In this article we will discuss about Taxonomy:- 1. Meaning of Taxonomy 2. Levels of Taxonomy 3. Periods 4. Role.
Meaning of Taxonomy:
The term taxonomy is derived from the Greek words (‘taxis’ means arrangement and ‘nomos’ means law). The term first proposed by the Swiss originated botanist Augustin Pyramus de Candolle in 1813 for the plant classification. He used the term in his famous book—Theory elementaire de la botanique (Elementary Theory of Botany). So taxonomy is the arrangement of the plants and animals on the basis of some laws.
Simpson (1961) has defined taxonomy as the theoretical study of classification including its bases, principles, procedures and rules. Ernst Mayr also defines taxonomy as the theory and practice of classifying organisms. So the science of classification is known as taxonomy.
Christoffersen (1995) has defined taxonomy as “the practice of recognising, naming, and ordering taxa into a system of words consistent with any kind of relationships among taxa that the investigator has discovered in nature”.
The process of taxonomy involves two distinct steps:
(i) Correct recognition and definition of the organisms and their relationships and
(ii) Application of suitable designations for the organisms and to different groups which include them.
The former is called classification which includes study of characters and grouping of individuals while the latter is termed as nomenclature.
Levels of Taxonomy:
There are three levels of taxonomy corresponding with three periods of taxonomy:
(i) Alpha taxonomy:
The level of taxonomy by which species are characterised and naming of the species is done.
(ii) Beta taxonomy:
The level of taxonomy by which the arrangement of species in their natural system of categories is made.
(iii) Gamma taxonomy:
The level of taxonomy which deals with the intra specific variations and evolutionary sequence and also a causal interpretation of organic diversity.
Mayr and Ashlock (1991) have divided the taxonomy into two levels:
The level which deals only the problems related to species.
The level which deals with the problems and principles of higher taxa (from subgenus and above) only.
Mayr and Ashlock (1991) recognise three schools of macro-taxonomy such as:
(i) Phenetics (or Numerical taxonomy),
(ii) Cladistics (Phylogenetic systematics) and
(iii) Evolutionary taxonomy (or Evolutionary systematics).
(i) Phenetics (or Numerical taxonomy):
It is an attempt to classify organisms based on overall characteristics rather than on evolution from a common ancestor. Phenetics helps to draw phyletic lineage (relationship) on the basis of similarities and dissimilarities. Pheneticists do not rely upon primitive (plesiomorphic) and derived (apomorphic) characters.
(ii) Cladistics (Phylogenetic systematics):
The term cladistics refers to taxonomy by which the organisms are ranked and classified according to the “recency of common descent”. The categorical status of the animal according to this theory depends upon the position of branching points on the phylogenetic tree.
Taxa based on entirely shared derived (synapomorphic) characters originated from a common ancestor. It is a method of phylogenetic analysis to identify monophyletic lineages or clades. Julian Huxley used the term ‘clade’ in 1958 and Cain and Harrison introduced the term ‘cladistic’ in 1960.
(iii) Evolutionary taxonomy (or Evolutionary systematics):
The whole concept is based on Darwinism. According to this concept each valid taxon is derived from a common ancestor but the common characteristic features of a realm of the biological world do not always include a common ancestry.
The evolutionary or Darwinian classification is a branch of biological classification in which the organisms are classified using a combination of phylogenetic relationship and overall similarity. This type of taxonomy considers taxa more important rather than single species.
Usually taxonomists agree to divide the taxonomy into two types:
(i) Classical taxonomy and
(ii) Neo-taxonomy or experimental taxonomy.
(i) Classical taxonomy:
The oldest form of taxonomy is called classical taxonomy or orthodox taxonomy. It is related to the description, naming and classification of the animals and plants based on the morphological features (related to external features including genitalia, anatomy, embryology and karyotype; etc.).
(ii) Experimental taxonomy or neo-taxonomy:
It is related to the genetical studies based on a common gene pool for a taxon and become helpful to distinguish two different taxa. Some modern procedures are applied to collect the data for morphology. The use of electron arid scanning electron microscope in different groups of invertebrates such as protozoans, helminthes, arthropods to study the fine structures that become helpful in morphotaxonomy.
The closely related two current aspects in taxonomy are taken into consideration, such as:
(i) Biochemical taxonomy and
(ii) Cytological taxonomy.
(i) Biochemical taxonomy:
It deals with taxonomic characters obtained from chemical analysis of enzymes, hormones, proteins with peptides, nucleic acids, amino acids and sugars.
The amino acid sequences of proteins become different in the different organisms and become helpful to distinguish the different species. Numerous techniques are applied to the study of constituent bio-molecules such as enzymes, hormones, nucleic acids, amino acids, etc. and help in the systematics.
To study the chemical substances, various procedures such as immunological, chromatography (paper chromatophy and column chromatography), electrophoretic method are applied in the study of systematics. The immunological data are used to distinguish two different taxa. The blood group genes are applied in the classification of pigeons and primates.
Chromatography is a kind of different techniques for the separation of a complex liquid mixture such as biological fluids (e.g., amino acids, steroid, carbohydrate, etc.) that pass through a column of adsorbing material (e.g., paper, magnesia) the components of the mixtures are adsorbed in separate layers in the column.
This technique is applied in various groups of arthropods, snails and the data is very much helpful in animal systematics.
It deals with taxonomic characters obtained from cytological studies. Cytotaxonomy is a branch of taxonomy dealing with the relationships and classification of organisms based on the structure, number of chromosomes. The position of the centromere is an important feature of chromosome structure which helps in taxonomic studies.
Periods of Taxonomy:
(i) First period:
This period may be extended from the time of Aristotle (384-322 B.C.) to Linnaeus (1707-1778). In this period Linnaeus strongly introduced binominal nomenclature for plants and animals and followed Aristotlian and Democritus principle in classification of animals. He also first introduced the hierarchic system of classification both in plants and animals following class, order, genus and species categories.
(ii) Second period:
In this period the evolutionary classification was introduced by Charles Robert Darwin (1809-82) and variation among the organisms is the main force in evolution which was discussed extensively. Darwin published his famous book “On the Origin of Species by Means of Natural Selection” in 1859.
In his book the theory of evolution by natural selection was his own creation although based on the work of Lamarck, Cuvier (1768-1832) and Erasmus Darwin (1731-1802), the grandfather of Charles Darwin. This theory helped a lot to the systematic zoology. E. Darwin’s book Zoonomia (1794) presented the laws of organic life. He suggested the struggle for existence in Zoonomia which was elaborated by Charles Darwin.
(iii) Third period:
This period includes the development of modern taxonomy which started about 1930. The study of genetics and population biology was started with typical taxonomy.
This period is remarkable with the publication of New Systematics by J. S. Huxley in 1940, Intraspecific variations were studied and the science of population genetics was started in 1908 by G. H. Hardy and W. Weinberg who independently discovered a principle concerned with the frequency of genes (allels) in a population in the light of evolutionary theory.
Role of Taxonomy:
1. It helps to recognise the exact species among a lot of different specimens.
2. It helps to describe the species in detail and groupings are done among the different species on the basis of resemblances and relationships.
3. It provides the international recognisation of the published species.
4. It provides most information that are obtained from different groups of organisms and are analysed critically to understand the causes of evolution that may lead to the formation of new species.
5. The data obtained from the different taxa help to construct the phylogeny.