Biological Classification – Systems of Classification

Biological Classification – Systems of Classification

Biological classification is the scientific procedure of arrangement of living organism into groups. It is done on the basis of their similarities and dissimilarities and placing the groups in a hierarchy of categories.

The first pioneer work on biological classification was done by Linnaeus. He classified living organisms into the two kingdoms, i.e Plantae and Animalia. But this classification system proved to be inadequate as there were many organisms that did not fall into any of the categories. Afterwards, the classification system for living organisms has undergone several changes. However, the place of plant and animal kingdoms remains consistent under all different systems.

Systematics / Taxonomy

Systematics is the study of identification, nomenclature, classification, and inter-relationships amongst living beings.The term systematics was coined by Linnaeus (father of taxonomy).

Taxonomy is a branch of biology that is connected with identification, nomenclature, and classification of organisms.The term taxonomy was coined by de Candolle (1813).


  • Atharvaveda (about 1500 B.C.) mentions about 2000 medicinal plants.
  • Aristotle (father of zoology) described a number of animals and studied their developmental stages. He wrote Historia Animalium (first book of Zoology)
  • Theophrastus (370-284) is known as the father of botany. He wrote Historia Plantarum. (first book of botany)
  • Pliny the Elder (23-79 AD) has described 2000 plants and animals.
  • Kasper Bauhin (1561-1624) was the first to give two-word names to organisms.
  • John Ray (1627-1705) introduced the word species.
  • Carolus Linnaeus (1707-1778) is known as the father of taxonomy. He gave the scientific system of binomial nomenclature (Philosophia Botanica), described 5900 species of plants (Species Plantarum) and 4326 species of animals (Systema Naturae).
  • Lamarck introduced the terms phylogeny the concept of evolution of dynamic nature of species in his book Philosophie Zoologique
  • Ernst Haeckel (1866) established the concept of phylogeny or developmental history of a race.
  • First natural system of plant classification was given by Bentham and Hooker (Genera Plantarum, 1862-1883).
  • First phylogenetic system of plant classification was given by Engler and Prantl (1887-1899) in their work Die
  • Naturlichen Pflanzenfamilien through its outline was proposed by Eichler (1883).

Classical Systematics or Old Systematics

It was first employed by Plato and Aristotle. Linnaeus and his followers also employed the same.

The main features are :

  1. Species is center stage of classical systematics. No work or very little work is done on intraspecific categories like sub-species, varieties, and population.
  2. A purely morphological definition of species is employed.
  3.  Most of the species of classical systematics are known by a single or at the most a few specimens.
  4. In classical systematics, the need to study large population was never felt. Instead, one or a few individuals were through to give information about all the traits of the species. It is known as the typological concept.
  5. The species is static and remains fixed. Variations are caused by imperfect expression of traits.

With knowledge about the origin of variations, a new concept came into existence. It is called the nominalistic concept of species. According to it, species have no existence. Only the individuals do so.


  • Taxonomy based on all available information and attempting to classify organisms, according to their origin, evolution and variations are called classical taxonomy.
  • A taxonomist engaged in studying origin, evolution, variations, and classification of organisms is called classical taxonomist.


Binomial Nomenclature

  • The system of providing distinct proper scientific names to organisms with each name consisting of two words, generic and specific.
  • Binomial nomenclature was developed by Linnaeus (Carl von Linne, a Swedish biologist), who gave certain principles (called Linnaean principles) for this in his book Philosophia Botanica (1751).
  • The standard references recognized for this are Species Plantarum (1753) and tenth edition of Systema Naturae (1758).
  • Binomial nomenclature is sometimes enlarged into trinomial nomenclature when the name of subspecies or variety is also incorporated, e.g., Brassica campestris botrytis.
  • Scientific names have been standardized through International Code of Botanical Nomenclature (ICBN, 1961) and International Code of Zoological Nomenclature (ICZN, 1964) and International Code of Bacterial Nomenclature (ICNB.)

Standardisation of Nomenclature: Attempts were made at first Botanical Congress held in August 1867 in Paris.

Type Specification

  • Holotype: Nomenclature type.
  • Isotype: Duplicate of holotype, as another branch of the same tree.
  • Paratype: Any other specimen described along with holotype.
  • Syntype: Any of two or more specimens cited by the author when there is no holotype.
  • Lectotype: Specimen selected from original material to serve as the nomenclatural type where there is no holotype.
  • Neotype: New nomenclatural type when the original material is missing.

Hierarchy of Categories (Taxonomic Hierarchy, Linnaean Hierarchy)

  • It is the sequence of arrangement of taxonomic categories in a descending order during the classification of an organism.
  • The hierarchy was first given by Linnaeus who used only five categories-class, order, genus, species, and variety.
  • The last one was discarded and three added so that now there are seven obligate categories – kingdom, division (= phylum), class, order family, genus, and species.
  • Sub and super-categories are interpolated according to requirement, e.g., superclass, sub-class.
  • A rank in the Linnaean hierarchy is called category, e.g., family Rosaceae, order Rosales, class Dicotyledoneae.
  • Character complexity increases from kingdom to species gradually.

(i)Species (John Ray, 1693):

It is a natural population or group of natural populations of individuals which are genetically distinct and
reproductively isolated with similar essential morphological traits.

Species is also a genetically closed system because its members do not interbreed with members of other species. Species is lowest or basic taxonomic category.

Interbreeding cannot be applied for delimitation of species in case of prokaryotes and some protests which lack
sexual reproduction.

The biological definition of species was given by R. Mayr. Species is of following types:

  • Morphospecies: Species erected on the basis of morphological characters only.
  • Taxonomic Species: A species having a definite binomial name.
  • Lannion (Linnaean Species): A taxonomic species distinguished on morphological grounds.
  • Macro species: A large polymorphic species with several subdivisions.
  • Microspecies (Jordanon, Hordan’s Species): A true breeding genetic unit below the rank of species.
  • Biospecies (Biological Species/Genetic species/Biological Concept of Species, Mayr): A species erected on the basis of reproductive and genetic isolation of a natural population.
  • Sibling Species: True species, which do not interbreed but difficult to separate on the basis of morphological characters alone.
  • Allopatric Species: Species having exclusive areas of geographic distribution.
  • Sympatric Species: Species having overlapping areas of geographic distribution.
  • Parapatric Species: Species with adjacent geographic ranges meeting in the very narrow zone of overlap.
  • Palaeospecies: Species known from fossils only.
  • Neontological Species: Living species.
  • Synchronic Species: Species belonging to same period or time.
  • Allochronic Species: Species belonging to different time periods.
  • Monotypic Species: Species without differentiation of subspecies/varieties.
  • Polytypic Species: Species with two or more subspecies/varieties.
  • Keystone Species: A species that plays a central role in the ecology of a place.

(ii) Genus (John Ray, 1963):

It is an assembly of related species which evolved from a common ancestor and have certain common characters
called correlated characters. A genus having only one species is monotypic. Others are polytypic.

(iii) Family (John Ray, 1993):

  • Taxonomic category, which includes one or more related genera. All genera of a family have some common or correlated features.
  • In plants, the family ends in the suffix -aceae and subfamily in -oideae while in animals the suffixes are –idea for family, -inae for sub-family, -ini for tribe (between subfamily and genus) and –oidea for superfamily.

(iv) Order (Linnaeus, 1735):

The order is a taxonomic category which includes one or more related families with some common features. It ends in suffix-ales in plants. Different suffixes are used in case of animals.

Cohort used differently by different authors as

  • Group of related species
  • Group between order and class and
  • The group above the rank of supper order.

(v) Class (Linnaeus, 1735):

It is a taxonomic category made of one or more related order. It ends in suffixes-phyceae,-opsida and –ae in plants.The suffix is not fixed in case of animals.

(vi) Division (Eichler) or Phylum (Cuvier, 1829):

The taxonomic category that includes one more related classes. The division is given the suffix-phyte and the subdivision-phytina.


  • It is a system of arrangement of organisms into groups on the basis of their similarities, differences, and relationships.
  • The finding of correct name and place of an organism in a system of classification is called identification.
  • Key is the tabulation of diagnostic characters of species, families, etc. in a dichotomous series for rapid identification.

Artificial Systems of Classification

  • They are systems of classification in which habit, habitat and a few morphological characters are used for groping of organisms.
  • All early systems of classification (e.g., given by Aristotle, Theophrastus, Pliny, Bauhin, Hohn Ray, Linnaeus, etc.) were artificial.
  • Aristotle divided animals into enaima (with red blood), anaima (without red blood), ovipera (egg laying), vivipera (giving birth to young ones).
  • Pliny & Elder divided animals into flight and nonflight animals.
  • Linnaeus divided angiosperms on basis of numerical value of essential organs, e.g., cryptogamia, monandria, diandria, polyandria.

Artificial systems are simpler and easier to practice in the field but they have several drawbacks:

  1. Grouping is based on external features which may appear due to parallel evolution, retrogressive evolution or progressive evolution.
  2. Organisms of different affinities may group in the same group, e.g.Vitis (dicot) and Asparagus (monocot) in the classification of Linnaeus.
  3. Closely related organisms get separated into different groups, e.g., Bat, Whale, and Panther.
  4. The traits used for the artificial system are liable to change.
  5. Natural relationships are not brought out.

Natural System of Classification

  • In it, organisms are arranged according to their natural affinities, especially reproductive and anatomical.
  • The characters are useful in bringing out homology and hence evolutionary tendencies.
  • Plant taxonomists use the term phylogenetic system for the one which brings out evolutionary tendencies. However, animal taxonomists consider natural systems to be phylogenetic.
  • Their modern-day taxonomy is called morpho-taxonomy, which consider both morphological traits and evolutionary relationships.
  • The natural classification of plants was first proposed by Schimper (1879) followed by Eichler (1883).
  • Benthan and Hooker (1862-1883) developed the first natural system of classification of higher plants in their “Genera Plantarum”.
  • Phylogenetic system of plant classification was first proposed by Engler and Prantl in “Die Naturlichen Pflanzen Familien”.
  • It was followed by Hutchinson, Takhtajan, Cronquist, etc.
  • Graphic representation of the evolutionary relationship of a group is called family tree or cladogram.
  • A family tree based on numerical or phenetic taxonomy is known as the Dendrogram.
  • Takhtajan has likened taxonomy without phylogeny to be bones without flesh.

Phylogeny: The term was introduced by Lamarck but the concept was established by Ernst Haeckel (1866).


It is the grouping of organisms of any level of Hierarchical classification, e.g., mammals, roses, crucifers, poppies, etc.The term was introduced by Adolf Meyer (1926) (for animal groups).

Monophyletic taxon/ clade belongs to a lower ranking category while polyphyletic taxon/ grade belongs to the higher
category. Study of the sequence of a clade is called Cladistics.

Need for Classification

  • Out of 1.7 million organisms, 1.2 million are animals and 0.5 million plants.
  • The largest group of organisms is insects with over 0.75 million species. Many plant and animal species have not yet been discovered.
  • Underwater reefs and tropical rainforests seem to possess innumerable undiscovered species.
  • Every year about 15000 new species are discovered.
  • It is estimated that total living organisms may range between 5-30 million species.
  • About 50-100 times more species have become extinct.

A good system of classification is a must for their identification, study and bringing out relationships amongst themselves.

Classification of Mango

Kingdom – Planta
Division – Spermatophyta
Subdivision – Angiospermae
Class – Dicotyledoneae
Order – Sapindales
Family – Anacardiaceae
Genus – Mangifera
Species – Indica

Classification of Tiger

Kingdom – Animalia
Phylum – Chordata
Subphylum – Vertebrata
Class – Mammalia
Order – Carnivora
Family – Felidae
Genus – Panthera
Species – Tigris

A Glance

  1. De Candolle (1813): Coined the term taxonomy.
  2. Carolus Linnaeus: Father of taxonomy, introduced binomial nomenclature, coined the term systematics, wrote Philosophia Botanica (ref. 1751). Species Plantarum (ref. 1753) and Systema Naturae (ref. 1758 tenth edition).
  3. First Book on Zoology: Historia Animalium by Aristotle (Father of Zoology).
  4. First Book on Botany: Historia Plantarum by Theophrastus (Father of Botany).
  5. Species: The word was first used by John Ray (1627-1705) for a population of interbreeding individuals.
  6. Biological Concept/Definition of Species: Mary (1942-1957) has defined a species as a group of structurally and functionally. Related organisms that share a common gene poll and interbreed freely but are reproductively isolated from individuals of other species.
  7. Correlated Characters: They are similar characters present in different members of a category that indicates a common ancestry, e.g., species in a genus, different genera in a family, different families in an order.
  8. DNA Hybridisation: It is the latest technique to known relationships by determining the ability of DNA strands of two different sources to form molecular hybrids in vitro. A variation called DNA printing is the technique to identify the source and known genetic relationship of the source. Father of DNA printing technique is Alec Jeffreys (Indian expert is Lalji Singh).


  1. Dogherty (1957): Distinguished prokaryotes and eukaryotes.
  2. Euglena: It has characters of both plants and animals. Nutritionally Euglena is mixotrophic.
  3. Autotrophs: Capable of synthesizing organic food. Two types – chemoautotrophs (energy from chemical
    reactions) and photoautotrophs (energy from solar radiations). Photoautotrophic nutrition is also called holophytic nutrition. Chemoautotrophs are anoxygenic. Photoautotrophs can be anoxygenic (photosynthetic
    bacteria) or oxygenic.
  4. Heterotrophs: Food from outside :
    • Saprobic: organic matter in the medium. If already soluble, the saprobes are chemoheterotrophs. If it is solubilized, the saprobes are called osmotrophs. Nutrition is of absorptive type
    • Holozoic/Ingestive/Phagotrophic
    • Parasitic.
  5. Eukaryotes: They constitute four kingdoms of Whittaker’s classification- protista, fungi, metaphyta, and metazoa.
  6. Decomposers: Mainly fungi (kingdom of decomposers), also many monera and protista.
  7. Algae: They are spread over three kingdoms-monera (cyanobacteria), protista and metaphyta.

Some Important points of Biological Classification

In the ancient (Vedic) times, the living organisms were classified into three classes and these were(BCECE 2015).

  1. Jivaja (Born from the worm or viviparous).
  2. Andaja (Born from egg or oviparous).
  3. Udbhija (Born from sprouts).

According to the five kingdom system of classification of living beings, Archaebacteria are monerans which live under extremely adverse conditions, Euglenoids are one of the protists, Phycomycetes is one of the classes of Fungi, and the algae belong to the kingdom Plantae(Kerala CEE 2011).

Mode of nutrition of kingdom – Animalia is heterotrophic. The prokaryotic kingdom monera lack nuclear envelope while Protists are eukaryotic (Kerala CEE 2012).

Mycoplasma is the smallest living cell which is devoid of the cell wall(AIIMS 2012).

Green phytoplanktons are photosynthetic and have chloroplast for carrying out photosynthesis. Since these are unicellular eukaryotic organisms, they are placed in the kingdom – Protista, e.g. Chlorella, Chlamydomonas, etc(Chhatisgarh PMT 2015).