Biological classification is a system which is used to organize and
codify all life on Earth. There are a number of goals to biological
classification, in addition to the obvious need to be able to precisely
describe organisms. Aristotle was one of the earliest to attempt to
classify plants and animals on a scientific basis. Simple morphological
characters were used by him to classify tthe plants into trees, herbs
and shrubs.Animals were divided into two groups, the red blooded and
non-red blooded animals.
Number of Kingdoms:
Variuos schemes dividing the organisms into two, three, four, five, and six kingdoms have been proposed from time to time..
Two Kingdom system of Classification
Carolus Linnaeus placed all the living organisms in two major kingdoms Kingdom Plantae for plants and Kingdom Animalia for animals. This classification was quite reasonable at that period of time since plants and animals could be very clearly distinguished. Plants were stationary, fixed to the soil, absorbed water for growth and could prepare their own food. Animals, on the other hand were capable of movement, and were feeding on plants and other animals for their growth and survival. Apart from this a few other significant differences, particularly at the level of cells, were established between plants and animals by scientists of the later period.
Kingdom Plantae
The original plant kingdom proposed by Linnaeus and subsequent taxonomists of that period included the bacteria, fungi, algae, liverworts, mosses, ferns, conifers and flowering plants. Plants in general are considered to exhibit the following distinguishing characters.
• Plants exhibit a great deal of variation in their form and size. Plant body is usually asymmetrical. However, in higher plants structures like leaves and flowers have a definite form and shape.
• Plants are rooted mostly and as such are incapable of locomotion. However, certain localized movements may occur in a plant body.
• Plants exhibit unlimited growth, almost throughout their life span.
• Plants exhibit absence of locomotion.
• Plants exhibit largely autotrophic nutrition or saprotrophic nutrition. Particulate matter cannot be absorbed and only materials present in a solution state can be absorbed.
• Plant body is composed of cells which have a distinct cell wall, a vacuole filled with sap and plastids of different kinds. The cells do not have centrioles and lysosomes.
• Plant cells can synthesize all the amino acids, coenzymes and vitamins necessary for its functions.
• Plants have reserve food as starch.
Classification of Kingdom Plantae
The Plant kingdom was initially classified as followsClassification of Kingdom Plantae
The Plant kingdom was initially classified as follows:
Merits and Demerits of Two Kingdom Classification
The two-kingdom classification received considerable recognition from biologists and was in use for quite a long period of time. However, as more and more information started emerging on the various groups of plants and animals, this system of classification was found to be inadequate. Studies made on the lower forms of life in particular, revealed an enormous variation of characteristics in those forms, not in agreement with the characteristics of the group in which they were placed.
The two kingdom classification has certain demerits such as:
• The higher organisms can be easily differentiated into plants and animals while there is no clear-cut distinction in the lower forms of life, into plants and animals eg.Euglena is a unicellular organism having certain features of animals and certain features of plants. It has a flagellum which is used for locomotion and food capturing. However, it has chloroplasts like plant cells. It also lacks a cell wall.
• Chlamydomonos is a unicellular alga. It is purely autotrophic but has a locomotor structures called flagella, like protozoans.
• Slime moulds are a type of fungi. They do not have a cell wall in the vegetative phase and ingest particulate matter like animals. However, they develop a cell wall in the reproductive phase which is similar to other fungi.
• The two kingdom system takes unicellular and multicellular organisms together. The bacteria were also considered as plants under this system of classification.
• Unicellular plants ( diatoms, dinoflagellates) and animals ( protozoans) resemble each other in the level of organization and reproduction by fission , however they are placed in different kingdoms.
• Fungi have been put under plantae despite the fact that they lack, cellulose, cell wall, chlorophyll. They are saprophytic or parasitic in their mode of nutrition unlike the plants.
• The eukaryotes have been put together with the prokaryotes .
• Viruses and Lichens could not be categorized because of their peculiar properties.
Thus, they resemble animals in one phase and plants in the other.
Three Kingdom system of Classification:
Earnst Haeckel (1866) a German zoologist suggested a third kingdom ie Protista , which included all the unicellular microorgnisms that are neither plants nor animals.Bacteria , algae, fungi, protozoa were all included under Protista by Haeckel.
According to Haeckel the three kingdoms are Protista, Plantae and Animalia.
The three kingdom system thus enabled to put those organisms under Proista which were neither plants or animals.
The characteristic feature of these are:
(i)The Protists are believed to have evolved from prokaryotic monerans and the precursors from which higher eukaryotic kingdoms- Plantae, Fungi and Animalia have evolved. Thus, the Protits exhibit following features:
(a) The cell organization is eukaryotic and possess nucleus, mitochondria, endoplasmic reticulum, Golgi bodies, and in some organisms plastids also
However, there were certain de-merits in the three kingdom system as well.
a) Acellular and multicellular organisms are kept together in Protista.
b) Bacteria and Fungi have been grouped with unrelated organisms.
Four Kingdom System Classification :
In 1958 Copeland , an American taxonomist , suggested that all prokaryotes , i.e. Bacteria, cyanobacteria etc be placed under the kingdom Monera ( Mychota). He named the four kingdoms as Monera ( Mycota), Protista, Plantae, and Animalia.
Copeland originally called the kingdom Monera as Mychota. It included all the prokaryotes ie Eubacteria ( including Cyanobacteria, formerly known as blue-green Algae) and Archaebacteria.The Actinomycetes ( filamentous bacteria ) are also included in the kingdom. The characteristic features of the Monerans are:
a) They are microscopic (i- few mms). Do not possess nucleus nd membrane bound organelles.
b) The Moneran cell wall is surrounded by rigid cell wall, with the exception of a few Monerans ( e.g. Mycoplasma).
c)Chemosynthesis as a mode of nutrition is found in many Monerans.
d) Some Monerans exhibit autotrophic mode of nutrtion. These prepare their own food by using either light energy ( photoautotrophs)or energy derived from chemical reactions chemoautotrophs.
e) Many Monerans are heterotrophs ( parasites or saprotrophs). These saprotrophs obtain their food from dead organic matter and absorb it in solution form as thier wall prevents the ingestion of complex organic material. Some of them live symbiotically with other forms of life.
f) They are cosmopolitan in distribution. They can sustain at places where life is possible. Some Monerans , like Archebacteria can flourish under extreme environmental conditions like lack of oxygen, high salt concentration, high temperature and acidic hydrogen ion concentration.
Five Kingdom System Classification :
In 1969 Robert H. Whittaker proposed the creation of a new kingdom called Fungi to include the fungi exclusively. This led to a five kingdom classification which is in practice today.
It is a phylogenetic system that is based on the following criteria:
(i) Complexity of cell structure: Prokaryotic Vs Eukaryotic organisation of cells
(ii) Complexity of organisms: Unicellularity Vs Multicellularit
(iii) Mode of nutrition: Autotrophic (holophytic) or heterotrophic [absorptive { saprozoic or parasitic or ingestive (holozoic)]. It is the main criteria for classification in this system.
(iv) Major ecological Role: Producer Vs Cosumers Vs Decomposers.
The Five kingdoms including various organisms are:
(a) Kingdom Monera includes prokaryotic, autotrophic, and heterotrophic organisms.
Kingdom Monera: includes prokaryotic, autotrophic and heterotrophic organisms.
Kingdom Protista: includes, eukaryotic, unicellular, autotrophic and heterotrophic organization.
Kingdom Fungi: includes eukaryotic, multicellular, heterotrophic organisms exhibiting absorptive (assimilative) type of nutrtion , as well as spore producing.
Kingdom Plantae or Metaphta: includes all chlorophyllous multicellulra photosynthetic organisms (plants) that occur on land , on sea shores, in lakes as well as in streams and the non- green relatives. Their photoautotrophic nutrition is also termed as holophytic nutrition. They include red, brwn and green algae, liverworts, mosses, ferns and seed plants with or without flowers.
The Five Kingdom System of Classification
Kingdom Animalia : includes all multicellular holozoic or phagocytic or ingestive eukaryotes.
These are known as Metazoa and includes sponges, Cnidarians, ( Hydra and Jellyfish) , worms, snails and other molluscs , arthropods, insects, starfishes, amphibians, reptiles, birds, and mammals.
Merits and Demerits of Five Kingdom Classification
The five-kingdom classification has certain merits and demerits. However, it is largely the most accepted system of modern classification mainly because of the phylogenetic placing of different groups of living organisms.
This system of classification looks more scientific and natural because of the following considerations:
• Separation of prokaryotes into an independent kingdom is justifiable because they differ from all other organisms in their general organization.
• Kingdom animalia has become more homogeneous by the exclusion of protozoa.
• Much coherance has been seen in the kingdom Plantae since the bacteria , fungi and some other algal forms have been
excluded from it.
• Some organisms like Euglena showing mixotrrophic mode of nutrition could be placed either in plant or animal kingdom easily.
However the creation of kingdom Protista including all unicellular eukaryotes has resoved the problem.
• Elevation of the group fungi to the status of a kingdom is justifiable since fungi totally differ from other primitive eukaryotes like algae and protozoans.
• The kingdoms Metaphyta and Metazoa are now more homogenous groups than they were in the two kingdom classification as it shows the phylogeny of different life styles.
• The five-kingdom classification gives a clear indication of cellular organization and modes of nutrition, the characters which appeared very early in the evolution of life. However, the five-kingdom classification has certain drawbacks also, particularly with reference to the lower forms of life.
• Kingdoms Protista and Monera still retain heterogeenosity, as both heterotroph and autotroph organisms with or without cell wall are included in both these kingdoms. The slime molds are different from other protista with which they have been combined.
• Multicellular green algae cannot be phylogenetically separated from unicellular algae. Therefore, unicellular Chlamydomonas is placed in kingdom Plantae rather than Protista.
• Viruses and Lichens do not find any place.
• Red and Green algae are not related to any other member of Kingdom Plantae.
Six kingdom system of classification
In 1977, Carl Woese extended Robert Whittaker's Five Kingdoms to replace Kingdom bacteria with two kingdoms, Eubacteria and Archaebacteria. Archaebacteria differ from Eubacteria in their genetic transcription and translation processes (in Archaebactera, transcription and translation more closely resembled eukaryotes). These distinguising characteristics were shown by molecular genetic analysis.
Number of Kingdoms:
Variuos schemes dividing the organisms into two, three, four, five, and six kingdoms have been proposed from time to time..
Two Kingdom system of Classification
Carolus Linnaeus placed all the living organisms in two major kingdoms Kingdom Plantae for plants and Kingdom Animalia for animals. This classification was quite reasonable at that period of time since plants and animals could be very clearly distinguished. Plants were stationary, fixed to the soil, absorbed water for growth and could prepare their own food. Animals, on the other hand were capable of movement, and were feeding on plants and other animals for their growth and survival. Apart from this a few other significant differences, particularly at the level of cells, were established between plants and animals by scientists of the later period.
Kingdom Plantae
The original plant kingdom proposed by Linnaeus and subsequent taxonomists of that period included the bacteria, fungi, algae, liverworts, mosses, ferns, conifers and flowering plants. Plants in general are considered to exhibit the following distinguishing characters.
• Plants exhibit a great deal of variation in their form and size. Plant body is usually asymmetrical. However, in higher plants structures like leaves and flowers have a definite form and shape.
• Plants are rooted mostly and as such are incapable of locomotion. However, certain localized movements may occur in a plant body.
• Plants exhibit unlimited growth, almost throughout their life span.
• Plants exhibit absence of locomotion.
• Plants exhibit largely autotrophic nutrition or saprotrophic nutrition. Particulate matter cannot be absorbed and only materials present in a solution state can be absorbed.
• Plant body is composed of cells which have a distinct cell wall, a vacuole filled with sap and plastids of different kinds. The cells do not have centrioles and lysosomes.
• Plant cells can synthesize all the amino acids, coenzymes and vitamins necessary for its functions.
• Plants have reserve food as starch.
Classification of Kingdom Plantae
The Plant kingdom was initially classified as followsClassification of Kingdom Plantae
The Plant kingdom was initially classified as follows:
Merits and Demerits of Two Kingdom Classification
The two-kingdom classification received considerable recognition from biologists and was in use for quite a long period of time. However, as more and more information started emerging on the various groups of plants and animals, this system of classification was found to be inadequate. Studies made on the lower forms of life in particular, revealed an enormous variation of characteristics in those forms, not in agreement with the characteristics of the group in which they were placed.
The two kingdom classification has certain demerits such as:
• The higher organisms can be easily differentiated into plants and animals while there is no clear-cut distinction in the lower forms of life, into plants and animals eg.Euglena is a unicellular organism having certain features of animals and certain features of plants. It has a flagellum which is used for locomotion and food capturing. However, it has chloroplasts like plant cells. It also lacks a cell wall.
• Chlamydomonos is a unicellular alga. It is purely autotrophic but has a locomotor structures called flagella, like protozoans.
• Slime moulds are a type of fungi. They do not have a cell wall in the vegetative phase and ingest particulate matter like animals. However, they develop a cell wall in the reproductive phase which is similar to other fungi.
• The two kingdom system takes unicellular and multicellular organisms together. The bacteria were also considered as plants under this system of classification.
• Unicellular plants ( diatoms, dinoflagellates) and animals ( protozoans) resemble each other in the level of organization and reproduction by fission , however they are placed in different kingdoms.
• Fungi have been put under plantae despite the fact that they lack, cellulose, cell wall, chlorophyll. They are saprophytic or parasitic in their mode of nutrition unlike the plants.
• The eukaryotes have been put together with the prokaryotes .
• Viruses and Lichens could not be categorized because of their peculiar properties.
Thus, they resemble animals in one phase and plants in the other.
Three Kingdom system of Classification:
Earnst Haeckel (1866) a German zoologist suggested a third kingdom ie Protista , which included all the unicellular microorgnisms that are neither plants nor animals.Bacteria , algae, fungi, protozoa were all included under Protista by Haeckel.
According to Haeckel the three kingdoms are Protista, Plantae and Animalia.
The three kingdom system thus enabled to put those organisms under Proista which were neither plants or animals.
The characteristic feature of these are:
(i)The Protists are believed to have evolved from prokaryotic monerans and the precursors from which higher eukaryotic kingdoms- Plantae, Fungi and Animalia have evolved. Thus, the Protits exhibit following features:
(a) The cell organization is eukaryotic and possess nucleus, mitochondria, endoplasmic reticulum, Golgi bodies, and in some organisms plastids also
However, there were certain de-merits in the three kingdom system as well.
a) Acellular and multicellular organisms are kept together in Protista.
b) Bacteria and Fungi have been grouped with unrelated organisms.
Four Kingdom System Classification :
In 1958 Copeland , an American taxonomist , suggested that all prokaryotes , i.e. Bacteria, cyanobacteria etc be placed under the kingdom Monera ( Mychota). He named the four kingdoms as Monera ( Mycota), Protista, Plantae, and Animalia.
Copeland originally called the kingdom Monera as Mychota. It included all the prokaryotes ie Eubacteria ( including Cyanobacteria, formerly known as blue-green Algae) and Archaebacteria.The Actinomycetes ( filamentous bacteria ) are also included in the kingdom. The characteristic features of the Monerans are:
a) They are microscopic (i- few mms). Do not possess nucleus nd membrane bound organelles.
b) The Moneran cell wall is surrounded by rigid cell wall, with the exception of a few Monerans ( e.g. Mycoplasma).
c)Chemosynthesis as a mode of nutrition is found in many Monerans.
d) Some Monerans exhibit autotrophic mode of nutrtion. These prepare their own food by using either light energy ( photoautotrophs)or energy derived from chemical reactions chemoautotrophs.
e) Many Monerans are heterotrophs ( parasites or saprotrophs). These saprotrophs obtain their food from dead organic matter and absorb it in solution form as thier wall prevents the ingestion of complex organic material. Some of them live symbiotically with other forms of life.
f) They are cosmopolitan in distribution. They can sustain at places where life is possible. Some Monerans , like Archebacteria can flourish under extreme environmental conditions like lack of oxygen, high salt concentration, high temperature and acidic hydrogen ion concentration.
Five Kingdom System Classification :
In 1969 Robert H. Whittaker proposed the creation of a new kingdom called Fungi to include the fungi exclusively. This led to a five kingdom classification which is in practice today.
It is a phylogenetic system that is based on the following criteria:
(i) Complexity of cell structure: Prokaryotic Vs Eukaryotic organisation of cells
(ii) Complexity of organisms: Unicellularity Vs Multicellularit
(iii) Mode of nutrition: Autotrophic (holophytic) or heterotrophic [absorptive { saprozoic or parasitic or ingestive (holozoic)]. It is the main criteria for classification in this system.
(iv) Major ecological Role: Producer Vs Cosumers Vs Decomposers.
The Five kingdoms including various organisms are:
(a) Kingdom Monera includes prokaryotic, autotrophic, and heterotrophic organisms.
Kingdom Monera: includes prokaryotic, autotrophic and heterotrophic organisms.
Kingdom Protista: includes, eukaryotic, unicellular, autotrophic and heterotrophic organization.
Kingdom Fungi: includes eukaryotic, multicellular, heterotrophic organisms exhibiting absorptive (assimilative) type of nutrtion , as well as spore producing.
Kingdom Plantae or Metaphta: includes all chlorophyllous multicellulra photosynthetic organisms (plants) that occur on land , on sea shores, in lakes as well as in streams and the non- green relatives. Their photoautotrophic nutrition is also termed as holophytic nutrition. They include red, brwn and green algae, liverworts, mosses, ferns and seed plants with or without flowers.
The Five Kingdom System of Classification
Kingdom Animalia : includes all multicellular holozoic or phagocytic or ingestive eukaryotes.
These are known as Metazoa and includes sponges, Cnidarians, ( Hydra and Jellyfish) , worms, snails and other molluscs , arthropods, insects, starfishes, amphibians, reptiles, birds, and mammals.
Merits and Demerits of Five Kingdom Classification
The five-kingdom classification has certain merits and demerits. However, it is largely the most accepted system of modern classification mainly because of the phylogenetic placing of different groups of living organisms.
This system of classification looks more scientific and natural because of the following considerations:
• Separation of prokaryotes into an independent kingdom is justifiable because they differ from all other organisms in their general organization.
• Kingdom animalia has become more homogeneous by the exclusion of protozoa.
• Much coherance has been seen in the kingdom Plantae since the bacteria , fungi and some other algal forms have been
excluded from it.
• Some organisms like Euglena showing mixotrrophic mode of nutrition could be placed either in plant or animal kingdom easily.
However the creation of kingdom Protista including all unicellular eukaryotes has resoved the problem.
• Elevation of the group fungi to the status of a kingdom is justifiable since fungi totally differ from other primitive eukaryotes like algae and protozoans.
• The kingdoms Metaphyta and Metazoa are now more homogenous groups than they were in the two kingdom classification as it shows the phylogeny of different life styles.
• The five-kingdom classification gives a clear indication of cellular organization and modes of nutrition, the characters which appeared very early in the evolution of life. However, the five-kingdom classification has certain drawbacks also, particularly with reference to the lower forms of life.
• Kingdoms Protista and Monera still retain heterogeenosity, as both heterotroph and autotroph organisms with or without cell wall are included in both these kingdoms. The slime molds are different from other protista with which they have been combined.
• Multicellular green algae cannot be phylogenetically separated from unicellular algae. Therefore, unicellular Chlamydomonas is placed in kingdom Plantae rather than Protista.
• Viruses and Lichens do not find any place.
• Red and Green algae are not related to any other member of Kingdom Plantae.
Six kingdom system of classification
In 1977, Carl Woese extended Robert Whittaker's Five Kingdoms to replace Kingdom bacteria with two kingdoms, Eubacteria and Archaebacteria. Archaebacteria differ from Eubacteria in their genetic transcription and translation processes (in Archaebactera, transcription and translation more closely resembled eukaryotes). These distinguising characteristics were shown by molecular genetic analysis.
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