Difference between revisions of "Species" - New World Encyclopedia

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Note: This is only a very rough draft, with notes. Please do not edit this article until the actual article is complete — i.e., when this notice is removed. You may add comments on what you would like to see included.
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[[Category:Public]]
  
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*Should add info from Ernst Mayr's publication "What is a species, and what is not" and from the Stanford Encylopedia of Philosophy, including the ontological status of species, species pluralism, and does the species category exist. Also add view of Stephen J. Gould that species do not change much for most of their existence and TF's perspective that species do not change.
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[[Image:Asian multicolored lady beetle.jpg|thumb|240px|Lady beetle]]
  
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*Start article with point that Species are the fundamental taxonomic units of biological classification, but that Biologists disagree on the definition of the term.  
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'''Species''' are the basic taxonomic units of biological classification. This grouping of organisms of "like kind" into discrete and stable units has been traced at least from the time of [[Plato]] and [[Aristotle]]. Nonetheless, today biologists lack consensus in how to define the term and what constitutes a species. Among the several definitions of species, the most commonly used is the biological species concept first coined by [[Ernst Mayr]]: Species are "groups of actually or potentially interbreeding natural [[population]]s which are reproductively isolated from other such groups." (However, see other [[#Definitions of species|definitions of species]] below.)
  
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This basic taxonomic unit is remarkably stable. Species tend to remain the same throughout their geological history. As noted by eminent evolutionist [[Gould, Stephen Jay|Stephen Jay Gould]], the macroevolutionary patterns of species are typically ones of morphological stability during their existence, a phenomena known as "stasis." In presenting the theory of [[punctuated equilibria]], Niles Eldridge and Gould noted: "Most species, during their geological history, either do not change in any appreciable way, or else they fluctuate mildly in morphology, with no apparent direction." Once a species appears, the fossil record does not change much during its existence, which may be several million years. This view accords well with the view of [[creationism]], which references a clear-cut boundary between species, as well as stability during their existence.
  
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In [[biology]], the most commonly used definition of '''species''' was first coined by [[Ernst Mayr]]. Species are "groups of actually or potentially interbreeding natural [[population]]s which are reproductively isolated from other such groups" (however, see other [[#Definitions of species|definitions of species]] below).  
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The concept of species is important. For one, [[environmental law]] is framed in terms of species.
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Indeed, many countries have laws proscribing special protection to species considered [[endangered species|endangered]] to prevent their extinction. The term species also is central to evolutionary studies, and is generally presented as the principal unit of [[evolution]]. Ernst Mayr maintains that one cannot even write about evolution, or most aspects of the philosophy of biology, without understanding the meaning of the biological species.
  
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In [[scientific classification]], a species is assigned a two-part scientific name.  The [[Genus]] is listed first (and capitalized) followed by a species epithet. 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 of the binomial (the specific epithet). See [[binomial nomenclature]] (also  [[binomen]], [[specific name]], [[binary name]], [[specific epithet]], [[Nomenclature Codes]]).
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It is difficult to even speculate how many species there are on the planet today. Estimates range from 10 million to 150 million. Less than 2 million species (perhaps only 2 percent or less of all species) have been identified.
  
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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." in the singular or "spp." in the plural in the place of the second part of the scientific name. Note that the word "specie" is NOT the singular of "species." It refers to coined money.
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Both the singular and plural forms of the noun are rendered by the word "species." The term derives from the Latin "specere" (to look at, to behold), with the meaning of "kind," "quality," "appearance," "shape," or "a peculiar sort." (Note that the word "specie" is not the singular of "species." It refers to coined money.)
  
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==Definitions of species==
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== Scientific name ==
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The definition of a species given above as taken from Mayr, is somewhat idealistic. Since it assumes [[sexual reproduction]], it leaves the term undefined for a large class of organisms that reproduce asexually. 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:
 
  
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* A '''typological species''' is 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 (ie: a 4-winged Drosphila born to a 2-winged mother is not a different species). Species named in this manner are called a ''morphospecies''.
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In [[taxonomy]], a species is assigned a two-part scientific name. The genus is listed first (and capitalized) followed by a species epithet (which is not capitalized). For example, humans belong to the genus ''Homo,'' and are in the species ''Homo sapiens.'' Tigers, lions, leopards, and jaguars are different species, but each are similar enough to belong to the same genus (''Panthera''). The name of the species is the whole ''binomial'' not just the second term of the binomial (the specific epithet). This system was set up by Swedish botanist [[Carolus Linnaeus]].
  
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* A '''morphological species''' is a group of [[organism]]s that have a distinctive form: 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. Although much criticised, the concept of morphological species remains the single most widely used species concept in everyday life, and still retains an important place within the biological sciences, particularly in the case of plants.
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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." in the singular or "spp." in the plural in the place of the second part of the scientific name.
  
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* The '''biological species''' or '''isolation species''' concept identifies a species as a set of actually or potentially interbreeding organisms. This is generally the most 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 many instances of allopatric (geographically isolated) populations. For example, it is possible to cross a [[horse]] with a [[donkey]] and produce offspring, however they remain separate species—in this case for two different reasons: first because horses and donkeys do not normally interbreed in the wild, and second because the fruit of the union is rarely fertile. The key to defining a biological species is that there is no significant cross-flow of [[gene|genetic material]] between the two populations.
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Groups within a species can be defined as being of a taxon hierarchically lower than a species. In [[zoology]], only the [[subspecies]] is used, while in [[botany]] the [[variety (biology)|variety]], [[subvariety]], and [[form (biology)|form]] are used as well.
  
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* A '''mate-recognition species''' is defined as a group of organisms that are known to recognise one another as potential mates. Like the isolation species concept above, it applies only to organisms that reproduce sexually.
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The idea of ''species'' has a long history. It is one of the most important levels of classification, for several reasons:
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*It often corresponds to what lay people treat as the different basic kinds of organisms—dogs are one species, cats another.
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* A '''phylogenetic''' or '''evolutionary''' or '''Darwinian species''' is 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.
 
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* See also '''microspecies''' under [[apomixis]], for species that reproduce without [[meiosis]] or [[mitosis]] so that each generation is genetically identical to the previous generation.
 
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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 judgement. 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.
 
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==Importance in biological classification==
 
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The idea of ''species'' has a long history. It is one of the most important levels of classification, for several reasons:
 
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*It often corresponds to what lay people treat as the different basic kinds of organism - dogs are one species, cats another.
 
 
*It is the standard [[binomial nomenclature]] (or [[trinomial nomenclature]]) by which scientists typically refer to organisms.
 
*It is the standard [[binomial nomenclature]] (or [[trinomial nomenclature]]) by which scientists typically refer to organisms.
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*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.
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*It is the only taxonomic level that has empirical content, in the sense that asserting that two animals are of different species is saying something more than classificatory about them.
  
 
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 and controversial.
 
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 and controversial.
  
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==Implications of assignment of species status==
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==Definitions of species==
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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]]).
 
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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.
 
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From a [[taxonomy|taxonomical]] point of view, groups within a species can be defined as being of a taxon hierarchically lower than a species. In [[zoology]] only the [[subspecies]] is used, while in [[botany]] the [[variety (biology)|variety]], [[subvariety]], and [[form (biology)|form]] are used as well.
 
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==The isolation species concept in more detail==
 
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In general, for large, complex, organisms that reproduce sexually (such as [[mammal]]s and [[bird]]s), 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 usually fertile, they are clearly separate species.
 
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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 to 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.
 
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===The isolation question===
 
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There are, essentially, two questions to resolve. First, is the proposed species consistently and reliably distinguishable from other species? Secondly, is it likely to remain so in the future? To take the second question first, there are several broad geographic possibilities.
 
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* 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. This is not to say that cross breeding does not take place at all, simply that it has become negligible. Generally, the hybrid individuals are less capable of successful breeding than pure-bred individuals of either species.
 
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* The proposed species are '''allopatric'''—they occupy different geographical areas. Obviously, it is not possible to observe reproductive isolation in allopatric groups directly. 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. A certain amount can be inferred from other experimental methods: for example, do the members of population '''A''' respond appropriately to playback of the recorded mating calls of population '''B'''? Sometimes, experiments ''can'' provide firm answers. For example, there are seven pairs of apparently almost identical marine snapping shrimp (''Altheus'') populations on either side of the [[Isthmus of Panama]], which did not exist until about 3 million years ago. Until then, it is assumed, they were members of the same 7 species. But when males and females from opposite sides of the isthmus are placed together, they fight instead of mating. 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.
 
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* The proposed species are '''parapatric'''—they have breeding ranges that abut but do not overlap. This is fairly rare, particularly in temperate regions. The dividing line is often a sudden change in habitat (an [[ecotone]]) like the edge of a forest or the snow line on a mountain, but can sometimes be remarkably trivial. The parapatry itself indicates that the two populations occupy such similar ecological roles that they cannot coexist in the same area. 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.
 
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* There is a '''hybrid zone''' where the two populations mix. Typically, the hybrid zone will include representatives of one or both of the 'pure' populations, plus first-generation and back-crossing hybrids. The strength of the barrier to genetic transmission between the two pure groups can be assessed by the width of the hybrid zone relative to the typical dispersal distance of the organisms in question. The dispersal distance of [[oak]]s, for example, is the distance that a bird or [[squirrel]] can be expected to carry an acorn; the dispersal distance of [[Numbat]]s is about 15 kilometres, as this is as far as young Numbats will normally travel in search of vacant territory to occupy after leaving the nest. The narrower the hybrid zone relative to the dispersal distance, the less gene flow there is between the population groups, and the more likely it is that they will continue on separate evolutionary paths. Nevertheless, it can be very difficult to predict the future course of a hybrid zone; the decision to define the two hybridizing populations as either the same species or as separate species is difficult and potentially controversial.
 
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* The variation in the population is '''clinal'''—at either extreme of the population's geographic distribution, typical individuals are clearly different, but the transition between them is seamless and gradual. For example, the [[Koala]]s of northern Australia are clearly smaller and lighter in colour than those of the south, but there is no particular dividing line: the further south an individual Koala is found, the larger and darker it is likely to be; Koalas in intermediate regions are intermediate in weight and colour. In contrast, over the same geographic range, black-backed (northern) and white-backed (southern) [[Australian Magpie]]s 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. The variation in Koalas is clinal (a smooth transition from north to south, with populations in any given small area having a uniform appearance), but the variation in magpies is ''not'' clinal. In both cases, there is some uncertainty regarding correct classification, but the consensus view is that species rank is not justified in either. The gene flow between northern and southern magpie populations is judged to be sufficiently restricted to justify terming them [[subspecies]] (not full species); but the seamless way that local Koala populations blend one into another shows that there is substantial gene flow between north and south. As a result, experts tend to reject even subspecies rank in this case.
 
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===The difference question===
 
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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".
 
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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.
 
  
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More typically, one or other of the following requirements must be met:
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Several different concepts are employed in identifying species:
  
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* It is possible to reliably measure a ''quantitative'' difference between the two groups that ''does not overlap''. A population has, for example, thicker fur, rougher bark, longer ears, or larger seeds than another population, and although this characteristic may vary within each population, the two do not grade into one another, and given a reasonably large sample size, there is a definite discontinuity between them. Note that this applies to ''populations'', not individual organisms, and that a small number of exceptional individuals within a population may 'break the rule' without invalidating it. The less a quantitative difference varies ''within'' a population and the more it varies ''between'' populations, the better the case for making a distinction. Nevertheless, borderline situations can only be resolved by making a 'best-guess' judgement.
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* '''Typological (or "morphological") species concept.''' Historically, species were viewed as collections of individuals that share a common [[phenotype]], including morphology, behavior, and ecological relationships with their environment. This method was used as a "classical" method of determining species. For example, continental North American savanna sparrows were differentiated from savanna sparrows from Sable Island, Nova Scotia, if they were sufficiently different in morphological characters. A [[chicken]] and a [[duck]] can be distinguished because they have different shaped bills and the duck has webbed feet. This traditional method is useful in the work of taxonomy, such as cataloging species and creating identification keys. It is also applicable in [[paleontology]], where morphology is all there is (such as snail shells in fossil beds). Indeed, the concept of morphological species remains the single most widely used species concept in everyday life, and retains an important place within the biological sciences, particularly in the case of plants. '''Shortcomings:''' However, there are also important shortcomings with the typological species concept of distinguishing species. For example, different phenotypes do not always constitute different species (i.e. a 4-winged Drosphila born to a 2-winged mother is not a different species). It is not uncommon to find pronounced morphological differences among individuals within one interbreeding population, due to individual genetic variation, sexual dimorphism (males, females, immatures), and life stage (catepillars and butterflies)—differences that may be much more evident than between clearly different species. Furthermore, there is the challenge of oversplitting taxa, whereby each variant is called a new species.
  
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* It is possible to distinguish a ''qualitative'' difference between the populations; a feature that does not vary continuously but is either entirely present or entirely absent. This might be a distinctively shaped seed pod, an extra primary feather, a particular courting behaviour, or a clearly different DNA sequence.
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* '''Biological (or "isolation") species concept.''' This concept identifies a species as a set of actually or potentially interbreeding organisms. Or, as stated by Ernst Mayr, "Species are groups of interbreeding natural populations that are reproductively isolated from other such groups." The biological species concept (BCS), which developed in the second half of the nineteenth century and was greatly advanced by Mayr in the twentieth century, involves thinking of species in terms of variable populations rather than fixed types. This is generally the most useful and common formulation for scientists working with living examples of the higher taxa, like [[mammal]]s, [[fish]], and [[bird]]s. '''Shortcomings:''' The BSC is meaningless for [[organisms]] that do not reproduce sexually. Furthermore, the key to defining a biological species is that there is no significant cross-flow of [[gene|genetic material]] between the two populations. But, biologists frequently do not know whether two morphologically similar groups of organisms are "potentially" capable of interbreeding, and BSC is thus impractical in many instances of allopatric (geographically isolated) populations. The BSC does not distinguish between the theoretical possibility of interbreeding and the actual likelihood of gene flow between populations. Does one successful hybridization invalidate species distinction? Note that it is possible to cross a [[horse]] with a [[donkey]] and produce offspring. However, they remain separate species—in this case for two different reasons: first because horses and donkeys do not normally interbreed in the wild, and second because the fruit of the union is rarely fertile. Similarly, how does one utilize the BCS to delineate paleospecies (extinct or fossil species)?
  
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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 [[eucalypt]]s, 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.
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* '''Mate-recognition species concept.''' A mate-recognition species is defined as a group of organisms that share a common fertilization system and are known to recognize one another as potential mates. '''Shortcoming:''' Like the BCS above, it applies only to organisms that reproduce sexually.
  
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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).
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* '''Phylogenetic species concept.''' The phylogenetic species concept, which has several versions, essentially defines a species as a group of organisms bound by a unique ancestry. Devised by [[paleontology|paleontologists]] Niles Eldredge and Joel Cracraft, it is an attempt to define species by their relationships to other species, involving uncovering their genealogical relationships. A formal definition given by Joel Cracraft is: "A species is the smallest diagnosable cluster of individual organisms within which there is a parental pattern of ancestry and descent." Thus, according to this concept, diagnosable geographic forms of the same basic "kind" of bird should be treated as distinct species, because these forms have evolved separately, and have unique evolutionary histories. For example, a population of sparrows that had a unique heritable character, such as a particular allele (form of a particular gene) would be considered a separate species from those that do not have that particular character. This method is applicable even to unusual reproductive modes other than sexual reproduction. '''Shortcomings:''' Application of this concept is problematic in those cases where morphologically different populations are connected by gene flow, and such morphological variation among populations is not uncommon within living species. For example, humans have substantial morphological variation from continent to continent. Fragmentary specimens collected by a paleontologist on different continents, which show clinal variation, would appear to be unique. Ernst Mayr also criticizes the phylogenetic species concept as nothing more than the revival of a purely morphological species concept.
  
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==Historical development of the species concept==
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* '''Evolutionary (or "Darwinian") species concept.''' An evolutionary species is 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. This "evolutionary species concept" (ESC) is often associated with [[George Gaylord Simpson]], a mammalian paleontologist, who stated "a species is a series of ancestor-descendent populations passing through time and space independent of other populations, each of which possesses its own evolutionary tendencies and historical fate." The ESC is the most popular concept among paleontologists, and is used extensively in comparative biology and phylogenetic systematics. It has similarities with the phylogentic species concept, but the ESC combines the genealogical basis of the phylogenetic species concept with the genetic basis of the biological species concept. Specifically, the ESC uses a wider range of characters to make the species determination. An evolutionary species is a lineage of interbreeding organisms, reproductively isolated from other lineages, that has a beginning, an end, and a distinct evolutionary trajectory (Wiley 1978). '''Shortcomings:''' Mayr criticized this concept for several reasons: (1) it replaces the clear-cut criterion of reproductive isolation of the BCS with such vague terms as "maintains its identity," "evolutionary tendencies," and "historical fate"; (2) it is applicable only to monotypic species, and geographic isolates would have to be treated as different species; (3) there are no empirical criteria by which evolutionary tendency of historical fate can be observed in a given fossil sample; and (4) the ESC does not help in the lower or upper demarcation of chronospecies, even though the concept was apparently introduced to deal with the time dimension.  
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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 element]]s. When early observers began to develop systems of organization for living things, they began to place formerly isolated species into a context. To the modern mind, many of the schemes delineated are whimsical at best, such as those that determined consanguinity based on color (all plants with yellow flowers) or behavior (snakes, scorpions and certain biting ants).  
 
  
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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 the time, it was still widely believed that there is no organic connection between species, no matter how similar they appear; every species was individually created by [[God]], a view today called [[creationism]].  This approach also suggested a type of idealism: the notion that each species exists 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.  
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* '''Ecological species concept''' defines a species as a group of organisms that share a distinct ecological niche. '''Shortcoming:''' This concept, which is based on the niche occupied by a species, is problematic because widespread species generally have local populations that differ in their niche occupation, which would require they be recognized as different species, even though based on all other criteria they would not be. As noted by Mayr (1996), "More fatal for the ecological species concept are the trophic species of cichlids (A. Mayer 1990)," which differentiate niche within a single set of offspring from the same parents. There are also common cases where two sympatric species seem to occupy the same niche.
  
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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.  [[Jean-Baptiste 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).
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These are just a few of more than a dozen common methods of delineating species among biologists. (One might also include, for example, Templeton's "cohesion species concept" that attempts to combine several components of species concepts, but which is likewise criticized as failing to resolve the resulting conflicts.) 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.
  
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Lamarck's most important insight may have been that species can be extraordinarily fluid; his [[1809]] ''Zoological Philosophy'' contained one of the first logical refutations of [[creationism]]. With the acceptance of the work of [[Charles Darwin]] in the 1860s, Lamarck's view of evolution was quickly eclipsed. It was not until the late [[20th century]] that his work began to be reexamined, and took its place as a fundamental stepping stone to the modern theory of [[adaptive mutation]]. Lamarck's long-discarded ideas of the goal-oriented evolution of species, also known the [[teleology|teleological]] process, have also received renewed attention, particularly by proponents of [[artificial selection]].
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== Number of species ==
  
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Charles Darwin and [[Alfred Russel Wallace|Alfred Wallace]] provided what scientists now consider the most powerful and compelling [[theory of evolution]].  Basically, Darwin argued that it is populations that evolve, not individuals.  His argument relies on a radical shift in perspective from 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.
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No one knows how many species exist today. Approximately 1.8 million species of [[animal]]s and [[plant]]s have been identified (excluding the diverse kingdoms of [[fungi]], [[bacteria]], and other unicellular organisms), but some biologists estimate there may be more than 150 million species of living things on the earth. Indeed, E. O. Wilson in his 1992 book ''The Diversity of Life,'' stated "How many species of organisms are there on earth? We don't know, not even to the nearest order of magnitude. The numbers could be as close to 10 million or as high as 100 million."
  
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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 will be eliminated.
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Of those that have been identified, more than half are [[insects]] (about 57 percent), and nearly half of all insect species are [[beetle]]s, meaning that beetles, with over 400,000 identified species, represent about 25 percent of all named species in the plant and animal kingdoms. This fact led to the famous quip from J. B. S. Haldane, perhaps apocryphal, who when asked what one could conclude as to the nature of the Creator from a study of his creation, replied: "An inordinate fondness for beetles" (Gould 1993).  
  
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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.
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There are also approximately 9,000 named species of [[birds]], 27,000 known species of [[fish]], and a ledger of about 4,000 or so [[mammal|mammalian]] species. These groups have been diligently catalogued, unlike insects, which rank among the most uncounted groups of organisms.
  
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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 [[gene]]s.
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== Speciation and extinction==
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There are two basic ways that are widely presented by biologists for the origin of a species: (1) the descendant species arises by transformation of the entire ancestral population; or (2) the descendant species branches off from the ancestral population. (Strict scientific [[creationism|creationists]], who do not accept the evolutionary theory of descent with modification, hold a third position—that species are individually and independently created by God, with no ancestral/descendant relationship among species. See article on [[creationism]].)
  
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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.
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[[Speciation]] is the term that refers to creation of new and distinct biological species by branching off from the ancestral population. Various mechanisms have been presented whereby a single evolutionary lineage splits into two or more genetically independent lineages. For example, ''allopatric speciation'' is held to occur in populations that become isolated geographically, such as by habitat fragmentation or migration. The isolated populations then undergo genotypic and/or phenotypic divergence. ''Sympatric speciation'' is held to occur when new species emerge in the same geographic area. Ernst Mayr's ''peripatric speciation'' is a type of speciation that exists in between the extremes of allopatry and sympatry. In ''parapatric speciation,'' the zones of two diverging populations abut but do not overlap. There is only partial separation afforded by geography, so individuals of each species may come in contact or cross the barrier from time to time. ''Polyploidy'' is a means by which the beginnings of new species are created in just two or three generations.
  
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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.
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For years, the prevailing evolutionary view involved the concept of "gradualism." This was a view of evolution as proceeding by means of slow accumulation of very small changes, with the evolving population passing through all the intermediate stages—sort of a "march of frequency distributions" through time (Luria et al. 1981). [[Charles Darwin|Darwin]] made frequent reference to gradualism in his ''Origin of Species.'' For example, he stated: "As natural selection acts solely by accumulating slight, successive, favorable variations, it can produce no great or sudden modification; it can act only by very short and slow steps. Hence the canon of 'Natura non facit saltum'…" He further stated that nature "can never take a leap, but must advance by the shortest and slowest step," and that "if it could be demonstrated that any complex organ existed, which could not possibly have been formed by numerous, successive, slight modifications, my theory would absolutely break down."
  
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Although the current scientific understanding of species suggests 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 nonetheless has some problems, however. 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 notion of species.
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However, recent models or theories have been developed that contrast with gradualism. These include [[punctuated equilibria]], theories of punctuational speciation, and theories of rapid transitions.
  
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[[Richard Dawkins]] defines two organisms as conspecific if and only if they have the same number of [[chromosome]]s and, for each chromosome, both organisms have the same number of [[nucleotide]]s (''[[The Blind Watchmaker]]'', p. 118).
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The theory of punctuated equilibria presents the view that the macroevolutionary patterns of species are typically one of morphological stability during their existence (stasis), and that most evolutionary change is concentrated in events of speciation—with the origin of new species usually occurring during geologically short periods of time, when the long-term stasis of a population is punctuated by this rare and rapid event of speciation. Crystallized and popularized by [[Stephen Jay Gould]] and Niles Eldredge, this view contrasts with what Gould and Eldredge saw as an a priori bias towards "phyletic gradualism," which was the terminology for the expected slow, steady, progressive transformation of an ancestral population into a new species. Such a transformation was to involve large numbers ("usually the entire ancestral population"), be "even and slow," and occur "over all or a large part of the ancestral species geographic range" (Eldredge and Gould 1972). This concept of phyletic gradualism was applied to the development of new species either by phyletic evolution (where the descendent species arises by transformation of the entire ancestral population) or by speciation (where the descendent species branches off from the ancestral population). According to the proponents of punctuated equilibria, the viewpoint of phyletic gradualism leads to the expectation that the fossil record should yield a gradually graded sequence of intermediary forms. However, the fossil record does not generally yield such a sequence of slightly altered intermediary forms, but instead the sudden appearance of species, and long periods where species do not change much. Notably, as a model developed to describe macroevolutionary trends, the theory of punctuated equilibria is not necessarily tied to any one mode of speciation. It can be affiliated with both allopatric speciation and sympatric modes, and is independent of whether natural selection is involved or not. Likewise, species may occur by gradual means, passing through all intermediate stages, or by rapid origin, such as in chromosomal speciation. It also does not exclude saltation initiation of major transitions.
  
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The classification of species has been profoundly affected by technological advances that have allowed researchers to determine relatedness based on genetic markers. The results have been nothing short of revolutionary, resulting in the reordering of vast expanses of the [[phylogenetic tree]] (''see also:'' [[molecular phylogeny]]).
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There are also punctuational models of speciation. This is a different concept from the theory of punctuated equilibria, in that punctuated equilibria refers to the macroevolutionary trend of stasis and rapid speciation, and punctuational models of speciation refer specifically to the mode of speciation. Punctuational models of speciation have been advanced in contrast with what is sometimes labeled the "allopatric orthodoxy." This terminology refers to the process of species origin involving geographic isolation, whereby a population completely separates geographically from a large parental population and develops into a new species gradually, until their differences are so great that reproductive isolation ensues. In this understanding, reproductive isolation is a secondary by-product of geographic isolation, with the process involving gradual allele substitution. Contrasted with this traditional view are the punctuational models for speciation, which postulate that reproductive isolation can rise rapidly, not through gradual selection, but actually without selective significance. In these views, natural selection does not play a creative role in initiating speciation, nor in the definitive aspect of reproductive isolation. Among these modes are "polyploidy" (where there is a multiplication of the number of chromosomes beyond the normal diploid number), and "chromosomal speciation" (large changes in chromosomes due to various genetic accidents).  
  
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==See also==
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There are also models for the origin of new designs via rapid transitions (such as bird feathers from reptilian scales, jaws from jawless fish, etc.).
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*[[Speciation]]
 
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*[[Cryptic species complex]]
 
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*[[Ring species]]
 
  
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==External links==
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[[Extinction]] is the disappearance of species (i.e. gene pools). The moment of extinction generally occurs at the death of the last individual of that species. Extinction is not an unusual event in geological time—species are created by speciation, and disappear through extinction.
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{{Wikispecies|Species}}
 
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*http://users.rcn.com/jkimball.ma.ultranet/BiologyPages/S/Speciation.html
 
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*[http://www.sciencedaily.com/releases/2003/12/031231082553.htm 2003-12-31, ScienceDaily: Working On The 'Porsche Of Its Time': New Model For Species Determination Offered] Quote: "...two species of dinosaur that are members of the same genera varied from each other by just 2.2 percent. Translation of the percentage into an actual number results in an average of just three skeletal differences out of the total 338 bones in the body. Amazingly, 58 percent of these differences occurred in the skull alone. "This is a lot less variation than I'd expected", said Novak..."
 
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*[http://www.sciencedaily.com/releases/2003/08/030808081854.htm 2003-08-08, ScienceDaily: Cross-species Mating May Be Evolutionarily Important And Lead To Rapid Change, Say Indiana University Researchers] Quote: "...the sudden mixing of closely related species may occasionally provide the energy to impel rapid evolutionary change..."
 
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*[http://www.sciencedaily.com/releases/2004/01/040109064407.htm 2004-01-09 ScienceDaily: Mayo Researchers Observe Genetic Fusion Of Human, Animal Cells; May Help Explain Origin Of AIDS] Quote: "...The researchers have discovered conditions in which [[pig]] cells and [[human]] cells can fuse together in the body to yield hybrid cells that contain genetic material from both species... "What we found was completely unexpected", says Jeffrey Platt, M.D."
 
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*[http://www.sciencedaily.com/releases/2000/09/000913211733.htm 2000-09-18, ScienceDaily: Scientists Unravel Ancient Evolutionary History Of Photosynthesis] Quote: "...gene-swapping was common among ancient bacteria early in evolution..."
 
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* [http://plato.stanford.edu/entries/species/ Stanford Encyclopedia of Philosophy entry]
 
  
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It should be noted that 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. This is not to say that cross breeding does not take place at all, simply that it has become negligible. Generally, the hybrid individuals are less capable of successful breeding than purebred individuals of either species.
  
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[[Category:Scientific classification]]
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==References==
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[[Category: zoological nomenclature]]
 
  
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*Eldredge, N., and S. J. Gould. 1972. Punctuated equilibria: An alternative to phyletic gradualism. In T. J. M. Schopf, ed. ''Models in paleobiology'' (pp. 82–115). San Francisco: Freeman, Cooper.
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*Gould, S. J., and N. Eldredge. 1977. Punctuated equilibria: The tempo and mode of evolution reconsidered. ''Paleobiology'' 3:115–151.
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*Gould, S. J. 1993. A special fondness for beetles. ''Natural history'' 1:4–12.
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*Luria, S. E., S. J. Gould, and S. Singer. ''A view of life.'' Menlo Park, CA: Benjamin/Cummings.
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[[et:Liik (bioloogia)]]
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*Mayr, E. 1996. What is a species, and what is not? ''Philosophy of Science'' 63:262–277.
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*Meyer, A. 1990. Ecological and evolutionary aspects of the trophic polymorphism in ''Cichlasoma citrinellum'' (Pices: Cichlidae)'' ''Biol. J. Linn. Soc.'' 39: 279-299.
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*Wiley, E. O. 1978. The evolutionary species concept reconsidered. ''Systematic Zoology'' 27:17–26.
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Latest revision as of 17:36, 14 October 2022


Lady beetle

Species are the basic taxonomic units of biological classification. This grouping of organisms of "like kind" into discrete and stable units has been traced at least from the time of Plato and Aristotle. Nonetheless, today biologists lack consensus in how to define the term and what constitutes a species. Among the several definitions of species, the most commonly used is the biological species concept first coined by Ernst Mayr: Species are "groups of actually or potentially interbreeding natural populations which are reproductively isolated from other such groups." (However, see other definitions of species below.)

This basic taxonomic unit is remarkably stable. Species tend to remain the same throughout their geological history. As noted by eminent evolutionist Stephen Jay Gould, the macroevolutionary patterns of species are typically ones of morphological stability during their existence, a phenomena known as "stasis." In presenting the theory of punctuated equilibria, Niles Eldridge and Gould noted: "Most species, during their geological history, either do not change in any appreciable way, or else they fluctuate mildly in morphology, with no apparent direction." Once a species appears, the fossil record does not change much during its existence, which may be several million years. This view accords well with the view of creationism, which references a clear-cut boundary between species, as well as stability during their existence.

The concept of species is important. For one, environmental law is framed in terms of species. Indeed, many countries have laws proscribing special protection to species considered endangered to prevent their extinction. The term species also is central to evolutionary studies, and is generally presented as the principal unit of evolution. Ernst Mayr maintains that one cannot even write about evolution, or most aspects of the philosophy of biology, without understanding the meaning of the biological species.

It is difficult to even speculate how many species there are on the planet today. Estimates range from 10 million to 150 million. Less than 2 million species (perhaps only 2 percent or less of all species) have been identified.

Both the singular and plural forms of the noun are rendered by the word "species." The term derives from the Latin "specere" (to look at, to behold), with the meaning of "kind," "quality," "appearance," "shape," or "a peculiar sort." (Note that the word "specie" is not the singular of "species." It refers to coined money.)

Scientific name

In taxonomy, a species is assigned a two-part scientific name. The genus is listed first (and capitalized) followed by a species epithet (which is not capitalized). For example, humans belong to the genus Homo, and are in the species Homo sapiens. Tigers, lions, leopards, and jaguars are different species, but each are similar enough to belong to the same genus (Panthera). The name of the species is the whole binomial not just the second term of the binomial (the specific epithet). This system was set up by Swedish botanist Carolus Linnaeus.

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." in the singular or "spp." in the plural in the place of the second part of the scientific name.

Groups within a species can be defined as being of a taxon hierarchically lower than a species. In zoology, only the subspecies is used, while in botany the variety, subvariety, and form are used as well.

The idea of species has a long history. 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 organisms—dogs are one species, cats another.
  • It is the standard binomial nomenclature (or trinomial nomenclature) by which scientists typically refer to organisms.
  • It is the only taxonomic level that has empirical content, in the sense that asserting that two animals are of different species is saying something more than classificatory about them.

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 and controversial.

Definitions of species

Several different concepts are employed in identifying species:

  • Typological (or "morphological") species concept. Historically, species were viewed as collections of individuals that share a common phenotype, including morphology, behavior, and ecological relationships with their environment. This method was used as a "classical" method of determining species. For example, continental North American savanna sparrows were differentiated from savanna sparrows from Sable Island, Nova Scotia, if they were sufficiently different in morphological characters. A chicken and a duck can be distinguished because they have different shaped bills and the duck has webbed feet. This traditional method is useful in the work of taxonomy, such as cataloging species and creating identification keys. It is also applicable in paleontology, where morphology is all there is (such as snail shells in fossil beds). Indeed, the concept of morphological species remains the single most widely used species concept in everyday life, and retains an important place within the biological sciences, particularly in the case of plants. Shortcomings: However, there are also important shortcomings with the typological species concept of distinguishing species. For example, different phenotypes do not always constitute different species (i.e. a 4-winged Drosphila born to a 2-winged mother is not a different species). It is not uncommon to find pronounced morphological differences among individuals within one interbreeding population, due to individual genetic variation, sexual dimorphism (males, females, immatures), and life stage (catepillars and butterflies)—differences that may be much more evident than between clearly different species. Furthermore, there is the challenge of oversplitting taxa, whereby each variant is called a new species.
  • Biological (or "isolation") species concept. This concept identifies a species as a set of actually or potentially interbreeding organisms. Or, as stated by Ernst Mayr, "Species are groups of interbreeding natural populations that are reproductively isolated from other such groups." The biological species concept (BCS), which developed in the second half of the nineteenth century and was greatly advanced by Mayr in the twentieth century, involves thinking of species in terms of variable populations rather than fixed types. This is generally the most useful and common formulation for scientists working with living examples of the higher taxa, like mammals, fish, and birds. Shortcomings: The BSC is meaningless for organisms that do not reproduce sexually. Furthermore, the key to defining a biological species is that there is no significant cross-flow of genetic material between the two populations. But, biologists frequently do not know whether two morphologically similar groups of organisms are "potentially" capable of interbreeding, and BSC is thus impractical in many instances of allopatric (geographically isolated) populations. The BSC does not distinguish between the theoretical possibility of interbreeding and the actual likelihood of gene flow between populations. Does one successful hybridization invalidate species distinction? Note that it is possible to cross a horse with a donkey and produce offspring. However, they remain separate species—in this case for two different reasons: first because horses and donkeys do not normally interbreed in the wild, and second because the fruit of the union is rarely fertile. Similarly, how does one utilize the BCS to delineate paleospecies (extinct or fossil species)?
  • Mate-recognition species concept. A mate-recognition species is defined as a group of organisms that share a common fertilization system and are known to recognize one another as potential mates. Shortcoming: Like the BCS above, it applies only to organisms that reproduce sexually.
  • Phylogenetic species concept. The phylogenetic species concept, which has several versions, essentially defines a species as a group of organisms bound by a unique ancestry. Devised by paleontologists Niles Eldredge and Joel Cracraft, it is an attempt to define species by their relationships to other species, involving uncovering their genealogical relationships. A formal definition given by Joel Cracraft is: "A species is the smallest diagnosable cluster of individual organisms within which there is a parental pattern of ancestry and descent." Thus, according to this concept, diagnosable geographic forms of the same basic "kind" of bird should be treated as distinct species, because these forms have evolved separately, and have unique evolutionary histories. For example, a population of sparrows that had a unique heritable character, such as a particular allele (form of a particular gene) would be considered a separate species from those that do not have that particular character. This method is applicable even to unusual reproductive modes other than sexual reproduction. Shortcomings: Application of this concept is problematic in those cases where morphologically different populations are connected by gene flow, and such morphological variation among populations is not uncommon within living species. For example, humans have substantial morphological variation from continent to continent. Fragmentary specimens collected by a paleontologist on different continents, which show clinal variation, would appear to be unique. Ernst Mayr also criticizes the phylogenetic species concept as nothing more than the revival of a purely morphological species concept.
  • Evolutionary (or "Darwinian") species concept. An evolutionary species is 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. This "evolutionary species concept" (ESC) is often associated with George Gaylord Simpson, a mammalian paleontologist, who stated "a species is a series of ancestor-descendent populations passing through time and space independent of other populations, each of which possesses its own evolutionary tendencies and historical fate." The ESC is the most popular concept among paleontologists, and is used extensively in comparative biology and phylogenetic systematics. It has similarities with the phylogentic species concept, but the ESC combines the genealogical basis of the phylogenetic species concept with the genetic basis of the biological species concept. Specifically, the ESC uses a wider range of characters to make the species determination. An evolutionary species is a lineage of interbreeding organisms, reproductively isolated from other lineages, that has a beginning, an end, and a distinct evolutionary trajectory (Wiley 1978). Shortcomings: Mayr criticized this concept for several reasons: (1) it replaces the clear-cut criterion of reproductive isolation of the BCS with such vague terms as "maintains its identity," "evolutionary tendencies," and "historical fate"; (2) it is applicable only to monotypic species, and geographic isolates would have to be treated as different species; (3) there are no empirical criteria by which evolutionary tendency of historical fate can be observed in a given fossil sample; and (4) the ESC does not help in the lower or upper demarcation of chronospecies, even though the concept was apparently introduced to deal with the time dimension.
  • Ecological species concept defines a species as a group of organisms that share a distinct ecological niche. Shortcoming: This concept, which is based on the niche occupied by a species, is problematic because widespread species generally have local populations that differ in their niche occupation, which would require they be recognized as different species, even though based on all other criteria they would not be. As noted by Mayr (1996), "More fatal for the ecological species concept are the trophic species of cichlids (A. Mayer 1990)," which differentiate niche within a single set of offspring from the same parents. There are also common cases where two sympatric species seem to occupy the same niche.

These are just a few of more than a dozen common methods of delineating species among biologists. (One might also include, for example, Templeton's "cohesion species concept" that attempts to combine several components of species concepts, but which is likewise criticized as failing to resolve the resulting conflicts.) 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.

Number of species

No one knows how many species exist today. Approximately 1.8 million species of animals and plants have been identified (excluding the diverse kingdoms of fungi, bacteria, and other unicellular organisms), but some biologists estimate there may be more than 150 million species of living things on the earth. Indeed, E. O. Wilson in his 1992 book The Diversity of Life, stated "How many species of organisms are there on earth? We don't know, not even to the nearest order of magnitude. The numbers could be as close to 10 million or as high as 100 million."

Of those that have been identified, more than half are insects (about 57 percent), and nearly half of all insect species are beetles, meaning that beetles, with over 400,000 identified species, represent about 25 percent of all named species in the plant and animal kingdoms. This fact led to the famous quip from J. B. S. Haldane, perhaps apocryphal, who when asked what one could conclude as to the nature of the Creator from a study of his creation, replied: "An inordinate fondness for beetles" (Gould 1993).

There are also approximately 9,000 named species of birds, 27,000 known species of fish, and a ledger of about 4,000 or so mammalian species. These groups have been diligently catalogued, unlike insects, which rank among the most uncounted groups of organisms.

Speciation and extinction

There are two basic ways that are widely presented by biologists for the origin of a species: (1) the descendant species arises by transformation of the entire ancestral population; or (2) the descendant species branches off from the ancestral population. (Strict scientific creationists, who do not accept the evolutionary theory of descent with modification, hold a third position—that species are individually and independently created by God, with no ancestral/descendant relationship among species. See article on creationism.)

Speciation is the term that refers to creation of new and distinct biological species by branching off from the ancestral population. Various mechanisms have been presented whereby a single evolutionary lineage splits into two or more genetically independent lineages. For example, allopatric speciation is held to occur in populations that become isolated geographically, such as by habitat fragmentation or migration. The isolated populations then undergo genotypic and/or phenotypic divergence. Sympatric speciation is held to occur when new species emerge in the same geographic area. Ernst Mayr's peripatric speciation is a type of speciation that exists in between the extremes of allopatry and sympatry. In parapatric speciation, the zones of two diverging populations abut but do not overlap. There is only partial separation afforded by geography, so individuals of each species may come in contact or cross the barrier from time to time. Polyploidy is a means by which the beginnings of new species are created in just two or three generations.

For years, the prevailing evolutionary view involved the concept of "gradualism." This was a view of evolution as proceeding by means of slow accumulation of very small changes, with the evolving population passing through all the intermediate stages—sort of a "march of frequency distributions" through time (Luria et al. 1981). Darwin made frequent reference to gradualism in his Origin of Species. For example, he stated: "As natural selection acts solely by accumulating slight, successive, favorable variations, it can produce no great or sudden modification; it can act only by very short and slow steps. Hence the canon of 'Natura non facit saltum'…" He further stated that nature "can never take a leap, but must advance by the shortest and slowest step," and that "if it could be demonstrated that any complex organ existed, which could not possibly have been formed by numerous, successive, slight modifications, my theory would absolutely break down."

However, recent models or theories have been developed that contrast with gradualism. These include punctuated equilibria, theories of punctuational speciation, and theories of rapid transitions.

The theory of punctuated equilibria presents the view that the macroevolutionary patterns of species are typically one of morphological stability during their existence (stasis), and that most evolutionary change is concentrated in events of speciation—with the origin of new species usually occurring during geologically short periods of time, when the long-term stasis of a population is punctuated by this rare and rapid event of speciation. Crystallized and popularized by Stephen Jay Gould and Niles Eldredge, this view contrasts with what Gould and Eldredge saw as an a priori bias towards "phyletic gradualism," which was the terminology for the expected slow, steady, progressive transformation of an ancestral population into a new species. Such a transformation was to involve large numbers ("usually the entire ancestral population"), be "even and slow," and occur "over all or a large part of the ancestral species geographic range" (Eldredge and Gould 1972). This concept of phyletic gradualism was applied to the development of new species either by phyletic evolution (where the descendent species arises by transformation of the entire ancestral population) or by speciation (where the descendent species branches off from the ancestral population). According to the proponents of punctuated equilibria, the viewpoint of phyletic gradualism leads to the expectation that the fossil record should yield a gradually graded sequence of intermediary forms. However, the fossil record does not generally yield such a sequence of slightly altered intermediary forms, but instead the sudden appearance of species, and long periods where species do not change much. Notably, as a model developed to describe macroevolutionary trends, the theory of punctuated equilibria is not necessarily tied to any one mode of speciation. It can be affiliated with both allopatric speciation and sympatric modes, and is independent of whether natural selection is involved or not. Likewise, species may occur by gradual means, passing through all intermediate stages, or by rapid origin, such as in chromosomal speciation. It also does not exclude saltation initiation of major transitions.

There are also punctuational models of speciation. This is a different concept from the theory of punctuated equilibria, in that punctuated equilibria refers to the macroevolutionary trend of stasis and rapid speciation, and punctuational models of speciation refer specifically to the mode of speciation. Punctuational models of speciation have been advanced in contrast with what is sometimes labeled the "allopatric orthodoxy." This terminology refers to the process of species origin involving geographic isolation, whereby a population completely separates geographically from a large parental population and develops into a new species gradually, until their differences are so great that reproductive isolation ensues. In this understanding, reproductive isolation is a secondary by-product of geographic isolation, with the process involving gradual allele substitution. Contrasted with this traditional view are the punctuational models for speciation, which postulate that reproductive isolation can rise rapidly, not through gradual selection, but actually without selective significance. In these views, natural selection does not play a creative role in initiating speciation, nor in the definitive aspect of reproductive isolation. Among these modes are "polyploidy" (where there is a multiplication of the number of chromosomes beyond the normal diploid number), and "chromosomal speciation" (large changes in chromosomes due to various genetic accidents).

There are also models for the origin of new designs via rapid transitions (such as bird feathers from reptilian scales, jaws from jawless fish, etc.).

Extinction is the disappearance of species (i.e. gene pools). The moment of extinction generally occurs at the death of the last individual of that species. Extinction is not an unusual event in geological time—species are created by speciation, and disappear through extinction.

It should be noted that 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. This is not to say that cross breeding does not take place at all, simply that it has become negligible. Generally, the hybrid individuals are less capable of successful breeding than purebred individuals of either species.

References
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  • Eldredge, N., and S. J. Gould. 1972. Punctuated equilibria: An alternative to phyletic gradualism. In T. J. M. Schopf, ed. Models in paleobiology (pp. 82–115). San Francisco: Freeman, Cooper.
  • Gould, S. J., and N. Eldredge. 1977. Punctuated equilibria: The tempo and mode of evolution reconsidered. Paleobiology 3:115–151.
  • Gould, S. J. 1993. A special fondness for beetles. Natural history 1:4–12.
  • Luria, S. E., S. J. Gould, and S. Singer. A view of life. Menlo Park, CA: Benjamin/Cummings.
  • Mayr, E. 1996. What is a species, and what is not? Philosophy of Science 63:262–277.
  • Meyer, A. 1990. Ecological and evolutionary aspects of the trophic polymorphism in Cichlasoma citrinellum (Pices: Cichlidae) Biol. J. Linn. Soc. 39: 279-299.
  • Wiley, E. O. 1978. The evolutionary species concept reconsidered. Systematic Zoology 27:17–26.


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