Taxonomic systems of Biodiversity

The initiation for eolving 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.

Taxonomy system


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’.

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 means 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 charateristics, 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 recognised 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


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

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.

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 recognised 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. 
Methods of taxonomy 

Phenetic 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 (OTU). 

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.

(Meyda Azzahra)
Source: global science