The Species

ByDr. Girish Chandra

 

The Species Concept

       (By Dr. Girish Chandra)

 SUMMARY

          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 are groups 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 elephus  elephus is the continental red deer and Cervus elephus scoticus is the red deer of Great  Britain

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.

 

Principles of Zoological Nomenclature

   Latin: nomen=name; calare=to call.

            Nomenclature provides names to species and higher taxa, to facilitate communication among zoologists.  According to Article 1 of the code:

            "Zoological nomenclature is the system of scientific names applied to taxonomic units of animals (taxa) known to occur in nature, whether living or extinct." The nomenclature should fulfill the following three basic requirements:

            Uniqueness: The name of a taxon is like the index number of a file. It gives immediate access to all information in literature, available about a particular taxon.  Every name must be unique because it is key to the entire literature.  Uniqueness has been achieved by adopting binominal nomenclature, as proposed by Linnaeus in the X edition of Systema Naturae in 1758.

            According to binominal nomenclature, each species name should consist of the first generic and second species name.  Species name should not duplicate under any genus, e.g. Panthera leo, Panthera tigris, Panthera pardus.  A combination of the two makes the name unique.

Universality: Scientific names should be known to all and be universally accepted.  Vernacular names would be difficult to keep track of, and scientists will have to learn names in several languages of the world.  To avoid this, zoologists have adopted by international agreement a single language, Latin, which is a dead language and therefore does not evolve and is acceptable to everybody.

            Stability:  Zoological names would lose their utility if they were changed frequently and arbitrarily. International Code of Zoological Nomenclature has been designed to bring about stability.  Taxonomists are bound to follow the rules given in the code before assigning names to taxa.  Most of the changes in names are due to taxonomists' errors.  Lot of name changing has taken place during the last 200 years.  International Code of Zoological Nomenclature safeguards against frequent name changing.

 

International Code of Zoological Nomenclature

(Adopted by the 15th International Congress of Zoology (London) and published on November 6, 1961)

 

            The object of the code is to promote stability and universality in the scientific name of animals, and to ensure that each name is unique and distinct.

            The Swedish naturalist Carl von Linne' (1707-1778), who changed his name to a binomen, Carolus Linnaeus, was the father a set of rules of nomenclature published in Critica Botanica (1737), Philosophia Botanica (1751) and in the 10th edition of Systema Naturae (1758).  The confusion that prevailed after Linnaeus was solved in the 5th International Congress of Zoology in Berlin in 1901.  The original code was, however, adopted in 1904 in the 6th International Congress of Zoology in Bern and published in 1905 in Paris as, "Regles Internationales de la Nomenclature Zoologique."

            The most recent version (a modified version of 1961 code) was published in 1964 in parallel French and English.  It was adopted by the 16th International Congress of Zoology, Washington (1963) with modifications in articles 11, 31, 39 and 60.

International Congress of Zoology is a legislative body, which adopts by voting the constitution and proposals put before it by the commission.

International Commission on Zoological Nomenclature is a judicial body elected by the International Congress of Zoology.  It is protector of the code and deals with the interpretations, disputes and implementation of the code.  Amendments have to be routed through the commission.

International Code of Zoological Nomenclature (1964) is the system of rules and recommendations authorized by the International Congress of Zoology.  The object of the code is to promote stability and universality in the scientific names of animals and to ensure that each name is unique and distinct.  Code does not restrict the freedom of taxonomic thought and action.

            Before the present code, the following codes were prevalent in Europe and U.S.A.:

1.      Strickland Code (1842) in Berlin.

2.      W.H.Dall Code (1877) in USA.

3.      Douville' Code (1881) in France.

Salient features of the "Code"

            The 1964 code consists of a Preamble, 86 Articles, 5 Appendices, a Glossary and a detailed Index, in parallel English and French.  Starting date of the code is 1st January 1758 (publication date of the10th edition of Systema naturae).

1.      Names must either be Latin or Latinized.

2.      Names of taxa higher than species should be uninominal.

3.      Name of a species is binomen.

4.      Name of a subspecies is a trinomen.

5.      Name of a subgenus is placed in parenthesis between genus and species, e.g. Xorides (Gonophonus) nigrus.

6.      Family name should end in DAE , e.g. Tipulidae.

7.      Genus name should be a noun in nominative singular or treated as such, e.g. Apis, Rana.

8.      Species name should be an adjective or noun in nominative singular agreeing in gender with the generic name, e.g. Drosophila obscura, Felis tigris etc. OR a noun standing in apposition to the generic name, e.g. Felis leo.

9.      Zoological nomenclature is independent of other systems.

10.  All names given to the species from time to time should be mentioned in synoymy.

11.  Author's name is not part of the name. It's use is optional and is suffixed, e.g. Cancer pagurus Linnaeus.

12.  Law of priority: The valid name is the oldest name published and available.

13.  Synonymy: Synonyms are different names assigned to the same taxon.  They should be mentioned along with the valid taxon, e.g. Erias vitella (=Erias fabia).

14.  Homonymy: Homonyms are identical names in spelling for different species of the same genus and for different genera of a family.  Junior homonym has to be rejected. Homonymy arises when an existing species' name is not known to the person assigning a name, or a species with identical name is transferred to the same genus.

15.  Holotype: Single specimen on which description of the species is based.  Red colored label is fixed on the specimen.

16.  Allotype: Specimen of the opposite sex to holotype. Also carries a red label.

17.  Paratype: All remaining specimens after the designation of holotype and allotype are assigned the status of paratypes.  They carry yellow labels.

18.  Syntypes: If no holotype is designated, all specimens that the author studied for the description of the species are called syntypes.

19.  Lectotype: In the absence of a holotype, one specimen from syntypes can be designated as Lectotype and rest of the specimens as Paralectotypes.

20.  Neotype: If all type-specimens are destroyed, a neotype, that fits the description very well, can be designated under exceptional circumstances.