Concept of the species
The species is a vague concept that there are a multitude of definitions in the literature. In its most simplistic, the concept of the species can distinguish different types of living organisms. Different definitions identify more precisely the criteria for distinguishing the species.
The definition most commonly cited is that of the biological concept of the case statement by Ernst Mayr (1942) "Species are groups of population natural, actually or potentially interbreeding, which are genetically isolated from other similar groups In this definition, he then added that this case will lead to viable and fertile offspring Thus, the species is the largest unit of population in which the gene flow is possible in natural conditions, individuals of a species is genetically isolated from other sets equivalent in terms of reproduction.
The biological concept of species is therefore based entirely on the reproductive isolation (or genetic isolation), that is to say, all biological factors (barriers) that would prevent members of two distinct species of generate viable and fertile offspring. According Theodosius Dobzhansky, it is possible to distinguish the barriers involved before the mating or fertilization and after intervening barriers Prezygotic barriers will prevent the coupling between two individuals of different species, or the fertilization of ova if mating takes place. If fertilization takes place after all, postzygotic barriers will prevent the zygote hybrid to become an adult viable and fertile. It is this reproductive isolation which will prevent the gene pool of each species to exchange freely with others and thus induce conservation of characters unique to each species
For some species, reproductive isolation appears so evident (between an animal and a plant for example) but in the case of closely related species, the barriers are much less clear. It is important to note that reproduction between individuals of the same species should be possible under natural conditions and that the offspring must be viable and fertile. For example, the horse and the donkey are two species interbreeding but their hybrids (mule, hinny) seldom are the offspring is not fruitful, there are indeed two different species. Similarly, some species can be artificially crossed but do not occur together in the natural environment.
Nevertheless, the biological concept of species has certain limitations. The reproductive isolation can not be determined in the case of fossil organisms and asexual (eg bacteria). Moreover, it is difficult to establish with certainty the ability of an individual to mate with other individuals. In many groups of plants (birch, oak, willow ...), there are many species that cross freely in nature without taxonomists do not consider a single case so far Many other definitions have therefore also underway to override the limits of the biological concept of species.
The concept of morphological species concept is the most commonly used in practice. It is to identify a species by its structural and morphological characteristics distinctive The advantage of this concept is that it is applicable both in asexual and sexed bodies and does not need to know the extent of gene flow. However, the major drawback of this concept lies in the subjectivity of the definition of species, which can lead to disagreements about the criteria used to define a species
Another definition is based on the notion of similarity (or conversely the degree of difference), a concept still used in paleontology, where there is no other option. Some authors even use these two principles to define the species.
Studying the DNA to search for similarities not visible directly on the physical (phenotype). But the quantitative criterion (number of identical genes) masks the qualitative criterion, by definition unmeasurable. Thus, the classification of type Orchids Ophrys shows a large number of species, apparently different (hence the perspective phenotype) while their genotypes were very close. The criterion of genetic similarity is used in bacteria (in addition to phenotypic similarities). It separates the species so that intraspecific genetic variation is much lower than interspecific variation.
The biological species is now most often defined as a reproductive community populations. If this definition is well suited to the animal kingdom, it is less obvious in the plant kingdom, which occur frequently hybridizations.
There are also species concept ecological link to the concept of niche ecological. A species is supposed to occupy a specific ecological niche. That is to associate a species of special living conditions. The definition proposed by Hutchinson soufffre problems collecting niche (several species whose niches are very close to see indiscernible).