonomists, genera are grouped into families, families into larger groups. Taxonomists use scientific hypotheses that species in the same genus have the same ancestors. The hypotheses are first based on observed similarity of physical attributes and behaviour, and where available, the DNA sequences of individuals and of species.
All species are given a two-part name, a "binomial". The first part of a binomial is the genus to which the species belongs. The second part is called the specific name (zoology) or the specific epithet (in botany, also sometimes in zoology). For example, Boa constrictor is one of four species of the Boa genus.
A usable definition of "species" and reliable methods of identifying particular species are important for stating and testing biological theories and for measuring biodiversity, though other taxonomic levels such as families can form the basis of broad-scale studies. Extinct species known only from fossils are generally difficult to assign precise taxonomic rankings, which is why higher taxonomic levels such as families are often used for fossil-based studies.
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. A kind was distinguished by its attributes; for instance, a bird has feathers, a beak, wings, a hard-shelled egg, and warm blood. A form was distinguished by being shared by all its members, the young inheriting any variations they might have from their parents. Aristotle believed all kinds and forms to be distinct and unchanging. His approach remained influential until the Renaissance.
When early modern 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). John Ray (1686), an English naturalist, was the first to give a biological definition of the term "species," as follows: "... no surer criterion for determining species has occurred to me than the distinguishing features that perpetuate themselves in propagation from seed. Thus, no matter what variations occur in the individuals or the species, if they spring from the seed of one and the same plant, they are accidental variations and not such as to distinguish a species... Animals likewise that differ specifically preserve their distinct species permanently; one species never springs from the seed of another nor vice versa".
In the 18th century, the Swedish scientist Carl Linnaeus classified organisms according to shared physical characteristics, and not simply based upon differences. He established the idea of a taxonomic hierarchy of classification based upon observable characteristics and intended to reflect natural relationships.
At the time, however, it was still widely believed that there was no organic connection between species, no matter how similar they appeared. This view was influenced by European scholarly and religious education, which held that the categories of life are dictated by God, forming a hierarchy, the scala naturae or great chain of being. However, whether or not it was supposed to be fixed, the scala (a ladder) inherently implied the possibility of climbing.
Species that could change
By the 19th century, naturalists understood that species could change form over time, and that the history of the planet provided enough time for major changes. Jean-Baptiste Lamarck, in his 1809 Zoological Philosophy, described the transmutation of species, proposing that a species could change over time. Textbooks ignore most of this, focusing on one aspect, that an organism could pass on an acquired trait to its offspring. They use the example of the giraffe's long neck supposedly having been created by generations of giraffes stretching to reach the leaves of higher treetops.
In 1859, Charles Darwin and Alfred Russel Wallace provided a compelling account of evolution and the formation of new species. Darwin argued that it was populations that evolved, not individuals, by natural selection from naturally occurring variation among individuals. This required a new definition of species. Darwin concluded that species are what they appear to be: ideas, provisionally useful for naming groups of interacting individuals. "I look at the term species", he wrote, "as one arbitrarily given for the sake of convenience to a set of individuals closely resembling each other ... It does not essentially differ from the word variety, which is given to less distinct and more fluctuating forms. The term variety, again, in comparison with mere individual differences, is also applied arbitrarily, and for convenience sake."
The commonly used names for plant and animal taxa sometimes correspond to species, as with lion, walrus, and camphor tree, but often they do not: for instance deer refers to a family which includes fallow deer and red deer.
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.) This commonly occurs when authors are confident that some individuals belong to a particular genus but are not sure to which exact species they belong, as is common in paleontology. Authors may also use "spp." as a short way of saying that something applies to many species within a genus, but not to all. If scientists mean that something applies to all species within a genus, they use the genus name without the specific name or epithet. The names of genera and species are usually printed in italics. Abbreviations such as "sp." should not be italicized.
Various codes have been devised for identifying particular species. For example:
- National Center for Biotechnology Information (NCBI) employs a numeric 'taxid' or Taxonomy identifier, a "stable unique identifier", e.g. the taxid of H. sapiens is 9606.
- Kyoto Encyclopedia of Genes and Genomes (KEGG) employs a three- or four-letter code for a limited number of organisms; in this code, for example, H. sapiens is simply hsa.
- UniProt employs an "organism mnemonic" of not more than five alphanumeric characters, e.g. HUMAN for H. sapiens.
- Integrated Taxonomic Information System (ITIS) provides a unique number for each species.
Standard definition: Mayr's Biological Species Concept
Most modern textbooks use Ernst Mayr's definition, known as the Biological Species Concept (BSC). It is also called a reproductive or isolation concept. This defines a species as "groups of actually or potentially interbreeding natural populations, which are reproductively isolated from other such groups". It can be argued that this definition is a a natural consequence of the effect of sexual reproduction on the dynamics of natural selection. Mayr's definition excludes unusual or artificial matings that result from deliberate human action, or occur only in captivity, or that involve animals capable of mating but that do not normally do so in the wild.
The Species Problem
It is difficult to define the word "species" in a way that applies to all organisms. The debate about how to define "species" is called the "Species Problem". The problem dates to On the Origin of Species, where Darwin wrote:
No one definition has satisfied all naturalists; yet every naturalist knows vaguely what he means when he speaks of a species. Generally the term includes the unknown element of a distinct act of creation.
A simple textbook definition, following Mayr's BSC, works well for most multi-celled organisms, but breaks down in several situations:
- Among organisms that reproduce asexually, as in single-celled organisms and parthenogenetic or apomictic multi-celled organisms.
- When biologists do not know whether two morphologically similar groups of organisms are capable of interbreeding.
- When hybridization and the extent of sexual reproduction vary widely.
- In ring species, when members of adjacent populations interbreed successfully but members of some non-adjacent populations do not.
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). In addition, horizontal gene transfer (HGT) makes it difficult to define the term species. All species definitions assume that an organism acquires its genes from one or two parents very like the "daughter" organism, but that is not what happens in HGT. There is strong evidence of HGT between very dissimilar groups of prokaryotes, and at least occasionally between dissimilar groups of eukaryotes, including some crustaceans and echinoderms.
The evolutionary biologist James Mallet concludes that
there is no easy way to tell whether related geographic or temporal forms belong to the same or different species. Species gaps can be verified only locally and at a point of time. One is forced to admit that Darwin's insight is correct: any local reality or integrity of species is greatly reduced over large geographic ranges and time periods.
Nikolai Vavilov developed ways to define and conceive of Linnaean species. He saw species as systems, each an integral entity consisting of closely interlinked components. He emphasized the variability within species, relativity of taxonomic criteria and the accumulation of genetic variation within a species. From the evolutionary point of view he compared species to knots in evolutionary chains. Building on V.L. Komarov's aphorism: "a species is a morphological system plus geographic distinctness", Vavilov defined a "Linnaean species" as "an isolated complex dynamic morph-physiological system bound in its origin to a certain environment and area".
In microbiology, genes can move freely even between distantly related bacteria, possibly extending to the whole bacterial domain. As a rule of thumb, microbiologists have assumed that kinds of Bacteria or Archaea with 16S ribosomal RNA gene sequences more similar than 97% to each other need to be checked by DNA-DNA Hybridization to decide if they belong to the same species or not. This concept was narrowed in 2006 to a similarity of 98.7%.
Typological or morphospecies
A typological species is a group of organisms in which individuals conform to certain fixed properties, so that even pre-literate people often recognize the same taxon as do modern taxonomists. The clusters of variations or phenotypes within specimens (i.e. longer or shorter tails) would differentiate the species. This method was used as a "classical" method of determining species, such as with Linnaeus early in evolutionary theory. However, different phenotypes are not necessarily different species (e.g. a four-winged Drosophila born to a 2-winged mother is not a different species). Species named in this manner are called morphospecies.
A single evolutionary lineage of organisms within which genes can be shared, and that maintains its integrity with respect to other lineages through both time and space. At some point in the evolution of such a group, some members may diverge from the main population and evolve into a subspecies, a process that may eventually lead to the formation of a new species if isolation (geographical or ecological) is maintained. The process through which species are formed by evolution is called speciation. A species that gives rise to another species is a paraphyletic species, or paraspecies.
Phylogenetic or cladistic species
A phylogenetic or cladistic species (PSC) is an evolutionarily divergent lineage, one that has maintained its hereditary integrity through time and space. A PSC is the smallest group of populations that can be distinguished by a unique set of morphological or genetic traits. Molecular markers may be used to determine genetic similarities in the nuclear or mitochondrial DNA of various species. For example, in a study done on fungi, studying the nucleotide characters using PSC produced the most accurate results in recognizing the numerous fungi species compared to other concepts used. Unlike the popular Biological Species Concept, PSC also does not rely on reproductive isolation, thus it is independent of processes that are integral in other concepts. PSC works for asexual lineages, and can detect recent divergences, which the Morphological Species Concept cannot. However, PSC does not work in every situation, and may require more than one polymorphic locus to give an accurate result. PSC may lead to splitting of existing species, for example of Bovidae, into many new ones.
An ecological species is a set of organisms adapted to a particular set of resources, called a niche, in the environment. According to this concept, populations form the discrete phenetic clusters that we recognize as species because the ecological and evolutionary processes controlling how resources are divided up tend to produce those clusters.
A genetic species is a set of individuals or populations with sufficiently similarity of DNA. Techniques to compare similarity of DNA include DNA-DNA hybridization, genetic fingerprinting and DNA barcoding. Richard Dawkins for example defined 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.
Evolutionarily significant unit
An evolutionarily significant unit (ESU) or "wildlife species" is a population of organisms considered distinct for purposes of conservation.
A mate-recognition species is a group of sexually reproducing organisms that recognize one another as potential mates. Expanding on this to allow for post-mating isolation, a cohesion species is the most inclusive population of individuals having the potential for phenotypic cohesion through intrinsic cohesion mechanisms; no matter whether populations can hybridize successfully, they are still distinct cohesion species if the amount of hybridization is insufficient to completely mix their respective gene pools.
Lumping and splitting of taxa
The naming of a particular species may 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 colloquially, 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.
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- These terms were translated into Latin as "genus" and "species", though they do not correspond to the Linnean terms thus named.
- 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.
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