A group of organisms, minerals, or other entities formally recognized as distinct from other groups. In biology, the species is a group of actually or potentially interbreeding natural populations, reproductively isolated from other similar groups such that exchange of genetic material cannot occur (the species barrier). Most species cannot interbreed with others; those that can, typically produce infertile offspring. This biological concept of the species cannot be applied to fossils, or to organisms that do not reproduce sexually. These species are defined on a comparative morphological basis.
For the movie, see Species (film).In biology, a species is one of the basic units of biodiversity. A species generally consists of all the individual organisms of a natural population which are able to interbreed, generally sharing similar appearance, characteristics and genetics due to having relatively recent common ancestors.
While definitions of "species" vary, actual species boundries are generally well defined; although the line between a species and a subspecies may be more difficult to define definitively. Examples of the millions of species include: Axolotl (Ambystoma mexicanum) and Drosera madagascariensis (a species of sundew). Some examples of non-species groups: the Amur Tiger is not a species, but a subspecies of Tiger; Deer does not refer to a species, but a family of 34 species, such as the Red Deer and Mule Deer. All extant humans belong to the same species: Homo sapiens, as well the same subspecies: H.
In scientific classification, a species is assigned a two-part name, treated as Latin. For example, humans belong to the genus Homo, and are in the species Homo sapiens. The name of the species is the whole binomial, not just the second term (which may be called specific epithet, for plants, or specific name, for animals). 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.
Since the advent of the theory of evolution, the conception of species has undergone vast changes in biology; The most commonly cited definition of "species" was first coined by Ernst Mayr. By this definition, called the biological species concept or isolation species concept, species are "groups of actually or potentially interbreeding natural populations which are reproductively isolated from other such groups". However, many other species concepts are also used (see other definitions of species below).
As a soft guide, however, the numbers of currently identified species can be broken down as follows:
287,655 plants, including: 15,000 mosses, 13,025 ferns, 980 gymnosperms, 199,350 dicotyledons, 59,300 monocotyledons;However the total number of species for some phyla may be much higher:
5-10 million bacteria;The scientific name of a species is properly typeset in italics. When an unknown species is being referred to this may be done by using the abbreviation "sp." Note that the word "specie" is not the singular of "species", but rather coined money.
Definitions of species
A species is a reproductively isolated population that shares a common gene pool and a common niche. This definition defines a species reproductively, genetically, and ecologically...
The definition of a species given above as taken partly from Mayr, is somewhat idealistic. 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 (ie: 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. A population or group of populations that differs morphologically from other populations. 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. 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. 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. 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. Species that reproduce without meiosis or fertilization so that each generation is genetically identical to the previous generation. Nevertheless, no species concept yet proposed is entirely objective, or can be applied in all cases without resorting to judgement. For most vertebrates, this is the biological species concept (BSC), and to a lesser extent (or for different purposes) the phylogenetic species concept (PSC). 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. 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. It is one of the most important levels of classification, for several reasons:
It often corresponds to what lay people treat as the different basic kinds of organism - dogs are one species, cats another. It is the only taxonomic level which has empirical content, in the sense that asserting that two animals are of different species is saying something more than classificatory about them.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. 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.
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. 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.
The isolation species concept in more detail
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. Because the result, a mule or hinny, is not usually fertile, they are clearly separate species. 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). 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. First, is the proposed species consistently and reliably distinguishable from other species?
The proposed species are sympatric—they occupy the same habitat. Observation of many species over the years has failed to establish even a single instance of two diagnostically different populations that exist in sympatry and have then merged to form one united population. Without reproductive isolation, population differences cannot develop, and given reproductive isolation, gene flow between the populations cannot merge the differences. Generally, the hybrid individuals are less capable of successful breeding than pure-bred individuals of either species. The proposed species are allopatric—they occupy different geographical areas. Often it is not possible to achieve certainty by experimental means either: even if the two proposed species interbreed in captivity, this does not demonstrate that they would freely interbreed in the wild, nor does it always provide much information about the evolutionary fitness of hybrid individuals. Until then, it is assumed, they were members of the same seven species. Even if the isthmus were to sink under the waves again, the populations would remain genetically isolated: therefore they are now different species. In many cases, however, neither observation nor experiment can produce certain answers, and the determination of species rank must be made on a 'best guess' basis from a general knowledge of other related organisms. The proposed species are parapatric—they have breeding ranges that abut but do not overlap. Because they do not crossbreed, it is safe to assume that there is a mechanism, often behavioral, that is preventing gene flow between the populations, and that therefore they should be classified as separate species. the decision to define the two hybridizing populations as either the same species or as separate species is difficult and potentially controversial. In contrast, over the same geographic range, black-backed (northern) and white-backed (southern) Australian Magpies do not blend from one type to another: northern populations have black backs, southern populations white backs, and there is an extensive hybrid zone where both 'pure' types are common, as are crossbreeds. In both cases, there is some uncertainty regarding correct classification, but the consensus view is that species rank is not justified in either.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.
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.
More typically, one or other of the following requirements must be met:
It is possible to reliably measure a quantitative difference between the two groups that does not overlap.Sometimes it is not possible to isolate a single difference between species, and several factors must be taken in combination. 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.
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
In the earliest works of science, a species was simply an individual organism that represented a group of similar or nearly identical organisms. 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.
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". 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. As such, the new emphasis was on determining how a species could change over time.
Lamarck's most important insight may have been that species can be extraordinarily fluid; Lamarck's ideas of the goal-oriented evolution of species, also known as a teleological process, were believed by Darwin and have even received some renewed attention, but the idea is hardly credible since experimental results fail to show the expected goal-oriented muation, there is no known mechanism to produce mutations biased towards the direction of higher fitness, and natural selection is anyway a more efficient process of optimization than a hypothetical Lamarckian process would be.
Charles Darwin and Alfred Wallace provided what scientists now consider as the most powerful and compelling theory of evolution. 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. 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.
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. 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. 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, although 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.
In recent years we have witnessed the drastic reduction in the size of breeding populations and the geographical range of many physically large mammals. In earlier times it was assumed that every species existed in at least a few thousand living individuals, except very rare relic, isolated groups. bird species are so stressed by habitat loss, and other effects of the modern world, that only a very few breeding males may contribute the genetic material to a small number of breeding females. Species in transition before our eyes. It is possible that this severe stress is essential to the creation of new species, and may have been a prime factor throughout biological history, from other population reducing influences.
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. 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. Hawksworth, "The magnitude of fungal diversity: the 1·5 million species estimate revisited" Mycological Research (2001), 105: 1422-1432 Cambridge University Press ^ Proceedings of the National Academy of Sciences, Census of Marine Life (CoML)
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