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

Apart from one specific character, the members of the Phylum may also show other common characters. Since a Phylum comprises enormous varieties of animals.

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.

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.

Each Class may further be divided into 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.

The Order is subdivided into 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 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.

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