The following points highlight the four important groups of species concept. The groups are: 1. Typological Species Concept 2. Nominalistic Species Concept 3. Evolutionary Species Concept 4. Biological Species Concept.
Group # 1. Typological Species Concept:
This concept says that the observed diversity of the universe reflects the existence of a limited number of underlying “universals” or types. Individuals are considered to be merely expressions of the same type. When two individuals or groups of individuals have sufficiently different characters then only they should be considered as different species.
Variation results in the revealing of characters which are already present but not expressed in each species. Thus, variation according to the typological species concept is considered to be an irrelevant phenomenon.
This concept dates back 2,300 years to the philosophies of Plato and Aristotle, and was the species concept of Linnaeus and his followers. Species was supposed to be a fixed or static unit which did not change and, as such, existed permanently and forever.
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Since this philosophical tradition is sometimes referred to as essentialism, the typological definition is also sometimes called the essentialist species concept. The species can be recognised by their essential natures or essential characters which are expressed in accordance with their morphology.
It is, therefore, also called the morphological species concept. Taxonomists almost unanimously accepted the essentialist species concept up to the early post-Linnaean period.
It thus includes four postulates:
(i) Species are similar individuals sharing the same essence.
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(ii) Each species is separated from all others by a sharp discontinuity.
(iii) Each species is completely constant through time.
(iv) Strict limits are present to the possible variation within any one species.
Two practical reasons exist for the present universal rejection of this concept:
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1. Individuals are frequently found in nature that are clearly conspecific with other individuals in spite of striking differences in structure owing to sexual dimorphism, age differences, polymorphism and other terms of individual variation. These were often described originally as different species.
For example:
(i) The male and female of river duck, the mallard, were originally placed under separate species. The males were described as Anas boscas and the females as Anas platyrhynchos.
(ii) In many groups of birds (humming bird, pea-hen etc.) females differ more from the males of their own species than from the females of other related species.
(iii) Larval stages of several chordates and invertebrates are markedly different from their parents or adult stage and, thus, were very often considered as separate species.
(iv) In case of the deep sea fishes the males are dwarf and attached to the body of the females. So they were considered as separate species.
Thus, in the above cases, they should be deprived of their separate species status, regardless of their degree of morphological differences, as soon as they are found to be numbers of the same breeding population. Different phases that belong to the same population cannot be considered as different species.
2. Sibling species differ hardly at all morphologically, yet are good biological species. Degree of difference is not the decisive criterion in the ranking of taxa as species. The typological species concept is still defended by a few writers.
In situations where there is a lack of biological information, a taxonomist may be forced to recognise a species provisionally on the basis of morphological evidences, but such species are subject to later reconsideration.
Group # 2. Nominalistic Species Concept:
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Nominalistic species concept is that of Occam and his followers, who believed that nature produces individuals only. Species are man’s own creation and have no actual existence in nature. They are mental concepts and nothing more. Species have been invented so that we may refer to great numbers of individuals collectively. This concept was popular in France during the 18th century.
The drawbacks of this concept are:
(i) No naturalist whether a primitive native or a trained population geneticist – can agree that species are man-made, when it is an established fact that they are the products of evolution.
(ii) Nominalists misinterpreted the relation between similarity and relationship. The members of any species are not grouped together as they are similar (as claimed by these workers), rather they are similar to each other because of common heritage. It is just like when two brothers are identical twins not due to their similarity but rather due to both being derived from a single zygote.
Group # 3. Evolutionary Species Concept:
The shortcomings of Biological species concept (in uniparental organisms where interbreeding fails), had led Meglitsch (1954), Simpson (1961), Grant (1971) and other authors – particularly paleontologists – in formulating the evolutionary species concept.
Simpson (1961) defined it as “an evolutionary species is a lineage (an ancestral- descendant sequence of populations) evolving separately from others and with its own unitary evolutionary role and tendencies”. Willey (1981), on the other hand, believed that each species is an internally similar part of a phylogenetic tree.
The drawbacks of this concept are:
(i) This definition is of a phyletic lineage and not of a species. It side-stepped the crucial role of why phyletic lines do not interbreed with each other.
(ii) This concept ignores the core of the species problem as to the causation and maintenance of discontinuities between contemporary species.
(iii) This concept has failed to solve the problem of how to deal with the relationship of descendant populations in a single lineage.
Group # 4. Biological Species Concept:
The biological species concept is also known as the Newer Species Concept, because it was accepted in the latter half of the nineteenth century after Darwin’s “Origin of Species” was published (in 1859) and also due to the fact that organic evolution was established.
It was after 1750 that an entirely new species concept began to emerge. But it was in 1905 that K. Jordan first clearly formulated the concept in all of its consequences. This concept combined the thoughts of the typological and nominalistic concepts by stating that the species have independent reality and are typified by the statistics of populations of individuals.
It, however, differs from both by stressing the populational aspect and genetic cohesion of the species and also by pointing out that species receives its reality from the historically evolved, shared information content of its gene pool.
Thus, the members of a species show the following properties:
(i) A reproductive community:
The member of an animal species recognises each other as potential mates and seek each other for the purpose of reproduction.
(ii) An ecological unit:
The species members form an ecological unit which, regardless of the individuals composing it, interacts as a unit with other species with which it shares the environment.
(iii) A genetic unit:
The species consists of a large, inter-communicating gene pool, whereas the individual is merely a temporary vessel holding a small portion of the contents of the gene pool for a short period of time.
These three properties raise the species above the typological interpretation of a “class of objects”. Thus, from this theoretical species concept, the species definition which results is — A species is a group of interbreeding natural population that is reproductively isolated from other such groups.
This species concept is called biological not because it deals with biological taxa, but because the definition itself is biological. It utilizes criteria that are meaningless as far as the inanimate world is concerned.
Biologically, a species is a potential gene pool. It is a Mendelian population which has its own devices, that is, isolating mechanisms which protect it against harmful gene flow from other gene pools. Gene of the same gene pool form harmonious combinations because they have become co-adopted by natural selection. Mixing the genes of two different species lead to high frequency of disharmonious gene combinations.
Mechanisms that prevent this are favoured by selection. Thus, the word species in biology is a relational term. ‘A’ is a species in relation to ‘B’ and ‘C’ because it is reproductively isolated from them. Since evolution is a regular process, the species is an arbitrary division of the continuous and ever-changing series of individuals in nature. Therefore, species is dynamic and multidimensional in nature.
The biological species concept has been able to solve the paradox caused by the conflict between the fixity of species of the naturalist and the fluidity of the species of the evolutionist. It was this conflict that made Linnaeus deny evolution and Darwin the reality of species.
The biological species concept combines the discreteness of the local species at a given time with an evolutionary potential for continuing change. The biological species concept has its importance in the fact that it is employed in the largest number of biological disciplines — ecology, physiology, behaviour biology etc.
Intraspecific categories designate groupings of population’s within species. Normally, the species is the lowest category used in routine taxonomy. The higher categories are groupings of species. In view of this key position of the species and the fact that in nature one encounters individuals and phena, the assigning of individuals and phena to species taxa is one of the key problems of taxonomy.