BIO-DIVERSITY: Taxonomic systems

The initiation for evolving taxonomic systems was provided by Aristotle (384-322 BC). He emphasized that animals can be classified according to their way of living, actions, habits and body parts. He observed insects, fishes, birds and whales. The insect orders like Coleoptera, Diptera were created by him. Due to his contributions, he is considered as the ‘father of biological classification.

For modern taxonomy, the first work was carried out by John Ray (1627 - 1705) of England. His most interesting systematic work ‘Synopsis Methodica Animalium Quadrupedum et Serpentini Generis'’ was published in 1693. He divided animals into those with blood and those without blood. He also classified animals based on gills, lungs, claws, teeth and other structures. He provided the first good definition of the species as ‘a reproducing unit’.

The great Swedish naturalist Linnaeus (Caroli Linnaei) (1707 - 1778) exerted an important influence on further advancement in taxonomy. Hence he has been called the father of taxonomy. In 1758 he published his famous book, systema naturae. He first introduced the hierarchic system, both in animal and plant kingdoms. He followed four categories namely class, order, genus, species for the animal world. His greatest contribution to taxonomy was the use of binomial nomenclature for all species of animals and plants.

Michael Adamson (1727 - 1806), a French botanist, stressed that classification should be based on as many characters as possible. His concept helped to develop a new type of taxonomy called ‘Numerical Taxonomy’.

Lamarck (1744 - 1829) made the first attempt to improve Linnaen system. He published seven volumes of his ‘Histoire Naturelle des Animaux sans Vertebres He arranged animals according to evolution. He displayed the groups of animals in the form of a branching tree. It was the beginning of the use of phylogeny in systematics.

Cuvier (1769 - 1832) insisted that extinct fossil forms should be included in the table of classification. He divided animals into four branches. They are Vertebrata-fishes to mammals, Mollusca-mollusca and barnacles, Articulata-annelids, crustaceans, insects and spiders and Radiata- echinoderms, nematodes and coelenterates.

Charles Darwin in 1859, published his famous work ‘Origin of species’. The new evolutionary concept of Darwin had an immediate acceptance among biologists. Due to the influence of evolutionary ideas, taxonomy was studied as an important evidence in favour of evolution. The taxonomists were encouraged to learn that evolution theory of Darwin gave meaning to their classifying activities. A large number of species were discovered and described.

The development of modern taxonomy started during 1930s. During this period taxonomy was based on population studies E. Mayr (1942) considered species as “groups of interbreeding natural popu­lations”. His book ‘New Systematics’ became a landmark in the history of taxonomy. The taxonomists were forced to accept species as a ‘population’. Hence the taxonomist started moving from the laboratory to the field. Morphological characters were studied along with other characters as behaviour, sound, ecology, genetics, zoogeography, physiology and biochemistry. Thus taxonomy was transformed into ‘biological taxonomy’.

A- Introduction to taxa and species

While grouping or arranging the organisms, a biologist faces three scientific ideas, namely taxonomy, systematics and classification. These disciplines though appear similar have slight deviations in their meaning.

The term taxonomy is a Greek word. Its components are taxis and nomos. While taxis mean arrangement, nomos means law. Thus taxonomy is defined as the “theory and practice of classifying organisms” (E. Mayr 1966).

The term systematics originates from the Greek word systema. It means ‘placing together’. Thus systematics means classification of living things in accordance with their natural relationships. G.G Simpson (1961) defines systematics as follows “Systematics is the scientific study of the kinds and diversity of organisms and of any and all relationships among them".

The term classification in meaning partly overlaps with taxonomy. However it simply means the activity of classifying. Thus according to Simpson “Zoological classification is the ordering of animals into groups on the basis of their relationships".

A certain amount of overlap in meaning between the terms systamatics, taxonomy and classification is unavoidable.

Taxon.

Based on specific characteristics, animals are grouped in various cat­egories. These categories are otherwise called taxa (singular: taxon). “A taxon is a taxonomic group of any rank that is sufficiently distinct to be worthy of being assigned to a definite category”.

The several taxa in animal taxonomy are the Phylum, Class, Order, Family, Genus and Species. This arrangement from Phylum to Species is designated as the hierarchic system of classification. In this system each taxon is based on specific characters of a group of organisms. Eventhough such an arrangement appears to be man-made, each taxon is a natural assem­blage. However, human error in identification and grouping may happen.

The taxon, ‘Phylum’ is the largest group. There are several such Phyla constituting the animal kingdom. Members of a Phylum are recognized by certain distinctive features as shown below.

          Characters                               Phylum

Single celled animals -                        Protozoa

Pore bearers -                                       Porifera

Common body cavity

and digestive cavity -                           Coelenterata

Flatworms -                                          Platyhelminthes

Thread-like worms -                             Nematoda

Metamerically segmented animal -      Annelida

Having jointed legs -                            Arthropoda

Soft bodied -                                        Mollusca

Spiny skinned -                                   Echinodermata

Having notochord -                             Chordata

Apart from one specific character, the members of the Phylum may also show other common characters. Since a Phylum comprises enormous varieties of animals, it is further subdivided as given below

Phylum                Phylum                Phylum

          Subphylum                                              Superclass

                       Classes       OR       Classes             OR          Classes

A Class is the next level in the hierarchy. There are only few Classes in a Phylum. The members of each Class are identified by some specific char­acter. Thus for example the Phylum: Protozoa comprises four Classes as follows. 

Class                                       Character

Rhizopoda                              with root like pseudopodia

Ciliata                                     having cilia

Flagellata                                having flagellum

Sporozoa                                 producing spores

Class                                                  Class

          Subclass                                                    Orders 

                         Superorders                         or

                                          Orders

An Order is another level in the taxonomic hierarchy. It is marked by some specific feature. A Class may have several Orders. For example, the Class: Insecta is subdivided into nearly 29 Orders. Each Order is identified by a specific character.

Order                  Character                         Example

Aptera                No wing                           Lepisma

Coleoptera         Horny wings                    Beetles

Lepidoptera       Scaly wings                     Butterflies

Diptera              Two winged                    Mosquitoes

Hymenoptera    Membranous wings        Wasps.

The Order is subdivided into Families

Order                                                      Order

               Superfamily                or                        Families 

                         Families

Each Family will contain several Genera (singular: Genus). Each Genus again is subdivided into Species.

In this hierarchy, the Species is considered as the most important taxon. A Species represents a natural unit. All other taxa remain arbitrary and are subjected to revision. A Species is considered a reality. It is the fundamental unit in taxonomy. Evolution basically operates at the Species level only. Hence the concept of Species has received much attention.

Concept of Species

Initially the Species was considered as a group of organisms showing similar or specific characters. However modern workers have identified three main concepts regarding Species.

          1-Typological Species concept - This concept has its beginning from the essentialism concept of Aristotle. According to this concept a Species is recognized by its essential characters expressed in morphology

          2-Nominalistic Species concept - According to this concept Species are man made ideas. Nature produces individuals and not Species. Thus a Species is considered as a mental concept.

          3-Biological Species concept - According to this concept, “Species are groups of interbreeding natural populations that are reproductively isolated from other such groups”. This concept is mostly accepted by present day taxonomists.

 B- Methods of taxonomy

Phonetic method or Numerical taxonomy

This method involves clustering or grouping of individuals of a taxon or several taxa. Based on overall similarity, identifications are being made. The desired size of the clusters or groupings is called the operational taxonomic unit (OUT).

The identification method involves measurement of taxon to taxon similarity or dissimilarity. It is measured using a scale of 0 to 1. ‘1’represents perfect identity. ‘-1’designates dissimilarity between taxa. In this method enormous amount of data are collected for related groups. Analyses are made, using statistical tools and computers.

 C- Cytotaxonomy

The characterization and identification of a cell’s complete chromosome set is referred to as karyotyping. It is the first stage in the process of using chromosomes in taxonomy.

Karyotypes within interbreeding populations of a species are usually constant. Between species there may be variation in chromosome number and size. Final stages of chromosomal aberrations such as inversions and translocations can give clues regarding intermediary stages.

D- Chemotaxonomy

Chemotaxonomy refers to the use of information about small molecules produced by the action of enzymes. Protein fractions in electrophoretic techniques, identification of amino acids in chromatography, prevalence of isoenzymes in tissue materials are all tools employed in chemotaxonomy. The occurrence of specific pheromones, colour pigments, toxins also help as keys in taxonomy.   

 F- Palaeotaxonomy

This method depends on identification and dating of fossils. Availability of a good complete fossil provides better chance for identification. In several fossils, their sections taken through laborious processes have provided the identification features.

The fossils are normally studied along with other accompanying fossils, its geographic location and other factors. Even though it is possible to assign a fossil to a genus or other higher level, fixing the species is not always possible.

 G- Nomenclature methods

Nomenclature forms the basis by which scientists can name and cross refer to organisms. It is an integral part of taxonomy. In fact, modern taxonomy started in 1753 with the publication of first part of Systema by Linnaeus. According to Linnaeus a Species is specified by the combination of both its specific and generic names. Since it requires two names, it is referred to as the binomial system. This system is now firmly established in Biology.

In modern times International Commissions are responsible for naming each major group of organisms. There are several such commissions. These commissions authorize the usage of scientific names in biology. Naming of animals is monitored by International Code of Zoological Nomenclature (ICZN) (International Commission of Zoological Nomenclature, 1985).

The rules are set out in the ‘codes’. The codes are modified by occasional science congresses. 

Basic principles of nomenclature

          1- Providing stability in the naming and classification of organisms is emphasized. Any taxon must have only one correct name

          2-If two or more names are already in use the correct name will be the one that was published earlier. This system is referred to as the law of priority.

          3- If two or more workers at one particular time describe the same organism using different names, it results in synonyms. However only one name will be held as a valid name. The validity is provided to the senior synonym.(law of priority)

          4- When names referring to two separate taxa of the same nomenclatural level are spelt the same, the two names are called homonyms. This situation arises when two separate authors used the same name to refer to two different taxa. This condition is called homonymy. In this situation the junior name is invalid and a new replacement has to be proposed.

          5- A material on which an original description is based gets a special status. It will form the basis for any future identity of a taxon. This idea is called the type concept. Thus the concept of a genus and species are fixed by their type genus or type species.

          6- Names that were used prior to those included by Linnaeus in the “Systema Naturae”, tenth edition, 1758 are not recognized.

          7- Scientific names must be either Latin or latinized. The name should be mentioned in italics.

          8- The genus name should be a single word beginning with a capital letter.

          9- The species name should be a single or compound word beginning with a small letter.

 K- Identification keys

Identification of animals is an integral part of taxonomy. Identification could be made through literature, keys, pictures and comparison with type specimens. Of these, the most commonly used method is, using of keys.

A key is essentially printed information or a computer software package. The construction of the key is an important job of a systematist.

A good key is strictly dichotomous and not having more than two alternatives at any point. The language of a key is telegraphic.

The key may be either bracketed or indented. In a bracketed key alternative contrastive characters are used for identification. The number on the right side indicates the next alternative character for consideration.

In an indented key a series of choices are provided for identifying a taxon. The user should choose from among the choices.

The following examples provide the keys for identification four species of frogs in Tamil Nadu, namely Rana hexadactyla, R. tigrina, R. cyanophlictis and R. limnochoris .     

The Bracketed key (Genus: Rana)

(1) Large size, snout - vent 100 - 200mm .......3

(1) Small size, snout to vent less than 100 mm ........2

(2) Pointed snout ...................  .......R. limnochoris

(2) Obtusely pointed snout .........  ...R. hexadactyla

 (3) 4^th toe longer than others ...........R. tigrina

  (3) 4^th toe not longer..................... R. cyanophlictis

The Indented key (Genus: Rana)

Large sized body

Skin smooth.................... R.hexadactyla

Skin with folds................. R. tigrina

Small size

Blunt snout...................... R. cyanophlictis

Pointed or round snout......... R. limnochoris

References

T. Sargunam Stephen, Biology (Zoology). First Edition – 2005, Government of Tamilnadu.