- 1 Why are Living Organisms Classified? – Biological Classification System and Its Types
- 2 Need for Classification
- 3 Advantages of Classification
- 4 Principle of Classification
- 5 Guidelines for Classification
- 6 Rules and Codes of Classification
- 7 Objectives of Classification
- 8 Types of Biological Classification
- 9 Different System of Classification
- 10 A. Two Kingdom System
- 11 B. Three Kingdom Classification
- 12 C. Four Kingdom Classification
- 13 D. Three-Domain System of Classification:
- 14 E. Five Kingdom Classification
One of the interesting Biology Topics is the study of animal behavior and how it is influenced by genetics and the environment.
Why are Living Organisms Classified? – Biological Classification System and Its Types
The arrangement of organisms into groups on the basis of their affinities or relationships is termed as classification. Classification is a broad term or a system of placing living organisms according to a particular sequence. To deal with enormous diversity in the living world, the scientific arrangement of living things into groups is required on the basis of common characteristics.
Biological classification is the scientific system of arranging organisms into groups and subgroups on the basis of their similarities or dissimilarities and placing the groups in a hierarchy of categories. Classification helps to establish the relationship amongst different groups of living organisms. With so many flora and fauna on earth, there must be a method to classify each organism to distinguish it from others, so it can be correctly identified. In science, the practice of classifying organisms is called taxonomy. On the basis of the specific characters, organisms are divided among specific classification units like species, genus, family, etc.
Need for Classification
The number of living beings, plants, and animals is enormous. Further many new forms are discovered and added every year. Plants, animals or microorganisms have great diversity in their form, structure, and mode of life. To find an organism with known characteristics from the vast number of organisms is almost impossible. They need to be divided into groups or subgroups. Arranging and grouping the organisms in a definite principle makes the study of living organisms easy. Just by selecting a few representatives from each group, a broad idea of life on this earth can be achieved. Therefore, a good system of classification is a must for their identification, study, and bringing out relationships among themselves. So need for classification are
- It is not possible to study every organism at a time.
- The study of one or two organisms of a group gives sufficient information about the essential features of the group.
- All the types of organisms are not found in one locality.
- Without a proper system of classification, organisms can not be identified.
- The organisms of the past can not be studied without classification.
- Classification helps to know the relationship of the different groups.
- From classification, we can know the evolutionary significance of different groups.
Advantages of Classification
- It makes the study of different organisms very convenient.
- It facilitates the identification of organisms.
- It shows relationships among various groups of organisms.
- It shows evolutionary trends in various groups of organisms by revealing a gradual increase in the complexity of structure in those organisms.
- It helps in interpreting the poorly developed structures of fossil organisms.
- The study of a few representatives of a group gives the general characteristics of that group as a whole.
- It is useful in foretelling missing or connecting links through which the evolution of one group has occurred from another.
- A proper system of classification is useful for studying fossils.
- It helps to know the organisms of other localities.
Principle of Classification
Classification is the arrangement of plants or animals in a series of groups or categories according to their natural relationship. The main objective of the classification of animals and plants lies in the placing of animals and plants of all categories in different groups in such a way that their origin, relationship, and structural features may be brought out easily. The principle of classification of plant kingdom is very simple based on only the habits of plants like trees, shrubs, and herbs.
According to Carolus Linnaeus, artificial systems of plants and animals were simply arranged in groups according to their superficial resemblances without paying any regard to evolutionary relationships. Thus, in such a system, closely related organisms are often placed in different groups, while quite different organisms are placed in the same group, because of the presence or absence of a particular feature. Natural classification may be defined as classification based on characters or groups of characters that indicate natural relationships. The phylogenetic system classifies organisms according to their evolutionary sequence. In this classification, natural affinities and relationships of organisms are taken into consideration in addition to several constant morphological characters. This system shows genetic relationships and helps to determine the ancestors and derivatives.
Guidelines for Classification
A modern taxonomist uses the following criteria and principles to determine the position of an organism in the natural system of classification. He studies the similarities and differences in organisms by examining many characteristics. These include
- Behaviour and Biochemistry
- Numerical Taxonomy
Considering all such characteristics of an organism, the determination of its position in classification would be more natural and scientific.
Rules and Codes of Classification
- Classification of organisms should be on the basis of their similarities and dissimilarities in such a way that their origin, relationship, and evolutionary sequence (phylogeny) may easily be understood.
- Taxonomy regulates all the rules, procedures, and principles of classification. It maintains the diagram, description, identification, nomenclature, and documentation of an organism.
- ICBN (International Code of Botanical Nomenclature) and ICZN (International Code of Zoological Nomenclature) maintain rules and regulations relating to the nomenclature of type specimens in the case of related organisms. The system of classification which is followed in the grouping of an organism is also mentioned.
- Classification of organisms in the present day gives more importance to cytology, embryology, DNA hybridization technique, biochemistry, paleontology, genetics, etc. in addition to their morphological characteristics.
Objectives of Classification
- To identify and describe different kinds of organisms.
- To place the organisms in various categories on the basis of their similarities and dissimilarities.
- To build a truly natural system of classification that gives an idea of the origin and evolution of the species.
- Helping an easy identification of species.
Types of Biological Classification
There are four types of Biological classification. These are
- Artificial System of Classification
- Natural System of Classification
- Phylogenetic System of Classification
- Phenetic System of Classification
1. Artificial Systsm of Classification:
This classification system is based upon superficial resemblances between organisms in traits, habitats, or morphological features irrespective of evolutionary history is called the Artificial system of classification, e.g., classification proposed by Carolus Linnaeus (1735-37) and other classifications prior to him.
2. Natural System of Classification:
This system of classification employs a number of characteristics including morphological, anatomical, physiological, etc. of organisms to find their natural relationship is called the Natural system of classification, e.g,, classification proposed by George Bentham and J. D. Hooker (1862-83). This system of classification is useful for practical value. In India, it is the most used system.
3. Phylogenetic System of Classification:
This system of classification is based on evolutionary sequence and the genetic relationship among the organisms in addition to other characters to trace their common ancestry is called a phylogenetic system of classification, e.g., classification proposed by Adolf Engler and Karl Prantl (1887-99). This system gives the phylogenetic relationship among organisms.
4. Phenetic System of Classification:
This system of classification is based on the overall similarities of organisms evaluated without regard to phylogeny due to the absence of fossil records is called a phenetic system of classification. This system of classification is also called numerical classification. Julian Huxley (1940) introduced the term new systematics. This system obtains supporting evidence from four major branches of taxonomy – Cytotaxonomy, Chemotaxonomy, Numerical taxonomy, and Cladistic taxonomy.
Different System of Classification
A. Two Kingdom System
Linnaeus (1758) classified all the living organisms under two large kingdoms i.e. kingdom Plantae (Plant Kingdom) and kingdom Animalia (Animal Kingdom).
1. Kingdom Plantae:
Kingdom Plantae includes bacteria, photosynthetic plants, and nonphotosynthetic fungi. They are distinguished by
- Presence of cell wall.
- Occurrence of inorganic crystals and central vacuole in the cell.
- Absorptive mode of nutrition.
- Well-defined growing points with unlimited growth.
- Absence of sex organs, nervous system, and excretory organs.
- Ability to manufacture food due to the presence of chlorophyll.
- Reserve food as starch.
- Absence of locomotion.
- Presence of branches without definite shape.
- Presence of external organs.
2. Kingdom Animalia:
Kingdom Animalia includes unicellular protozoans and multicellular animals or metazoans. They are distinguished by
- Absence of cell wall.
- Absence of inorganic crystals and central vacuole in the cell.
- Holozoic mode of nutrition.
- Well-defined growing points with limited growth,
- Presence of sense organs, nervous system, and excretory organs.
- Inability to manufacture food due to the absence of chlorophyll.
- Reserve food as glycogen.
- Presence of locomotion.
- Definite shape and size.
- Presence of internal organs.
- Presence of muscular tissues.
Objections Against Two Kingdom Classification:
- First-formed organisms were neither plants nor animals.
- Fungi differ in structure, physiology, and reproductive details from plants. A group of fungi called slime molds have ingestive type of nutrition and resemble animals.
- At the lower level of organization, there are several instances where the distinction between plants and animals disappears, e.g., Euglena has both holophytic and saprobic nutrition. Sponges are branched, fixed, and irregular in outline just like plants.
- Unicellular algae like Chlamydomonas are motile and possess regular shape, definite growth, and photosensitive organelles.
- Only with the invention of the electron microscope, did viruses and prokaryotes become well-defined.
- Even viruses were well studied as a branch of botany though they have no relation with plants.
- Lichens are peculiar groups of dual organisms that are formed by an association between an alga and a fungus. They have no plant character, nor any animal character.
- Groups of unicellular algae (euglenoids, diatoms, and dinoflagellates) and protozoa show a sufficient degree of resemblance.
B. Three Kingdom Classification
It was proposed by E. Haeckel (1866). He separated out all unicellular organisms into a separate Kingdom Protista (fungi, protozoans, algae, bacteria, and slime moulds). Thus he proposed three kingdoms namely Protista, Plantae, and Animalia. Later on, the kingdom protista was reserved only for unicellular organisms. The three-kingdom classification is an improvement over the kingdom classification in separating unicellular organisms. The system is, however, defective in keeping prokaryotes and eukaryotes together.
C. Four Kingdom Classification
It was proposed by Copeland (1956). With the discovery of the electron microscope, it became clear that bacteria and related organisms have a different nuclear structure as compared to others. They are prokaryotes in contrast to others which have a true nucleus and are called eukaryotes. Copeland created a separate kingdom Monera (= Mychota) for them. This divided the living world into four kingdoms-Monera, Protista, Plantae, and Animalia. In this system, fungi continued to remain with the Plantae.
D. Three-Domain System of Classification:
Robert H. Whittaker (1969), proposed the Five Kingdom Classification in which he included all prokaryotes (Archaebacteria and Eubacteria) in a single kingdom Monera. Later Carl Woese and Co-workers, (1977) after a prolonged and detailed study, argued that there should be a new category of classification of life, the Domains a classification category above kingdom. They divide cellular life forms into three domains.
- Domain I: Archaebacteria (now Archae)
- Domain II: Eubacteria (now Bacteria)
- Domain III: Eukarya or Eukaryotes
The principles of the domain system are as follows:
- Presence or absence of a well-developed nucleus.
- In all previous classification systems, all the microorganisms that lack nuclei were grouped into a single category. But here, they are grouped into two different domains, viz., Domain Archae and Domain Bacteria.
- They used the nucleotide sequences of 16s rRNA, other RNA, and proteins as an Evolutionary Chronometer. It is assumed that the organisms showing more similarities in those sequences are more related to one another.
- All living beings have evolved from a single ancestor which contains poorly developed genetic machinery. It is often called a progenitor. Sometimes, it is also referred to as the Last Universal Common Ancestor (LUCA).
- The chemical nature of lipid molecules, present in the plasma membranes, was also considered.
- Another important principle is the antibiotic susceptibility of organisms.
The three-domain system is as follows:
Domain I: Archaebacteria (Archae)
- Archaea are prokaryotic in nature.
- Peptidoglycan is absent in the cell wall.
- The cell membrane is composed of branched hydrocarbon chains, attached to glycerol by ether linkages.
- Archaea are not sensitive to some antibiotics that affect the Bacteria but are sensitive to some antibiotics that affect the Eukarya.
- They contain unique types of rRNA which are distinctly different from Bacteria and Eukarya.
- They can reproduce in stronger acidic media or at higher temperatures.
Domain Archae contains a single kingdom – Archaebacteria. They are of the following types:
- Methanogens (Methane-makers): They use CO2, H, and N to produce energy to live and as a result, produce methane gas. They live in swamps, marshes, the gut of cattle termites, etc. They are decomposers and can be used in sewage treatments, e.g., Methanococcus jannaschii.
- Extreme Halophiles (salt-lover): They may live in stronger salt-containing media, e.g., Ferroplasma acidophilus.
- Extremophiles (heat/cold-lover): They may live in temperatures above 60°C (e.g., Thermus aquaticus) or under polar ice (Bacteria) (e.g., Cenarchaeum symbiosum), etc.
Domain II: Eubacteria
- Members are prokaryotic in nature.
- Cell walls contain peptidoglycan.
- Cell membrane is composed of unbranched fatty acid chains, attached to glycerol by ester linkages.
- They are sensitive to traditional antibacterial antibiotics but resistant to most antibiotics that affect Eukarya.
- Several loops are present in rRNA.
Bacteria contains single kingdom Eubacteria which have different groups.
- Proteobacteria: Chemoheterotrophic, gram-negative bacteria, very closely related to eukaryotic mitochondria, e.g., Rhizobium.
- Cyanobacteria: Chlorophyll present and capable of photosynthesis, very closely related to the eukaryotic chloroplast, e.g. Nostoc.
- Spirochaetes: Gram-negative, spiral bacteria, e.g., Treponema.
- Firmicutes: Gram-positive bacteria, capable of endospore formation, e.g. Bacillus subtilis.
- Chlamydias: Parasites, e.g., Chlamydia.
Domain III: Eukarya (Eukaryotes)
- Eukaryotic in nature
- Peptidoglycan is absent in the cell wall if the cell wall is present.
- The cell membrane is composed of unbranched fatty acid chains attached to glycerol by ester linkages.
- They are resistant to traditional antibacterial antibiotics but are sensitive to most antibiotics that affect eukaryotic cells.
- RNA contains molecular regions, distinctly differently different from the rRNA of Archaea and Bacteria.
Eukarya is divided into the following kingdoms:
- Protista: These are simple, predominately unicellular eukaryotic organisms, e.g., slime moulds, euglenoids, algae, and protozoans.
- Fungi: Unicellular or multicellular eukaryotic organisms, having cell walls (composed of chitin) but tissue organization is absent. Incapable of photosynthesis, therefore, either parasitic or saprophytic in nature, e.g., fungi, club fungi, yeasts, and moulds.
- Plantae: Multicellular eukaryotic organisms having cellulosic cell walls, stored product is principally starch. Chlorophyll is present, therefore autotrophic, e.g., bryophytes, pteridophytes, gymnosperms, and angiosperms.
- Animalia: Multicellular eukaryotic organisms that lack cell walls. Cells are organized into tissue. Do not carry out photosynthesis. Holozoic in nature. Glycogen is the stored product. Examples are multicellular invertebrates and vertebrates.
E. Five Kingdom Classification
Robert H. Whittaker (1969), an American taxonomist proposed this classification of living organisms on the basis of five criteria for distinguishing the different kingdoms:
- Complexity of cell structure: prokaryotic and eukaryotic.
- Complexity of body organization: unicellular and multicellular.
- Modes of nutrition: Autotrophs or heterotrophs.
- Ecological lifestyles like producers (Plantae), decomposers (fungi), and consumers (Animalia).
- Phylogenetic relationships.
- Whittaker’s five kingdoms are Monera, Protista, Plantae, Fungi and Animalia.
Monera: Kingdom of Prokaryotes
The kingdom includes all prokaryotes – mycoplasma, bacteria, actinomycetes, and cyanobacteria (blue-green algae).
- They are basically unicellular prokaryotes and are primitive living forms.
- Modes of nutrition are saprobic, parasitic, chemoautotrophic, photoautotrophic, and symbiotic.
- The cell wall is generally present. It contains peptidoglycan and polysaccharides.
- The plasma membrane is the only differentially permeable membrane, membrane-bound cell organelles like mitochondria, Golgi bodies, lysosomes, etc. are absent.
- Well-organised nucleus absent genetic material is naked DNA, present inside the cytoplasm. DNA is devoid of histone protein, circular and supercoiled known as nucleoid.
- Flagella if present are single-stranded, and made of flagellin protein.
- Reproduction is by asexual methods, multiplication takes place mainly by amitosis.
- Gametes are absent, gene recombination may take place by conjugation, transduction, etc.
Protista: Kingdom of Unicellular Eukaryotes
It includes dinoflagellates, euglenoids, diatoms, protozoans, and slime moulds.
- They are unicellular and colonial eukaryotes.
- Mostly aquatic forming plankton.
- Modes of nutrition are photosynthetic, saprobic, parasitic, ingestive, or holozoic, etc.
- Plasma membrane as well as internal membrane around cell organelles are present.
- The tissue system is absent.
- Flagella, if present is 9 + 2 stranded, made of tubulin protein.
- Well well-organised nucleus is present, and DNA is associated with histone proteins.
- Asexual reproduction is quite common.
- Sexual reproduction is present but an embryo stage is absent.
Fungi: Kingdom of Multicellular Decomposers
It includes moulds, mildews, yeasts, rust fungi, morels, mushrooms, puff-balls, bracket fungi, etc.
- They are achlorophyllous, spore-producing, multicellular, or multinucleated eukaryotic organisms.
- They are heterotrophic either saprobic or parasitic; symbiotic association is found with some algae and higher plants, e.g., lichens, and mycorrhiza.
- The vegetative body of the fungus is filamentous and is called mycelium. Branches of mycelium are called hyphae.
- The cell wall contains chitin and noncellulosic polysaccharides. Cellulose is found in a few cases.
- A double envelope organization occurs in fungal cells. Nuclei are small and show intranuclear spindle formation.
- Golgi bodies or dictyosomes are unicisternal.
- Reserve Food is glycogen and fat.
- Tissues are poorly developed.
- Reproduction is both asexual and sexual.
- There is an evolutionary tendency towards degeneration of sex organs and employment of somatogamy. An embryo stage is absent.
Plantae: Kingdom of Multicellular Producers or Metaphyta
It includes all photosynthetic eukaryotic multicellular plants. The important constituents are multicellular green algae, brown algae red algae, bryophytes, pteridophytes, gymnosperms, and angiosperms.
- Most of the members are restricted to land, sea shores, lakes, ponds, and streams.
- They are multicellular and eukaryotic. Tissues and tissue systems are present except for certain cases.
- Growth is usually indefinite, body form is generally irregular due to the presence of branches.
- Responsiveness is poor.
- Organs are commonly external.
- The mode of nutrition is autotrophic.
- Photosynthetic organs contain chloroplastids in their cells. Due to photosynthetic activity, they are called producers.
- Plants are sedentary or fixed. Some aquatic forms are free-floating.
- Food reserve is usually starch and fat.
- Mature cells have central vacuoles and inorganic crystals often occur in the cell. The cell wall is made of cellulose.
- Some plants are heterotrophic saprobic or Parasitic forms of plants parasitic.
- A few plants that can trap and kill insects, and absorb nitrogenous substances for their nitrogen nutrition are called insectivorous plants.
- Reproduction takes place by vegetative, asexual, and sexual methods. Vegetative reproduction is common.
- An embryo stage is present except in algal forms.
- The life cycle exhibits an alternation of generations.
Animalia: Kingdom of Multicellular Consumers or Metazoa
This kingdom includes all multicellular animals, they are the maximum in number and most diverse types of organisms.
- Kingdom Animalia or metazoa includes all animals of the two kingdom classifications except protozoa.
- Groups include sponges, coelenterates, worms, mollusks, arthropods, starfishes, fishes, amphibians, reptiles, birds, and mammals.
- Insects, a group of arthropods include the largest number and varieties of organisms.
- Organisms are eukaryotic, without cell walls and photosynthetic pigments.
- Body form is regular, only in a few cases do irregular forms occur.
- Body organization is cellular, tissues, organs, and organ systems are present.
- Organs are internal.
- Growth is definite, it does not occur throughout life.
- Responsiveness is sharp.
- Cells do not possess central vacuoles. Instead, small vacuoles may occur in the cytoplasm.
- The centrosome is present.
- Mode of nutrition is heterotrophic being holozoic or ingestive. A few animals are parasitic, living inside the body of other animals or plants.
- Animals are motile as they have to search for their food. Sponges and corals are of sedentary habit.
- Most organisms possess muscle cells for their mobility and nerve cells for the conduction of impulses. They possess a good coordination system.
- Asexual reproduction occurs only in lower forms.
- Sexual reproduction involves the formation of gametes inside gonads. Meiosis occurs at the time of gametogenesis.
- There is distinct embryological development.
- Gametes represent only the haploid stage in the life cycle. The life cycle is, therefore, diplontic.
- Ecologically animals are consumers.
Important Features of Five Kingdoms with Examples:
|Monera||1. Unicellular and Prokaryotic.||Archaebacteria, Eubacteria, Cyanobacteria.|
|2. Autotrophic mode of nutrition.|
|3. Asexual reproduction.|
|4. Multiplication by amitosis.|
|Protista||1. Unicellular, eukaryotic, and aquatic.||Dinoflagellates, Euglenoids, Diatoms, Protozoans, and Slime moulds.|
|2. Autotrophic and heterotrophic modes of nutrition.|
|Fungi||1. Multicellular and eukaryotic.||Bread mould, Yeast, Mushrooms.|
|2. No true tissues.|
|3. Cell wall made up of chitin.|
|4. Heterotrophic, saprobiotic, parasitic mode of nutrition (absorptive).|
|5. Asexual and sexual reproduction.|
|Plantae||1. Multicellular, non-motile forms.||Green algae, Brown algae, Red algae, Bryophytes, Pteridophytes, Gymnosperms, Angiosperms.|
|2. Eukaryotic, cell wall made up of cellulose.|
|3. Well-developed tissues.|
|4. Photosynthetic autotrophic mode of nutrition.|
|5. The life cycle exhibits alternation of generations.|
|Animalia||1. Multicellular, motile forms.||All animals except protozoa, i.e., sponges, cnidarians, worms, insects, mollusks, starfish, fishes, amphibians, reptiles, birds, and mammals.|
|2. Eukaryotic with well-developed tissues.|
|3. Heterotrophic mode of nutrition.|
|4. Sexual reproduction, control, and co-ordination system.|
|5. Distinct embryological development.|
Advantages of Five Kingdom Classification:
- The creation of the kingdom Monera for prokaryotes is fully justified because they have their own level of structural and biochemical organization.
- The classification brings out phylogeny within the whole living world.
- Many intermediate forms of unicellular eukaryotes have been considered both among plants and animals.
- The creation of a separate kingdom protista for them is also a better approach.
- Separation of fungi into a separate kingdom is a wise step. Because they have their own biochemical, physiological, and structural organization and have never been related to plants.
- In this classification, animal and plant kingdoms are more homogeneous.
- The system is based on levels of organization and modes of nutrition. The latter became established very early during evolution.
Disadvantages of Five Kingdom Classification:
- Kingdom protista is a highly heterogeneous group with several lines of evolution.
- The placement of algae in three kingdoms seems to be elusive.
- The placement of certain unicellular algae and unicellular fungi in kingdom plantae and kingdom fungi respectively is not justified.
- Archaebacteria are quite different from other bacteria in structure, composition, and physiology.
- Red and brown algae are not related to other members of kingdom plantae.
- Viruses have not been included in this system of classification.
Important Differences of Artificial Classification and Natural Classification:
|Artificial Classification||Natural Classification|
|1. It is an old type of classification.||1. It is a more developed classification.|
|2. It is based on some morphological features of the organisms.||2. It is based on many morphological features of the organisms and their interrelationship.|
|3. The relationship among organisms is not clear.||3. Relationship among organisms is established.|
|4. Proposed by Theophrastus, Linnaeus, and others.||4. Proposed mainly by Bentham and Hooker.|
Important Differences of Natural Classification and Phylogenetic Classification:
|Natural Classification||Phylogenetic Classification|
|1. Similarities and dissimilarities of characters among organisms, this classification is based.||1. It is based on evolutionary lines.|
|2. In this system fossils have little role.||2. In establishing evolutionary relationships fossils play an important role.|
|3. For practical purposes it is useful.||3. It is not useful for practical purposes.|
|4. Bentham and Hooker proposed this system.||4. Engler, Prantle, Hutchinson, etc. proposed this system.|