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Species

Species

There is little agreement among scientists about the definition of the word "species." However, most biologists would agree that a species is a detectable, naturally occurring group of individuals or populations that is on an evolutionary path independent from other such groups. Several more detailed definitions have been articulated over the years; two that have gained prominence are the biological species concept (BSC) and the phylogenetic species concept (PSC).

In 1942 biologist Ernst Mayr defined the biological species concept as follows: "Species are groups of actually or potentially interbreeding natural populations, which are reproductively isolated from other such groups." This definition places emphasis on restriction of gene flow among groups. Reproductive isolation means that individuals from two groups are unable to interbreed successfully, that is, produce healthy, fertile offspring. So according to this definition, an individual is a member of a particular species if it can breed successfully with members of that species but not with members of other species.

Interbreeding between two different groups is called hybridization and is viewed differently by different scientists. In animals, hybrid offspring of two different species are thought to be unhealthy or infertile as adults, but in plants hybrid offspring are often thought to be more vigorous than their parents. As a result, plant biologists and animal biologists differ regarding the significance of interbreeding in answering species questions, and most plant biologists are not proponents of the BSC.

Objections to the BSC include the fact that the extent of hybridization can range from very little to extensive, making its interpretation subjective. Also, it requires guesswork regarding the species status of groups that do not occur in the same place and thus have no opportunity to interbreed, and it cannot easily be applied to organisms in the fossil record or to those that lack sexual reproduction. Furthermore, it is now known that hybridization can occur between two independent groups that are not each other's nearest relatives. Thus, putting two hybridizing groups into one species could misrepresent evolutionary history by excluding other more closely related (and often reproductively isolated) groups.

A phylogenetic species concept was articulated by Joel Cracraft in 1987 as follows: "a species can be defined as an irreducible cluster of organisms, within which there is a parental pattern of ancestry and descent, and which is diagnosably distinct from other such clusters." This definition views a species as being the smallest possible grouping of organisms in time and space that can be differentiated from other groupings, with the basis for the differentiation being inherited. So an individual is a member of a species if it shares the inherited characteristics of the species, irrespective of whether it can hybridize with a member of another species. The primary objection to this definition is that it is too vague.


MAYR, ERNST (1904)

German-born U.S. evolutionary biologist who helped found the "modern synthesis," the melding of evolutionary theory with genetics. Mayr's greatest contribution was to explain how new species can arise. When a population is isolated, on an island, for example, it can evolve separately from the rest of the species. Mayr's views have defined evolutionary biology for nearly three-quarters of a century, and he has won two prestigious prizes, the Balzan Prize and the Japan Prize.


These are but two examples of the numerous definitions from a century of ongoing debate about the definition and meaning of species. Scientists often approach the species question differently depending on what organisms they are studying and the way in which they are studying them. Traditionally, organisms have been grouped into species based on aspects of their appearance or particular behaviors. More recently, analysis of deoxyribonucleic acid (DNA) has joined the list of techniques for differentiating or grouping organisms. Additionally, there are specific criteria used for different groups. In plants, for instance, plant chemistry, insect associations, and number of chromosomes may be important indicators of species status. As another example, scientists studying bacteria may use such characteristics as shape, biochemistry, and conditions favoring growth to help them answer species questions. Thus, there is no simple, universally agreed-upon definition of species.

see also Biodiversity; Buffon, Count; Evolution; Speciation; Taxonomy, History of

Ann E. Kessen and Robert M. Zink

Bibliography

Keller, Evelyn Fox, and Elisabeth A. Lloyd, eds. Keywords on Evolutionary Biology. Cambridge, MA: Harvard University Press, 1992.

Otte, Daniel, and John A. Endler. Speciation and Its Consequences. Sunderland, MA: Sinauer Associates, 1989.

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species

species (sing. and pl.) Literally, a group of organisms that resemble one another closely. The Latin word species means ‘appearance’ or ‘semblance’. In taxonomy, it is applied to one or more groups (populations) of individuals that can interbreed within the group but that do not, under natural conditions, exchange genes with other groups (populations); it is an interbreeding group of biological organisms that is isolated reproductively from all other organisms (see BIOSPECIES). A species can be made up of groups in which members do not actually exchange genes with members of other groups (though in principle they could do so), as e.g. at the two extremes of a continuous geographical range. However, if some gene flow occurs along a continuum, the formation of another species is unlikely to occur. Where barriers to gene flow arise (e.g. physical barriers, such as sea, or areas of unfavourable habitat) this reproductive isolation may lead either by local selection or by random genetic drift to the formation of morphologically distinct forms (‘races’ or ‘subspecies’). These could interbreed with other races of the same species if they were introduced to one another. Once this potential is lost, through some further evolutionary divergence, the races may be recognized as species, although this concept is not a rigid one. Most species cannot interbreed with others; a few can, but produce infertile offspring; a smaller number may actually produce fertile offspring. The term cannot be applied precisely to organisms whose breeding behaviour is unknown. See MORPHOSPECIES; PALAEOSPECIES; REPRODUCTIVE ISOLATING MECHANISMS.

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species

species (sing. and pl.) Literally, a group of organisms that resemble one another closely: the term derives from the Latin speculare, ‘to look’. In taxonomy it is applied to one or more groups (populations) of individuals that can interbreed within the group but cannot exchange genes with other groups (populations), or, in other words an interbreeding group of biological organisms which is isolated reproductively from all other organisms (see biospecies). A species can be made up of groups in which members do not actually exchange genes with members of other groups (though in principle they could do so), as, for example, at the two extremes of a continuous geographical range. However, if some gene flow occurs along a continuum, the formation of another species is unlikely to occur. Where barriers to gene flow arise (e.g. physical barriers, such as sea, or areas of unfavourable habitat) this reproductive isolation may lead by either local selection or random genetic drift to the formation of morphologically distinct forms termed races or subspecies. These could interbreed with other races of the same species if they were introduced to one another. Once this potential is lost, through some further evolutionary divergence, the races may be recognized as species, although this concept is not a rigid one. Most species cannot interbreed with others; a few can, but produce infertile offspring; a smaller number may actually produce fertile offspring. The term cannot be applied precisely to organisms whose breeding behaviour is unknown. See morphospecies and palaeospecies.

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species

species (sing. and pl.) Literally, a group of organisms that resemble one another closely: the term derives from the Latin speculare, ‘to look’. In taxonomy, it is applied to one or more groups (populations) of individuals that can interbreed within the group but cannot exchange genes with other groups (populations), or, in other words an interbreeding group of biological organisms that is isolated reproductively from all other organisms (see BIOSPECIES). A species can be made up of groups in which members do not actually exchange genes with members of other groups (though in principle they could do so), as, for example, at the two extremes of a continuous geographical range. However, if some gene flow occurs along a continuum, the formation of another species is unlikely to occur. Where barriers to gene flow arise (e.g. physical barriers, such as sea, or areas of unfavourable habitat) this reproductive isolation may lead by either local selection or random genetic drift to the formation of morphologically distinct forms termed races or subspecies. These could interbreed with other races of the same species if they were introduced to one another. Once this potential is lost, through some further evolutionary divergence, the races may be recognized as species, although this concept is not a rigid one. Most species cannot interbreed with others; a few can, but produce infertile offspring; a smaller number may actually produce fertile offspring. The term cannot be applied precisely to organisms whose breeding behaviour is unknown. See MORPHOSPECIES and PALAEOSPECIES.

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species

spe·cies / ˈspēsēz; -shēz/ • n. (pl. same) 1. (abbr.: sp., spp.) Biol. a group of living organisms consisting of similar individuals capable of exchanging genes or interbreeding. The species is the principal natural taxonomic unit, ranking below a genus and denoted by a Latin binomial, e.g., Homo sapiens. ∎  Logic a group subordinate to a genus and containing individuals agreeing in some common attributes and called by a common name. ∎  a kind or sort: a species of invective at once tough and suave. ∎  used humorously to refer to people who share a characteristic or occupation: a political species that is becoming more common, the environmental statesman. ∎  Chem. & Physics a particular kind of atom, molecule, ion, or particle: a new molecular species. 2. Christian Church the visible form of each of the elements of consecrated bread and wine in the Eucharist. ORIGIN: late Middle English: from Latin, literally ‘appearance, form, beauty,’ from specere ‘to look.’

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species

species
1. A group of organisms that resemble each other more than they resemble members of other groups and cannot be subdivided into two or more species. The precise definition of what constitutes a species differs depending on which species concept is applied. According to the biological species concept, a species comprises a group of individuals that can usually breed among themselves and produce fertile offspring. However, many other species concepts have been proposed, including the phylogenetic species concept and various typological species concepts. Typically, a species consists of numerous local populations distributed over a geographical range. Within a species, groups of individuals become reproductively isolated because of geographical or behavioural factors (see isolating mechanism), and over time may evolve different characteristics and form a new and distinct species.

2. A rank, or category, used in the classification of organisms. Similar species are grouped into a genus, and a single species may be subdivided into subspecies or races. See also binomial nomenclature.

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species

species, in biology, a category of classification, the original and still the basic unit in the demarcation of plant and animal types. The species marks the boundary between populations of organisms rather than between individuals. Because related species are not absolutely permanent (see evolution), a precise definition of the term is difficult. On the basis of genetics, scientists now include in a species all individuals that are potentially or actually capable of interbreeding and that share the same gene pool. The latter term refers to that collection of characteristics whose combination is unique in the species, although each individual of the group may not display every single one of the characteristics (see genetics). In the few cases where members of different species can interbreed, the offspring are usually sterile (e.g., the mule). Groups distinguished by lesser differences than those marking a species are called variously subspecies, varieties, races, or tribes.

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Species

Species

The term species, in the most general sense, refers to the various kinds of living things. Thus, species are generally recognized as distinct, fully differentiated groups of organisms. However, most modern definitions of species also recognize them as reproductive communities and acknowledge that mating occurs between members of each species but does not occur (or occurs only rarely) among members of different species. Species are therefore generally recognized as genetically differentiated, reproductive communities within which there is a pattern of ancestry and descent among organisms. Although most scientists accept this general definition, there are two somewhat different criteria that are often employed in the recognition of species, and the application of the two criteria does not always lead to the same conclusions.

The Role of Interbreeding

The first major criterion for the designation of species is the actual occurrence of interbreeding among the various organisms and populations within a species and the absence of such interbreeding between species. However, patterns of interbreeding are difficult to observe directly, particularly among plants that may live for hundreds of years. For this reason, indirect evidence regarding patterns of interbreeding is usually provided by the study of the patterns of differentiation among populations in genetically determined characteristics. In a trivial sense, oak trees and daisies are regarded as belonging to different species because they are distinguished by numerous characters and because hybrids between them are never observed. Thus, it is reasonable to conclude that they are not part of the same reproductive community. However, there are many recognized species of daisies, and many recognized species of oaks, and these species often are delimited by subtle differences. Consequently, one might examine several populations of daisies and observe that the plants are identical except for one character: the occurrence of a line of hairs along the undersides of the leaves. If all of the individuals in some populations have the line of hairs, and all of the individuals in other populations lack these hairs, there is evidence that two population systems exist and that there is no gene flow between them. In contrast, if each population that is examined includes individuals with the line of hairs and other individuals without the line of hairs, it can be concluded that this is simply a character that varies within a single species, like blood types in humans. Under this criterion, two species can be recognized even when the differences between them are not readily observable. For example, there are many documented cases in which two or more population systems differ from each other by genetically determined differences that can only be detected by biochemical tests. Though they are difficult to distinguish, the species are recognized as distinct genetic communities.

Reproductive Isolating Barriers

The second major criterion of species status is the existence of a genetically determined barrier to gene flow between species. Such a barrier, known as a reproductive-isolating mechanism or a reproductive isolating barrier (RIB), prevents members of two different species from interbreeding, even if they occur in the same location. For example, the pollen that is produced by plants of one species may not germinate when placed on the stigmas of plants of another species and, thus, there can be no reproduction or gene flow between them. In this case, the RIB is the pollen/stigma incompatibility. Because this barrier is genetically determined, the two are regarded as reproductively isolated, and, as a result, two species are recognized.

Generally, any species boundary due to a reproductive isolating barrier also serves to prevent interbreeding as defined by the first criterion, but there are many instances in which the first criterion is satisfied while the second is not. The line of hairs on the underside of the leaves, which distinguishes two species of daisies under the first criterion in the example just described, does not by itself prevent interbreeding from occurring between the two kinds of daisies. The two daisy species may fail to exchange genes not because of a genetic mechanism but because they occur on different sides of a mountain range. Some biologists argue that if a RIB is not identified, the two kinds of daisies (one with the line of hairs, the other lacking it) should be grouped together and recognized as belonging to the same species. Others argue that the two populations are, indeed, persisting as separate and fully differentiated reproductive communities. Although they may have the potential to interbreed, the available evidence suggests that this does not occur, so they should be recognized as different species. Whatever position one takes on this matter, it should be noted that most species that have been recognized by science have, in fact, been delimited according the first criterion.

see also Cultivar; Hybrids and Hybridization; Speciation; Taxonomy; Variety.

Jerrold I. Davis

Bibliography

Futuyma, D. J. Evolutionary Biology. Sunderland, MA: Sinauer Associates, 1997.

Grant, V. Plant Speciation, 2nd ed. New York: Columbia University Press, 1981.

Mayr, E. The Growth of Biological Thought. Cambridge, MA: Belknap Press of Harvard University Press, 1982.

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species

species outward form (surviving spec. theol. of the elements in the Eucharist); kind (gen. and spec.). XVI. — L. (sg. and pl.) speciēs, f. spec- of specere look, behold.
So specie phr. in s., in kind; in the real form XVI; in actual coin XVII; hence sb. coined money XVII; abl. sg. of speciēs. specific XVII. — late L. specificus. specification XVII. — medL. specify XIII. — OF. or medL. specificāre. specimen †experiment; †pattern; typical example. XVII. — L. specimen. specious †fair to look upon XIV; attractive or plausible but lacking in genuineness XVII. — L. speciōsus.

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species

species. Name given to each of 5 types of process in strict counterpoint. The species are: 1. Added voice (i.e. the counterpoint melody) proceeds at same pace as cantus firmus (a note to a measure). 2. Added voice proceeds at 2 or 3 times pace of cantus firmus. 3. Added voice proceeds at 4 or 6 times pace of cantus firmus. 4. Added voice proceeds (as in 2) at rate of 2:1, but 2nd note is tied over to 1st note of following measure (syncopation). 5. Added voice uses mixture of processes of other 4 species and also introduces shorter notes (florid counterpoint).

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species

species Group of physically and genetically similar individuals that interbreed to produce fertile offspring under natural conditions. In biological classification, species is the lowest level. Each species has a unique two-part Latin name, the first part being the capitalized genus name and the second part, the species. So far, more than 1.5 million plant and animal species have been identified. See also binomial nomenclature; taxonomy

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species

species (spee-shiz) n. the smallest unit used in the classification of living organisms. Members of the same species are able to interbreed and to produce fertile offspring. Similar species are grouped together within one genus.

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species

species (sing. and pl.)
1. A class of particular chemical individuals all of which are similar, e.g. ions, atoms, or molecules.

2. See CLASSIFICATION.

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Species

Species

a group of individuals of common parentage; a sort, kind, or variety.

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species

speciesbiz, Cadíz, Cadiz, fizz, frizz, gee-whiz, his, is, jizz, Liz, Ms, phiz, quiz, squiz, swizz, tizz, viz, whizz, wiz, zizz •louis, Suez •scabies •Celebes, heebie-jeebies •showbiz • laches • Marches • breeches •Indies • undies • hafiz • Kyrgyz •Hedges • Bridges • Hodges • Judges •Rockies • walkies •Gillies, Scillies •pennies • Benares •Jefferies, Jeffreys •Canaries •Delores, Flores, furores •series • miniseries • Furies •congeries • Potteries • molasses •glasses • sunglasses • missus • suffix •falsies • fracases • galluses •Pontine Marshes • species •subspecies • conches • munchies •treatise •civvies, Skivvies •Velázquez • exequies • obsequies •Menzies • elevenses •cosies (US cozies), Moses •Joneses

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