Species, as we know them, are groups of individuals that look alike, live together, and interact with one another and with the other kinds in a similar way. This is a very vague definition of species as nature has provided so many variations in animals that it is very difficult to precisely define a species. Various concepts to explain as to what actually constitutes a species have been given from time to time. Some of them are given below.
1. Typological or morphological species concept
This is also called essentialism and was put forth by Plato and Aristotle in 350 BC, was later followed by Linnaeus and is still followed by majority of taxonomists. According to this concept universe has a limited number of types and individuals are not given any recognition. Species can be recognized by their most essential morphological characters. Therefore, species are groups of individuals that resemble each other in most essential visible morphological characters OR morphologically distinct organisms constitute a species. This concept emphasizes only the appearance of the animals.
2. Nominalistic species concept
This concept was put forward by Buffon and Lamarck in mid 18th century in France. According to this concept, only individuals exist and species are man-made aberrations or abstractions. Followers of this concept treat species as individuals on higher plane. Numerical taxonomists of today essentially follow this concept by taking all characters of individuals and feeding them into a computer to get a classification. Such classifications are likely to be erroneous as they give equal importance to all characters.
3. Genetic species concept
Simpson (1961) advocated that genetically identical individuals should be called Biospecies. M. Florkin (1964) gave a definition as follows: “Species are groups of individuals with more or less similar combinations of sequences of purine and pyramidine bases in their macromolecules of DNA and with a system of operators and repressors leading to the biosynthesis of similar amino acid sequences.”
This definition is too complicated to be followed. Moreover, no two individuals (with the possible exception of monozygotic twins) are genetically identical.
4. Evolutionary species concept
This concept takes into account the lineage of a species. The following definitions have been given:
Meglitsch (1954) said, “Natural population evolving as a unit in actuality and retaining this capacity in case artificial barriers are removed.”
Simpson (1961) gave this definition-“Species is a lineage evolving separately from others and with its own unitary evolutionary role and tendencies.”
Wiley (1978): “Species is a single lineage of ancestral descendant populations of organisms which maintain its own evolutionary tendencies and historical fate.”
Biologically the above concept is acceptable but it is difficult to find evidence of lineage for lack of fossil evidence in most cases.
5. Biological species concept
First proposed by K. Jordan (1905), this concept combines elements of typological, nominalistic and genetic aspects and lays emphasis on the reproductive isolation. Ernst Mayr (1969) proposed the following definition, which is now universally accepted, “Species aregroups of actually or potentially interbreeding natural populations that are reproductively isolated from other such groups.”
According to this concept, a species forms a reproductive community, an ecological unit and a genetic unit, which share a gene pool, and protect their gene pool from other species, by isolating mechanisms. However, there are several difficulties in applying this concept, three of them are important:
1. Insufficient information
2. Uniparental reproduction
3. Evolutionary intermediaries
All the species concepts discussed above have some difficulty or the other in applying them in practice. Therefore, now taxonomists apply typological concept in everyday practice in identifying and classifying animals but biological concept gets priority.
The Supra- and Infraspecific categories
Polytypic species (Rassenkreis): Rensch (1929) proposed the German term Rassenkreis (which literally means “Circle of races” for those species, which have two or more geographically isolated interbreeding populations.
Superspecies (Artenkreis): This German term literally means “Circle of species”. They are monophyletic groups of closely related and largely or entirely allopatric species, distributed over a wide range. Mayr (1931) introduced the term “Superspecies” for such closely related but distinct species that have apperently evolved from a common ancestor. Examples of superspecies are paradise magpies (Astrapia) in the mountains of New Guinea, e.g. Astrapia nigra, A. splendidissima, A. mayeri, A. rothschildi and A. stephaniae. Species of Anopheles maculipennis complex in Europe are also considered superspecies.
Sibling species: They are true sympatric species that are morphologically identical or nearly so but are reproductively isolated. For example, Drosophila pseudo-obscura and D. persimilis are sibling species.
Subspecies: This term was used by Schlegel in 19th century as a replacement for variety. Subspecies is a geographically isolated (allopatric) population of a species that differs morphologically from other populations of the species but does not exhibit reproductive isolatA trinomen is used to designate a subspecies. For example, Cervus elephuselephus is the continental red deer and Cervus elephus scoticus is the red deer of Great Britain
Variety: This was the only subdivision of species recognized by Linnaeus, who used it to identify any variation from the “type” and for individual variants or for anything that did not fit in a fixed pattern of a species. The term is no longer used and is not recognized as a valid taxonomic category.
Deme: This is a minimal interbreeding local population unit of a species which share a single gene pool. Demes live in most suitable areas that are separated by regions of unsuitable conditions and are affected by the gene flow from adjacent areas.
Cline: This is an evolutionary concept proposed by J.H. Huxley (1939) and is defined as a gradation in measurable characters. Cline is formed by a series of contiguous populations in which a given character changes gradually. Two opposite ends of the series may be very different but difficult to be called subspecies due to the absence of geographical isolation. The terms geocline (geographic), ecocline (ecological) and chronocline (succession lines) are also used.
Race: This is not a recognized taxonomic category. They are local populations of a species which are affected by the local conditions and therefore develop morphological differences. For example human races.
Hypodigm: All specimens personally known to taxonomists at the time of describing species and used by him collectively as a sample on which his inferences are based.