Biological Classification | Taxonomy, Species, Kingdom System

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The Animal kingdom comprises of over one million types of animals. Every organism has some characters similar to its own type, but it differs in a few. This diversity is one of the most important features of organism which led to organic evolution.

Animals show diversity of form, structure, physiology, mode of life, habitat, and behaviour etc. Animals range from simple single celled animals to most complex multicellular forms. At the same time a large number of animals are now extinct and their fossils are found.

This necessitates the categorisation of animals into different groups based on their similarities and dissimilarities. This arrangement in orderly manner, helps in easy identification of the animals and lays the foundation stone for modern classification. Systematics can be defined as “the science of the diversity of organisms”.

“Taxonomy is described sometimes as a science and sometimes as an art, but really it’s a battleground.”


(i) It is not possible for any person to study all the animals. So it will be easy to know about different groups of animals by studying representatives from each group. So classification is of great convenience to the biologists. in Classification helps in storing information.
(ii) Classification makes it easy to exchange the information regarding organisms.
(iii) Classification establishes natural relationship among the animals.


Taxonomy is the branch of biological science, They deals with identification, nomenclature and classification of organisms

(i) Identification. It deals with the structure and function of a particular animal and it’s position in the animal kingdom.
(ii) Nomenclature. It is the application of distinctive names to each of the groups or Taxon in internationally accepted system ot classification.
(iii) Classification. Classification is a method arranging organisms into related groups according to their resemblances and differences Simpson 1961 state that “Classification is the ordering of animals in grOups or sets, on the basis of their relationships, i.e., of association by continuity, Similarity or both.


Aristotle, (384-322 B.C.) Father of Zoology, made the earliest serious effort to classify animals for the first
time. He divided the animal kingdom into two groups on the basis of colour of blood.

  1. Anaima (or Invertebrates). These animals do not contain red blood.
  2. Enaima (or Vertebrates). These animals contain red-blood. This group is again sub-divided into two groups basing on reproduction.
    (a) Oviparous. These are egg laying animals like fishes, amphibians, reptiles and birds etc.
    (b) Viviparous. These animals give birth to young ones. Ex-mammals.

Charaka “Father of Ayurbeda”(1st Century A.D.) also identified some plants and animals in his book “Charak Samhita”.

John Ray (1627-1 705), for the first time introduced the term species in classification. But later a Swedish biologist, Carolus Linnaeus (1707-1778) gave a comprehensive scheme of scientific classification. He published two famous books such as “Genera Plantarum (1737) and “Species Plantarum” (1757). Linnaeus published his tenth edition of “Systema Naturae” (1758) listing about 4236 species which are still accepted today. His natural system of classification is the basis of modern classification.

George Cuvier (1769-1832), “Dictator of Biology” established the modern system of classification.

He divided the animals into four branches, such as, (i) Vertebrate (from fish to mammals). (ii) Moliusca (Different molluscs and Barnacles). (iii) Articulata (Annelids, crustaceans, spiders,insects) (iv) Radiata (Rotifers, Cnidarians, Nematodes, Echinodermata).

Haeckel and Lankester laid the basic principles of zoological classification. These principles are still followed. Julian Huxley (1940) emphasized on the fact that systematics should be related to new discoveries made in the branches of cyto-genetics, embryology, ecology, anatomy,physiology and biochemistry of organisms. H.F. Copeland (1956) proposed the four kingdom classification.i, such as: (i) Monera (Ex-virus) (ii) Protista (Ex-protozoa, fungi), (iii) Metaphyta (Ex-green plants) (iv) Metazoa (Ex-Multicelluar animals). But Robert H. Whittaker (1969)classified the living organisms in a five kingdom classification such as ; Monera, Protista, Fungi, Plan tae and Animalia. In 1981 Margulis and Schwartz proposed a modifed version of Whittaker’s systemm of classification and arranged the kingdoms on evolutionary basis. i.e. (i) Monera (ii) Protocista (ii) Plantae (iv) Mycota, and (v) Animalia.


Taxonomists have proposed various systems of classification.


The systems of classification proposed before Linnaeus, were discarded as they are not based on scientific basis. Carolus Linnaeus, for the first time, arranged the animals according to their morphological and sexual characters. But it suffered a serious drawback, because the closeiy related species were far separated and unrelated species were grouped in a singe group.was regarded as the first scientific system used for classification. So Linnaeus is regarded as the “Father of Classification”. Later systems were developed on the basis of Linnean system.


Only in the last century, this system was proposed.It is based on both, the morphological characters and their natural relationships. This system was not completely accepted, as it did not emphasize the evolutionary relationship among the species.

    This system is based on morphological characters, natural affinities and evolutionary relationships among the species. It also reflects their genetic relationship with their ancestors and other animals which evolve from them.


The above systems mainly deal with pure taxonomic studies basing on morphological characters and evolutionary trends. The modern or experimental taxonomy is based on several aspects.

(I) Cytotaxonomy. It is the utilisation of cytological informations like, (a) shape, size and number of chromosomes of the species and (b) their behaviour particularly during meiosis, in placing the animals in taxonomic groups. It has been found to be effective in finding interrelationships among species and genera.

(ii) Chemotaxonomy. It deals with the chemical basis of taxonomy. Study of chemical composition shows the presence of specific chemical substances in the cells of a genus or a family or a class. This factor establishes interrelationship among the members of the group. Some factors like, structure of proteins, lipids, DNA, and genetic codes are studied to place the animals in proper taxonomic groups.

(iii) Numerical Taxonomy. It deals with the application of mathematical techniques in solving the taxonomic problems.


Classification consists of a number of groups or Taxon of Species is the lowermost taxon of any cassification.

What is Species Concept or define Species Concept

Species. It is “a group of interbreeding population, occurring in nature, having many characteristics in common and differing other groups, from which they are reproductively isolated.”

Define the General Characteristics of species

(a) A species is a population individuals having morphological similarity
(b) There is free geneflow in a common gene pool of a species.
(c) They can interbreed (flow of gene).
(d) It is reproductively isolated rom other species
(e) It occupies a particular ecological niche.
(f) It helps in evolution by giving rise to new specie

For example Panthera tigris is the name of a species of a tiger. But Panthera has other species like leo (Panthera leo i.e., lion).

The entire course of evolution depends on origin of new species which forms the fundamental unit. So Linnaeus considered the species to be a distinct group With constant
characteristic features.

Species’ forms the lowest taxon of biological classification. The modern taxonomists considered and described the species from different aspects.

  1. Genetic Species. It is defined as “a group of genetically identical individuals”.
  2. Morphological Species. Linnaeus considered the species to be morphologically identical group. It can be defined as “a group of individuals that resemble each other in most of their morphological characters and that the adjacent local population within the group differ only in variable characters that intergrade marginally.”
  3. Biological Species. According to Mayr (1940) “Species is a group of actually or potentially interbreeding natural population that is reproductively isolated from other such groups.” Merrel (1962) mentioned that The species is a natural, biological unit, tied together by the bond of mating and sharing a common gene pool. For this reason it has objective reality.”

What are the types of species?

A species can be of two types:
(1) Polytypic species. A species wi htwo or more sub-species is called as polytypic species.
(2) Monotypic. It is not sub-divided into sub-species.

What are the common features of species?

Genus (PI-Genera). It comprises of two or more species, having many common features.

what is family?

Family. This group includes one or more genera with some similarities and have differences in many characteristics.
For example the family includes the genus Panthera (lion, tiger, etc.) and the genus Felis jungle cat, Leopard, Golden cat,Fishing cat etc.). The elated family Canidae includes dogs, foxes, etc.

What is order?

Order. It consists of a number of families. For example families like Felidae, Canidae are included under one order i.e.,Carnivora.

What is class?

Class. It consists of more than one orders. For example Class Mammalia includes nearly sixteen living orders such
as carnivora, primate etc.

Phylum. (PI-Phyla)It includes one or more classes having some common general characters.

Example. Different classes like Elasmo- branchii, Dipnoi, Teleostei, Amphibia, Reptilia, Aves and Mammalia are included under one phylum, chordata.

Kingdom. It occupies the highest taxonomic position and includes many phyla. It includes all the animals of the world, i.e. Kingdum Animalia.

Schematic Representation of Taxonomic Hierarchy








To make the classification more meaningfull, now-a-days sub-divisions of each category are described.

The taxonomic hierarchy represents the stem, branch, small branch, twigs, cluster of leaves and single leaf of a tree. The leaf represents a species and so on.


In the earlier part of this chapter, we have discussed the need and the systems of classification of living organisms. We shall discuss here grouping of these living organisms to particular kingdom and how the discovery of new organisms, particularly micro-organisms changed the earlier concept of two kingdoms classification to three kingdoms, four kingdoms and ultimately to five kingdoms concept.


TWO KINGDOM SYSTEM-Plantae and Animalia

Prior to the discovery of micro-organisms it was not difficult to divide the living world into two divisions plants and animals. Linnacus divided the living world into two kingdoms: Plantae and Animalia based on characteristics such as motility, green colour, presence or absence of leaves. flowers, eyes, mouth, legs etc. Plants were regarded as non-motile and animals were motile.

Kingdom I. Plantae.

The living organisms included under this kingdom were, bacteria, fungi, algae, liverworts, mosses, ferns, cornifers and flowering plants. They are distinguished by (I) presence of cell wall (ii) occurrence of inorganic crystals in the cells (iii) presence of large central vacuole in the cell (iv) absorption of inorganic nutrients from outside (v) well defined growing points with unlimited growth (vi) absence of excretory organs, sense organs, nervous system and muscular tissue (vii) ability to manufacture food due to presence of chlorophyll (viii)n storage of reserve food as starch. (ix) shape is not well defined (x) absence of locomotion.

Kingdom II. Animalia.

This kingdom includes protozoan, sponges, jelly fishes, worms, insects, crabs, centipedes, mnillipedes, spiders, snails, starfishes, sharks, frogs, lizards, birds and mammals. They possess characters opposite to those of plants such as
(i) absence of cell wall (ii) no inorganic crystals in their cells (iii) no central vacuole in the cell (iv) ingestive or
holozoic type of nuitrition (V) no definite growing point but growth uniform and limited (vi) in presence ol excretory system, nervous system, sense organs and muscle tissues
(vii) inability to synthesize food (vii) storage of food as liver and muscle glycogen and at (ix) body shape well defined (x) locomotion with respect to change of place occurs.

What are the Merits and Demerits?

The two kingdom classification has been discarded due to the following objections

(i) There are many living organisms which can neither be put under plants nor under animals,
(ii) Placement of fungi under plants has been objected because of their difference in structure, physioloey and productive process.
(iii) At lower levels of organisation, the distinction between plants and animals disappears. For example, Euglena has both holophytic and holozoic nutrition. Some sponges are fixed corns, branched and irregular in morphology like plants.
(iv) Inclusion of prokaryotes including viruses under plants is unjustified.
(U) Lichen is a peculiar group having neither plant character nor animal character. Hence their inclusion under plants has been objected.
(v) Microorganisms like viruses, prokaryotes (bacteria) and eukaryotes (uncellular) cannot be placed in one group because of their wide variation in size of the body.
(vii) Unicellular algae like euglenoids, diatoms and dinoflagellates and protozoans show sufficient resemblance. Therefore separating them into plants and animals is difficult.


THREE KINGDOM SYSTEM-Plantae, Animalia and Protists :

Ernst Hacckel (1866) proposed a new kingdom protista taking into account all the micro-organisms and by separating them from the other two kingdoms: Plantae and Animalia. During his period many microbes were not known because of the lack of the knowledge in microscopy. He separated unicellular organisms from plant and animal kingdoms and grouped them under protists. However, the three kingdom classification could not serve the purpose of taxonomists as many intermediate forms between plants and animals were later on discovered and it could not distinguish between primitive and advanced protists.


FOUR KINGDOM SYSTEM- Monera, Protista, Metaphyta, Metazoa:

In order to solve the problem of classification created by protists H.F. Copeland (1956) proposed a new system of classification. His system is known as four kingdom system. According to his four kingdom classification, the living world can be divided into the following kingdoms:

What is four kingdom classification?

  1. Kingdom Monera. It includes prokaryotic unicellular, colonial organisms such as bacteria, blue-green algae.
  2. Kingdom Protocista (Protista). It includes lower eukaryotic unicellular or multicellular organisms such as algae other than blue-green algae, protozoans, fungi.
  3. Kingdom Meta phyta. It comprises of higher multi-cellular eukaryotes having cell wall,chloroplast, tissue differentiation form limited to intermediate type. They possess root or root like organs of absorption. All terrestrial and aquatic plants belong to this kingdom.
  4. Kingdom Metazoa. It includes higher multicellular organisms without cell wall and chloroplast. Tissue and organ systems are well developed. All multicellular animals belong to this kingdom. FTVE KINGDOM SYSTEM Monera, Protista, Fungi, Plantae, Animalia

It is an improvement over four kingdom system of classification. It was first proposed by an American taxonomist, Robert H. Whittaker (1969) who separated Fungi from the plants. He divided the living world into five kingdoms-Monera, Protista, Fungi, Plantae and Animalia and.As viruses are on the border line of living and non living, they have been omitted from the classification. Whittaker based his scheme of classification of five factors such as

(i) Complexity of Cell Structure-prokaryotes (cell without nucleus as in bacteria and blue-green algae) and eukaryotes (cell having distinct, nucleus as in protozoa, fungi plants and animals)
(i) Complexity of Organism’s Body-unicellularity and multicellularity.
(iii) Mode of Nutrition-autotrophic (organisms like green plants can manufacture their own food), Heterotrophic cannot synthesize their own food and derive it either from living (parasites) or dead organic matter (saprophytes) or feed on solid organic matter (holozoic)
(iv) Life Styles-producers, consumers and decomposers.
(v) Phylogenetic Relationship-evolutionary history of an organism.


KINGDOM MONERA (The prokaryotes)

(i) Monera (monos-single) includes unicellular,microscopic organisms. However, some are colonial and filamentous.
(ii) An organised nucleus is absent. Nuclear membrane, plastids and mitochondria lacking.
(iii) Presence of a rigid cell wall which is made of peptidoglycan and polysaccharides other than cellulose.
(iv) Mode of nutrition may be autotrophic (can manufacture their own food) or heterotrophic (cannot synthesize their own food and hence absorb nutrients from the surrounding medium) or parasitic.
(v) A double stranded circular DNA is present. It is not associated with histone proteins.
(vi) Centriole absent and ribosomes are 70S type.
(vii Some are motile and others are non-motile. Flagella, whenever present, without 9+2internal microtubular structure.
(viii) Many fix atmospheric nitrogen into ammonia, enriching the soil.
(ix) Asexual reproduction is by binary fission or budding.
Kingdom monera includes two major groups, Eubacteria (true bacteria) and Archaebacteria (ancient bacteria).

Examples.Anabaena, Streptomyces, Escherichia coli, Azotobacter.


KINGDOM PROTISTA (Unicellular eukaryotes).

(I) Kingdom Protista (protisto=primary) includes different types of unicellular, aquatic eukaryotes, usually they are microscopic.
(ii) Their cells possess typical eukaryotic cell organelles such as nucleus, mitochondria,endoplasmic reticulum,Golgi bodies, plastids if photosynthetic).
(iii) They are free living, solitary or colonial. Some are parasitic or commensal or symbiotlc
(iv) Locomotion is being performed by either cilia, flagella or pseudopodia. Flagella or cilia with 9+2 internal microtubular structure.
(v) They have diverse modes of nutrition such as autotrophic (synthesize their own food), holozoic (feed on solid organic matter), saprophytic (absorb soluble food), parasitic (ive on other organisms), Digestion is carried on by food vacuoleintracellular digestion)
(vi) Excretion and respiration occurs through the general surface ofthe body by the process of diffusion. In fresh water protozoans such as in amoeba osmoregulation (regulation of water content of the cell) is carried out with the help of contractile vacuole.
(vii) Asexual reproduction takes place through various means such as by binary fission multiple fission, sporulation, budding etc. Sexual reproduction takes place by conjugation, syngamy, automixis etc.
(viii) Two types of life cycles are observed in protista. They are haplontic and diplonticlife ucycles. In he former the individuals are halpoid (n) and produce haploid (n) gametes in later life cycle the individuals are diploid (2n) and gametes are haploid (n)
(Detailed account of the kingdom Protista is given after the present description)


KINGDOM FUNGI (multicellular decomposers).

Fungi were once considered as plants because they have cell walls and produce spores, but they are now separated from plants on the basis of nutrition. They have the following general features:
(I) They include diverse kinds of eukaryotic, unicellular or multicellular organisms without chlorophyll.
(ii) They are termed as saprobes when they live on decaying matter, and parasites when they feed upon the tissues of living organisms.
(iii)The plant body or thallus consists of delicate thread like hyphae (hypha in singular).The hyphae collectively form the mycelium.
(iv) The cell wall is made of chitin and the reserve food is stored in the form of glycogen.( Some species show symbiotic assoclation with some algae and higher plants.
(v) They reproduce in three ways. Vegetaive reproduction in fungi takes place through fragmentation, fission, budding, o1dia and chlamydospore formation. Asexual
reproduction occurs by forming zoospores, aplancspores and conidia. Sexual reproduction takes place through plasmogamy, karyogamy and meiosis.

Kingdom fungi is divided into 5 major phyla such as Zygonmycetes (the conjugation fungi), Ascomycetes (the sac Tung), Basialomycetes (the club fungi), Oomycetes and Myxomycetes.
Examples. Rhizopus, Penicillium, Agaricus, Lichens, Mucor

WHAT IS KINGDOM PLANTAE (Mulucelular producers)?

KINGDOM PLANTAE (Mulucelular producers)

Kingdom plantae includes all multicellular plants Occuring on land as weu as in water They bear the following characters…..
(i) Shape of the plant body is not delinite except certain lower forms.
(ii) The cells have plastids containing photosynthesizing pigments.
(iii) The cell wall is composed of cellulose, A large central vacuole is present within the cell. But devoid of centriole
(iv) Plants are fixed or immobile and donot have the power of locomotion.
(v) Majority of the species are autotrophic, some are heterotrophic and few species are parasitic.
(Vi) They are the major producers orn land and aquatic habitat, Reserve food is stored in the form of starch.
(vii) Reproduction takes place by vegetative, asexual and sexual methods,
(viii) Growth of the plant body takes place throughout the life.

Kingdom plantae includes major phyla like chlorophyta (Green algae), Rhodophyta (Red algae), Phaeophyta (Brown algae), Bryophyta (moss like plants) and Tracheophyta (land plants).


KINGDOM ANIMALIA (Multicellular consumers).

The animal kingdom is the most complex of ali five kingdoms. It includes diverse forms of animals ranging from sponges to the chordates. They have the following criteria:
(i) The animals have a definite shape of the body.
(ii) The cell lacks plastids, cell wall and central vacuole
(iii) Centriole is present near nucleus.
(iv) They are mobile (except sponges) and perform locomotion. They have well developed muscle and nerve cells.
(v) Nutrition is hoiozoic, some are parasitic.
(vi) Normal method of reproduction is sexual type. However, asexual method of reproduction is observed in lower forms.
(vi) The period of growth is limited upto particular age unlike plants.

Merits of Five Kingdom Classification

(i) It is always advantageous to place the prokaryotes in a separate kingdom Monera as they differ from all other organisms with regard to their cellular organisation, modde of reproduction and physiological activities.
(ii) It is wise to separate fungi from other plants and put them in a separate kingdom because of their structural and physiological and nutritional peculiarities.
(iii) In old classifications the intermediate or transitional forms (e.g. Euglena) were included both among plants and animals. Under five kingdom classification such forms are rightly included in the kingdom Protista only.
(iv) This system of classification is more natural in comparison to the old systems and easily help in establishing phylogenetic relationship among species.
(v) After the three ancient kingdoms such as Monera, Protista and Fungi are separated the remaining kingdoms Plantae and Animalia are more homogenous than before.

Demerits of Five Kingdom Classification

(i) Viruses are not included under this system of classification.
(ii) According to the five kingdom classification the unicellular algae are separated from the multicellular algae and are placed under two separate kingdoms such as kingdom protista and kingdom plantae respectively. However, many taxonomists feel that these two categories of algae are to be put under a single kingdom i.e. kingdom protista.
(iii) Inclusion of slime moulds under the kingdom protista is doubtful.
(iv) Placement of same type of organisms in Monera and Protista is confusing. These two separate kingdoms include organisms having cell wall and without cell wall,unicellular and filamentous photosynthetic and non-photosynthetic organisms
(v) Classification of the living world based on five kingdom system include large number of phyla, classes, sub classes, orders etc. is very difficult and painstaking to remember on the part of the students.


Recently Carl Woese, Kandler and Wheel is (1990) have proposed a six kingdom classification based on the sequence of ribosomal RNA genes present in all living organisms. They created 3 domains (Eukarya, Bacteria and Archae) above the
kingdom level. Domain Eukarya includes four kingomds: Protista, Fungi, Plantae and Animalia Domain Bacteria contains the kingdom Eubacteria and the domain Archaea includes the kingdom Archaebacteria (ancient bacteria).


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