"Biol. A category of classification lower than a genus or subgenus and above a subspecies or variety; a group of animals or plants which possess in common one or more distinctive characters, and do or may interbreed and reproduce their characters in their offspring; a distinct kind or sort of animal or plant."
"A species is an evolved or evolving, genetically distinctive, reproductively isolated, natural population. All of these attributes are necessary, and no others would seem to be essential."
"Species are groups of actually (or potentially) interbreeding natural populations which are reproductively isolated from other such groups."
"A group of organisms judged by taxonomists (by diverse criteria) to be worthy of formal recognition as a distinct kind."
"Arten sind Gruppen von Individuen, die sich in ihren geschlectlich entwickelten Formen nich mehr dergestalt kruezen konnen, dass sich die aus dieser Kruezung hervorgehenden vollkommen ausgebildeten Tiere unbeschrankt miteinander fortzuplanzen vermogen."
The stage of the evolutionary process at which the fixation of the discontinuity takes place is fundamentally important, and the attainment of this stage signifies the advent of species distinction. Dobzhansky (1935) has therefore proposed to define species as that stage of evolutionary process, "at which the once actually or potentially interbreeding array of forms becomes segregated in two or more separate arrays which are physiologically incapable of interbreeding." This definition differs from those hitherto proposed in that it lays emphasis on the dynamic nature of the species concept.
"The species is the most inclusive Mendelian population; its chief characteristic is that its members do not (no matter how good may be the opportunity) interbreed with members of other Mendelian populations. Populations whose members do not interbreed because they are kept apart by geographic barriers may not be classes as species on this evidence alone; for example, a number of Mendelian populations living on several widely separated oceanic islands may all belong to the one species, even though there is virtually no chance of interbreeding in nature because of the distances separating the islands. On the other hand, if it were found, when they came together that they still did not interbreed, then they would be correctly classed as separate species."
"A species is an aggregation of individuals which may resemble each other within some range of variation and always arises from a common reproductive sequence."
"It has been shown, first of all, that species do not exist as natural biological entities. Each is fitted to live in the environment in which it is found as a key fits in a lock."
Species: "A grade or rank assigned by systematists to an assemblage of organic forms which they judge to be more closely related by common descent than they are related to forms judged to be outside the species."
"A species includes a group of individuals very closely related to one another, which have descended from the same ancestors and can give rise to offspring which are fertile among themselves; such individuals usually differ from one another to only about the same degree that offspring of the same parents may differ."
"Species of helminths may be defined tentatively as a group of organisms the lipid-free antigen of which when diluted to 1 : 4000 or more, yields a positive precipitin test within one hour with a rabbit anti-serum produced by injecting 40 mg. of dry weight, lipid-free antigenic material and withdrawn ten to twelve days after the last of four intravenous injections administered every third day."
"The smallest natural populations permanently separated from each other by a distinct discontinuity in the series of biotypes"
"No single absolute test for a species is yet known, and it is debatable if such is ever likely to be found, but as a working hypothesis the following criteria should be considered: a species is morphologically definable in that it has a sum-total of characters, and every individual within it has constant resemblances with every other individual within it, and constant differences from every individual of other species, even when the individuals are grown under diverse conditions; species are isolated one from another, sometimes geographically, sometimes by habitat preferences, sometimes by having different flowering periods, usually by not crossing naturally to produce completely fertile offspring; species may show chromosomal differences. A species is an isolated group of individuals whose sum of characters tends to keep constant by natural in-breeding."
"A species is a group of individuals that are morphologically and physiologically similar (although comprising a number of groups of the lowest taxonomic category), which has reached an almost complete biological isolation from similar neighboring groups of individuals inhabiting the same or adjacent territories. Under biological isolation we understand the impossibility or nonoccurrence of normal hybridization under natural conditions."
"....a population of individuals prevented from interbreeding with all other populations by physiological differences (in the widest sense) whether or not structural differences are also present. A species is thus regarded as the stage in evolution at which physiological isolating mechanisms become virtually complete."
"A species is a group of individuals which, in the sum total of their attributes, resemble each other to a degree usually accepted as specific, the exact degree being ultimately determined by the more or less arbitrary judgement of taxonomists."
"Concept of a Linnean species as a definite, discrete, dynamic system differentiated into geographical and ecological types and comprising sometimes an enormous number of varieties."
"Present author (1931) formulated the following ideas as essential for a working concept of species:1. Common structural characteristics which unite certain individual organisms into one group, and a common genetic basis for the group represented by a specific chromosome complement.
2. Characteristic features which distinguish such entities from one another, one of these features frequently being the chromosome complement.
3. Relative stability combined with more or less variability within the group. This stability is made possible by a high degree of regularity in chromosome distribution from cell to cell and from parent to offspring, while inherited variations arise from occasional changes of one sort or another in the chromosomes.
4. Common descent of all individuals of the group from one or more preexisting species is made possible by the known mechanisms of heredity and genetic variation.
5. Syngamy or free intercrossing and high interfertility among the individuals of the group are just what would be expected in organisms in which almost all of the genes in all of the chromosomes are homologous.
6. Absence of free intercrossing and usually high, if not complete, sterility in hybrids between different species (with a few exceptions) are logical results of the accumulation of genic and larger chromosomal differences between diverging groups of individuals within a species.
7. The existence in many species of subspecific groups, occupying different but usually overlapping geographic areas. These subspecies differ more from one another in structure and interfertility or both than do the individuals composing each subspecies, but the subspecies are sometimes still connected with one another by intergrading forms. This is the necessary result of genetic variability within the species, plus the influence of environmental variability, isolation, and natural selection or random fixation. It is a stage in one process of species formation."
"Species are separated from each other by gaps of genetic discontinuity in morphological and physiological characteristics which are maintained by the absence or rarity of gene interchange between members of different species.""In order....to make valid inferences as to the specific status of allopatric as well as sympatric, population systems, one must determine not only whether they can cross and produce fertile hybrids under the optimum conditions of a cultivated garden plot but, in addition, whether they could coexist in the same territory and hybridize under natural conditions."
"The wisest course would seem to avoid defining species too precisely and to be tolerant of somewhat different species concepts held by other workers. The one principle which is unavoidable is that species are based on discontinuities in the genetic basis of the variation pattern rather than on the amount of difference in their external appearance between extreme or even 'typical' individual variants."
"If we accept this latitude in our species definitions, then we can recognize the existence as species-isolating mechanisms of purely spatial isolation, strictly ecological isolation of sympatric forms, or various combinations of these two isolating factors. And the latter are by far the most common in nature."
"A species is a name in a book." "Species are judgments." "A species is what a competent taxonomist thinks it is." "A species is a group of individuals which, in the sum total of their attributes, resemble each other to a degree usually accepted as specific, the exact degree being ultimately determined by the more or less arbitrary judgement of taxonomists."
"In general, it is becoming clear that we must use a combination of several criteria in defining species. Some of these are of limiting nature. For instance, infertility between groups of obviously distinct mean type is a proof that they are distinct species, although once more the converse is not true. Thus, in most cases, a group can be distinguished as a species on the basis of the following points jointly:
1. a geographical area consonant with a single origin;
2. a certain degree of constant morphological and presumedly genetic difference from related groups:
3. absence of intergradation with related groups.""In most cases a species can thus be regarded as a geographically definable group, whose members actually interbreed or are potentially capable of interbreeding in nature, which normally in nature does not interbreed freely or with full fertility with related groups, and is distinguished from them by constant morphological differences."
"Thus we must not expect too much of the term species. In the first place, we must not expect a hard-and-fast definition, for since most evolution is a gradual process, borderline cases must occur. And in the second place, we must not expect a single or a simple basis for definition, since species arise in many different ways."
"Only deep attachment to the word species seems to prevent recognition that the needs of biologists would best be served by having a separate term, such as syngen, for the unit of evolution when it is known not to be the same as the unit of identifications. When they are known to be the same, then they are at once both a species and a syngen. The reverence now accorded, often unjustly, to the term species would in time be transferred to the term syngen, or whatever term may be adopted, because of its greater evolutionary significance. The taxonomist and the geneticist would both be free to perform their different tasks with a logical and consistent terminology, neither being frustrated by conflict with the other. The major remaining problem is whether the two terms can be generalized.""Generalization of the term species for the unit of identification offers no great difficulty in principle. It simply requires a verbal statement which embodies current sound taxonomic practice. The first requirement is that the species be readily and visibly identifiable as a group of organisms manifesting discontinuity in morphology with all other groups. Secondly, the level of visible discontinuity ideally is the simplest one judged to be untransgressible by genetic means, either a recombination or mutation. Groups of organisms that are judged to show minimal, irreversible, visible divergence thus are assigned to different species."
"The generalization of syngen is more difficult. Failure to recognize the equivalent of the syngen in organisms that cannot outbreed is due to the identification of the method of ascertainment with the thing ascertained. The discreteness and reality of the latter is based upon the genetic complexity of the features that mark off a syngen from closely related syngens. This is what prevents gene flow between them, and it is the reason that the test of gene flow serves for ascertainment. Conceptually therefore a syngen can be defined by the complexity of its genetic distinctness from other syngens. A syngen, like a species, has thus passed the threshold of irreversible readily recognized visible differentiation. This level of biological organization is difficult to discover in any kind of organism, but it can be done with any kind. With outbreeders, the method is to test for gene flow. With asexual organisms, the method is to compare different strains of a species from every possible point of view and, with the fullest possible array of facts, to arrive at judgements on discontinuities and the probable complexity of their genetic basis. In spite of the difference in method imposed by the difference in reproduction, corresponding levels of biological organization are thereby defined. In both cases, the level or organization includes the groups of individuals that can potentially contribute to the further evolution of the group. Differences in the sharpness of delimitation found in sexual organisms, as will now appear."
"The species may be defined as a collection of populations that actually or potentially interbreed under natural conditions. The species is therefore the basic unit within which evolution takes place. Or is it? The biological concept of species, built so painstakingly over many decades, is being questioned by modern genetic studies of populations just as the typological concept was challenged earlier by phenotypic studies. In this chapter (27), we shall first examine geographical variation in populations and then try to define the evolutionary unit. We must remember, however, that observed geographical patterns are thin slices in time through populations. Whereas geographical variation represents divergence within a population over time, genetic monitoring of populations over periods of a decade or so has shown that the time scale for some change can be extremely short, and that populations can be genetically dynamic."
"We intuitively recognize a species as a group of closely similar organisms, such as humans, horses or carrots. The scientific definition has varied historically, but one that is often cited today is 'a group of morphologically similar organisms of common ancestry that under natural conditions are potentially capable of interbreeding.' There are a number of practical problems with this definition, such as ascertaining common ancestry and determining the meaning of 'similar' and 'potentially capable of interbreeding.' But, for the moment, this definition should give some notion of what is meant by a 'species.'
Here are some recent additions to the list from Michael Gudo, a German Paleontologist and Graduate Student:
A species is »an ancestral-descendant sequence of populations evolving separately from others and with its own evolutionary role and tendencies.
Species are distinguished on phenetic differences, be they morphological, physiological, biochemical, or genetic. All characters that vary among the specimens under study are scored and a taxon-charakter matrix constructed. This matrix is then subjected to multivariate analysis in order to identify discrete clusters of individuals based on total morphological variation. The smallest clusters recognisable (either by eye or by using a cut-off level) then constitue a species.
Species are lineages which occupy an adaptive zone minimally different from that of any other lineage and which evolve separately.
Species are the smallest diagnostable cluster of self-perpetuating organisms that have unique sets of characters.
Arten haben keinen ontologischen Status. Sie besitzen nicht den Charakter von Dingen, sondern den von Klassen. (Translation from Willmann 1985)
Arten im Sinne von Klassen werden durch bestimmte Eigenschaften definiert. Sie sind Kollektive von Organismen, die die in der Definition herausgestellten Eigenschaften aufweisen ? typologische Abstracta, dienlich nur der Zuordnung einzelner Organismen, d.h. logische Konzepte.« (Willmann 1985). »Nur Arten im Sinne des Biospezies-Begrifs sind natürliche, also real-objektive Einheiten, und nur solche Einheiten sollten und können die Objekte der Biologie als eines naturwissenschaftlichen Zweiges ein.« (Willmann 1985)
"In der Neontologie besteht im wesentlichen Übereinkunft darin, daß Arten (hypothetisch-)reale Einheiten darstellen, wenn man sie als geschlossene Fortpflanzungsgesellschaften begreift. Wenn aber solche reale "Biospezies" oder "Arten im Sinne des Biospezies-Konzeptes" reale und objektiv bestehende Naturgegebenheiten sind, dann muß das entsprechende Konzept zwangsläufig auch für die Paläontologie verbindlich sein."
This one is not a species definition, but a very nice comment on how paleontologists work with species:
Obwohl Arten als die (basic units of evolution) angesehen werden, operierte man in der Paläontologie mit einem Artkonzept in einer Form, als wäre die Evolutionstheorie nie entwickelt worden (Simpson 1961: 152, Translation from Willmann 1985).
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