Systems of Classification –Types of Taxonomic system & Kingdom System Classification

Most biologists estimate that there are between 3 million to 30 million species on the earth. Out of which, about 1.7 million species of living organisms have been identified, described, and given scientific names. Animals constitute about 1.2 million and plants constitute about 0.5 million, and the remaining may be microbes and other organisms.

Every year about 15,000 new organisms are discovered. It is almost impossible to study each individual. Hence many attempts have been made to develop systems of classification to classify living organisms since ancient times. Taxonomy is the branch of science that deals with the practice of describing, naming, and classifying organisms. Let’s read the article to study the various systems of classification proposed by different taxonomists.

Definition of Classification

Classification is the arrangement of organisms into different groups on the basis of their similarities and dissimilarities. The grouping is done from less inclusive to more inclusive groups until the organisms are assembled into a final most inclusive group.
The sequence of classification follows a fixed system of hierarchy of categories. The classification process includes assigning appropriate positions and ranks to a new taxon. These ranks are listed as follows:

Learn About Phylum Porifera Here

Why is Classification Required?

The classification of living organisms is needed because of the following reasons:

1. It is not sufficient to study one or more organisms to know the essential and specific features of the group.
2. Classification helps in knowing the relationship among the different groups of organisms.
3. It helps in studying the evolutionary relationship between the organisms of different groups.

Types of Classification in Taxonomic System

Taxonomists have devised different systems of classification for living organisms from time to time. These systems can be categorised into three main types:

1. Artificial System of Classification
   A. Basis of artificial classification: The artificial system of classification is based on one or a few superficial and observable characteristics. This system of classification was common before and also during the period of Linnaeus. The chosen characteristics maybe those of habit, habitat, phenology, number of plant parts (cotyledons), floral characters (particularly the number of stamen and carpels).
   B. Scientists contribution: A number of scientists followed the artificial system of classification of different living organisms. A few can be discussed as follows:
   i. Theophrastus (370-285 BC), often referred to as ‘Father of Botany’, classified plants on the basis of their habitat, form, and texture into four categories: trees, shrubs, under shrubs, and herbs.
   ii. Aristotle (384-322 BC) arranged animals into four major groups like insects, birds, fishes, and whales.
   Aristotle also divided animals into two groups, one having red blood (Enaima) and the others without red blood (Anaima).
   iii. John Ray (1627-1705) divided plants into dicotyledons and monocotyledons based on the number of cotyledons in the seed.
   iv. Carolus Linnaeus’ system was based on just a few reproductive features. He used number, union, length, and certain other characters of stamens as the basis of the classification of plants. Based on these characteristics, he proposed classes as Monandria (1 stamen), Diandria (2 stamens), and Triandria and Polyandria (3 or more stamens).
   

1. C. Shortcomings of an artificial system of classification:
   i. This system is not universally applicable since quite diverse and unrelated animals were grouped together. For example, whales and fish were put in an aquatic group, bats and birds were put together.
   ii. Closely related organisms which are not similar in all aspects except one or two are separated. For example, whales and bats have all the core features of mammals but are placed in separate groups, respectively, due to their habitat and flying features.
   iii. This system fails to give an idea about natural and phylogenetic relationships amongst different taxa.
2. Natural System of Classification
   A. Basis of the natural system: The natural system of classification is based on natural affinities. It also uses more number of characters rather than a single character for determining the similarities.
   B. Scientists contribution: The following scientists contribute to the natural system of classification:
   i. A.L. de Jussieu (1748-1836)classified the plants on the basis of the number of cotyledons along with the presence and absence of petals and position of petals and stamens.
   ii. A.P. Cndole classified the plants on the basis of the characteristics and arrangement of vascular tissues.
   iii. The most important and the last of the natural systems for classification of seed plants was proposed by George Bentham and Joseph Hooker (1862-1863). This work was published in ‘Genera Plantarum’.
   C. Advantages of the natural system of classification
   i. It establishes natural relationships amongst organisms and the members that are closely related are placed in a group together.
   ii. It can be used for the remarkable prediction of rank and categories of newly identified plant species.
   iii. It contains more exclusive information about taxonomic hierarchy.
   D. Disadvantages of the natural system of classification
   i. The complexity level of classifying organisms is higher than the artificial system.
   ii. Less known plants cannot be classified through this system since their characteristics are not fully explained.
   iii. A single property of interest cannot be taken into consideration for classification.
3. Phylogenetic System of Classification
   A. Basis of the phylogenetic system: This system of classification is based on the evolutionary relationships and genetic relationships amongst organisms. In such a system, organisms belonging to the same group are believed to have evolved from common ancestry and may be represented in the form of a family tree called a cladogram.
   B. Phylogenetic vs Natural & Artificial system
   i. The phylogenetic system differs from the natural and artificial systems of classification because it not only considers one or a few characteristics for classifying organisms but along with the number of significant characteristics, the evolutionary relationships are also taken into consideration.
   ii. Moreover, the phylogenetic system is not only used for the identification of organisms. Instead, it is used for studying relationships with other organisms. Hence it does not attempt to rank the organism.
   C. Scientists Contribution
   i. Darwin’s theory of Origin of Species is the basis of the phylogenetic system of classification.
   ii. Engler and Prantl (1884-1930)arranged families and orders of the flowering plants in ascending series based on the complexity of floral morphology.
   iii. The characters like one whorl of perianth or no perianth, unisexual flowers, and pollination by winds were considered to be primitive compared to perianth with two whorls, bisexual flowers and pollination by insects.
   iv. Monocots are more primitive than dicots.
   v. As a whole, this system considered the evolution of angiosperms from a single source, and the sequence of order and families show parallel evolution.
4. Phenetic system of classification: It is based on numerical methods for the assessment of similarities and differences between the species. In this system, as many traits as possible are used for these comparisons, without extra importance being given to anyone. Marvin Takhtajan (1980), Arthur Cronquist’s system and Dr Rolf Dahlgren’s system (1983) are based on numerical methods of classification that are represented by a dendrogram. A dendrogram is a branching tree that represents the taxonomic and evolutionary relationships between organisms.

Types of Kingdom Classification System

Organisms are beings classified into the different kingdoms that constitute two, three, four, five, and six kingdom systems of classification in a hierarchical manner. These kingdom classification systems can be discussed as follows:

1. Two-Kingdom System of Classification: Linnaeus divided all the living organisms into two kingdoms in 1758. These kingdoms and their divisions can be mapped as follows:

A. Features of kingdom Plantae and Animalia

B. Shortcomings of the two-kingdom system of classification: Two kingdom system of classification was accepted for a long time. However, some difficulties arose from this classification with the discovery of new organisms. Some of these difficulties are as follows:
i. The first formed bacteria were not given a proper place in two-kingdom systems of classification.
ii. Eukaryotes and prokaryotes have different nuclear structures, therefore, they cannot be placed together in a group.
iii. It is not easy to recognise lower organisms as plants or animals. For example, Euglena has features of both plants and animals. It has chloroplast and follows a mixotrophic (dual) mode of nutrition. Also, it exhibits locomotion like animals by twisting and turning its flagellum.
iv. Fungi do not show structural, physiological, and reproductive similarities with other plants. Therefore they cannot be placed in kingdom Plantae.
v. Lichens are included amongst plants despite being dual organisms where there is a symbiotic association between fungus and green algae.
vi. Tunicates resemble plants in having cellulose branching and anchorage.
vii. Sponges and corals are fixed like plants and superficially resemble them in being irregular and slow to react.

Fig: Two-Kingdom System of Classification

2. Three Kingdom System of Classification: Ernst Haeckel classified living organisms into three kingdoms in 1866. These are named Protista, Plantae, and Animalia. Protista included all those organisms that lack the capability of tissue differentiation. These are algae, protozoans, bacteria, and slime moulds. Later kingdom Protista was reserved only for unicellular organisms.
Limitations of the three kingdom system of classification
i. Prokaryotes and eukaryotes were not separated.
ii. Both unicellular and multicellular organisms were placed together.
iii. Viruses are not included in this system as they show the characteristics of both living or nonliving.

Fig: Three-Kingdom System of Classification

3. Four Kingdom System of Classification: Copeland added one more kingdom called Monera to the three kingdoms explained by Haeckel. The prokaryotes that have primitive nuclei are placed in the kingdom Monera. This system of classification was proposed in 1956.
The drawback of the four kingdom system of classification
i. Fungi are placed with plants. However, they are saprophytic in nature.
ii. The position of viruses and symbiotic organisms, for example, lichens, is not justified in this system of classification.

Fig: Four-Kingdom System of Classification

4. Five-Kingdom System of Classification: R.H. Whittaker, an American taxonomist, proposed a five-kingdom system of classification in 1969. In this system, the separation into five kingdoms is made on the following criteria:
i. The complexity of cell structure
ii. The complexity of body structure
iii. Mode of nutrition
iv. Reproduction
v. Phylogenetic relationship
The five kingdoms are Monera, Protista, Fungi, Plantae, and Animalia. The comparative study of characteristics of these kingdoms can be summed up as follows:

A. Advantages of five-kingdom classification
i. It exhibits a phylogenetic relationship between diverse groups.
ii. The separation of prokaryotes in a separate kingdom of Monera is fully justified because they differ from all other organisms in their structural organisation.
iii. Inclusion of unicellular eukaryotes into an independent kingdom Protista has removed irregularity to putting some of the eukaryotes in plants and animals.
iv. Fungi being grouped in a separate kingdom is justified as they have a heterotrophic mode of nutrition.
v. Kingdom Animalia has become more homogenous with the exclusion of protozoa.
B. Limitations of five-kingdom classification
i. Monera and Protista are heterogeneous groups. Both include photosynthetic and heterotrophic forms and also with and without a cell wall.
ii. Both unicellular and multicellular algae have been placed under kingdom Plantae.
iii. The three higher kingdoms (Fungi, Plantae, and Animalia) are polyphyletic because several multicellular lines originated from Protists.
iv. Viruses have not been given any place in the classification.

Fig: Five-Kingdom System of Classification

5. Six-Kingdom System of Classification: This system of classification was proposed by Carl Woese in 1978. It is also named a three-domain system as the organisms are classified into three domains: Archaea, Bacteria, and Eukarya.
i. Eubacteria include prokaryotes that contain peptidoglycan in their cell wall. For example, gram (+)ve and gram (-)ve bacteria, mycoplasma.
ii. Archaea are characterised by possessing cell walls without peptidoglycans and includes three kingdoms; Methanogens, Halophiles, and Thermoacidophiles.
iii. The third domain Eukarya includes all eukaryotes placed by Whittaker in the kingdoms- Protista, Fungi, Plantae, and Animalia.

Fig: Three-Domain System of Classification

Summary

To deal with the enormous diversity in the living world, the scientific arrangement of living things into groups is required on the basis of common characteristics. This arrangement of organisms is called classification. External and internal structures along with the structure of cell, ecological and evolutionary information form the basis of modern taxonomic and systematic studies. The workings of several taxonomists proposed three main systems of classification namely artificial system, natural system, and phylogenetic system.

The study of these main categories leads to classifying the organisms into different kingdoms based on their similarities. Earlier Linnaeus proposed two kingdoms for classifying the organisms into plants and animals which is further being improved by different taxonomists and later a five-kingdom system of classification was proposed by R.H. Whittaker that justifies the position of prokaryotes in Monera, eukaryotes in Protista, Fungi, Plantae, and Animalia.

Frequently Asked Questions (FAQs) on Systems of Classification

Q.1. What are the three systems of classification?
Ans: The three systems of classification are as follows:
1. Artificial system of classification
2. The natural system of classification
3. The phylogenetic system of classification.

Q.2. What is the importance of classification?
Ans: Classification is the method of arranging organisms into different groups on the basis of similarities and differences, therefore makes the study of a wide variety of organisms easier.

Q.3. What is the basis of classification?
Ans: The basis of classification is as follows:
1. The complexity of the cell
2. The complexity of the body structure
3. Mode of nutrition
4. Reproduction
5. Phylogenetic relationship

Q.4. Who proposed the natural system of classification?
Ans: The natural system of classification was the latest proposed by Bentham and Hooker.

Q.5. Who proposed the phylogenetic system of classification?
Ans: Ernst and Prantl proposed the phylogenetic system of classification.

Learn About Six Kingdom Classification Here

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