The biological world is enormously diverse. There are so many varieties of living organisms on the earth, therefore it would be almost impossible to study them individually. Hence, there must be an approach that enables us to get an idea of a group of organisms by studying a particular individual or type. The basis of classification of living organisms includes complexity, level of organisation, number of cells, mode of nutrition, etc.

Taxonomy deals with the systematic arrangement, naming, and identification of organisms. Hence, it gives an idea of the range of diversity of organisms and puts light on their relatedness to different characteristics. What is the basis of the classification of these living organisms? Let’s read the article to know about the main features that are the foundation stones of the classification of living organisms.

What is Classification?

Definition: The arrangement of living organisms into a hierarchical series of taxonomic groups on the basis of similar and dissimilar characteristics is called biological classification.

What is the Need for Classification?

Classification of different organisms into the respective groups is essential because of the following reason:

1. Classification supports the systematic study of a wide variety of living organisms in a simple and easier way.
2. Classification is essential to understand the interrelationship among the different groups of organisms.
3. Classification establishes a base for the development of other biological sciences. For example, biogeography deals with the study of the geographical distribution of plants and animals. The basis of biogeography is completely dependent on the information supplied by the classification of organisms according to their habitat.

Early History of Classification

Different biologists classified living organisms according to their perspectives. The contribution of different biologists for the classification of living organisms can be discussed as follows:

1. Theophrastus often referred to as “Father of Botany”, classified plants based on their habitat, form, and texture into four categories namely, herbs, shrubs, and trees.
2. Aristotle had classified the organisms based on their habitat. The habitat was recognised as a criterion to group animals into the land, air, and water. In this system, quite diverse and unrelated animals were grouped together. For example, whales and fishes were put in a group of aquatic animals, likewise, birds and bats are placed together though are completely different in the body organisation and physiology.
   Aristotle also divided the animals into two groups, one having red blood (Enaima) and the others without red blood (Anaima).
3. Linnaeus used number, union, length, and certain other characters of stamens as the basis of the classification of plants. For example, he proposed Monandria (1 stamen), Diandria (2 stamens), Triandria, and Polyandria (3 and more stamens). It is an artificial system of classification in which very distantly related or superficial characters are considered as the basis of classification.
   Linnaeus also proposed binomial nomenclature and formalised the two-kingdom system of classification.
4. George Bentham and J.D. Hooker proposed the natural system of classification of seed plants. The classification is based on natural affinities among the organisms.
5. Another system of classification is the phylogenetic system that is based on the evolutionary sequence as well as the genetic relationship among the organisms. It was proposed by Engler and Prantl.

Basis of Biological Classification of Organisms in the Present Era

R.H. Whittaker, an American taxonomist, proposed a five-kingdom system of classification. It is the most suited system of classification of living organisms into five kingdoms, namely Monera, Protista, Fungi, Plantae, and Animalia. The following are the basis of the classification of living organisms into five kingdoms:

1. The complexity of Cell Structure: The cell is the fundamental unit of each living being. The complexity of cells mainly refers to the structure of the nucleus.
   i. The cells that have a well-defined nucleus (having a nuclear membrane, nucleoplasm, nucleolus, chromatin network, etc.) are called eukaryotic cells. The cells that lack a well-defined nucleus and membrane-bound cell organelles are called prokaryotic cells. They have genetic materially suspended in the cytoplasm.
2. Number of Cells: Organisms are classified into unicellular and multicellular organisms based on the number of cells. All living activities of a unicellular organism are performed by a single cell, whereas multicellular organisms exhibit division of labour and hence show a greater efficiency of work. In multicellular organisms, all cells are not of the same type, as they are modified according to their functions.
3. Mode of Nutrition: The organisms can be classified as autotrophs and heterotrophs based on their mode of nutrition. Autotrophs can synthesise their food by performing photosynthesis or chemosynthesis. Heterotrophs cannot synthesise their food hence obtain food from other sources. Plants are autotrophs (except a few insectivorous plants), and animals are mainly heterotrophs.
4. Level of Organisation: The level of organisms show the following hierarchy:
   Cell ? Tissue ? Organ ? Organ System ? Organism
   i. The simplest organisms have a cellular level organisation (the body is made up of one or more cells). E.g. Poriferans.
   ii. Some organisms exhibit tissue-level organisation. The body of these organisms comprises a group of cells called tissues. These organisms exhibit division of labour. E.g. Cnidarians.
   iii. Complex organisms, such as plants, advanced invertebrates, vertebrates, exhibit the organ and organ system level of organisation.
   iv. Most complex organisms like humans have a digestive system, respiratory system, excretory system, reproductive system, nervous system, and endocrine system for performing the function of digestion, respiration, reproduction, excretion, and coordination, respectively.
5. Phylogenetic Relationship: Phylogenetic relationship refers to the evolutionary and genetic relationships among organisms. It reflects the phylogeny or pattern of descent of the group. Each group is believed to have evolved from a single common ancestor, and the subunits used in the classification of the group are supposed to reflect the evolutionary branches that diverge from one another during the history of the group.
6. Reproduction Type: The ability to produce offspring from their parents is called reproduction. Reproduction is of two types, namely, asexual and sexual reproduction. Most primitive organisms preferably reproduce through asexual means. Such as, Amoeba reproduce through fission. Advanced invertebrates and all vertebrates have sex organs and reproduce sexually, involving the fusion of gametes.
   Plants reproduce asexually through vegetative parts and also exhibit sexual reproduction through the flower.

Fig: Overview of Classification of Organisms

Position of Living Organisms in Five Kingdoms Respective to Basis of Classification

The above-explained basis of classification truly justifies the position of different organisms into the five kingdoms. This can be described as follows:

1. Kingdom Monera: This kingdom is single-celled, prokaryotic organisms. Such as bacteria. They exhibit autotrophic (photoautotrophic or chemoautotrophic) or heterotrophic modes of nutrition.
2. Kingdom Protista: This kingdom comprises unicellular, eukaryotic organisms. The organisms can be classified into photosynthetic protists, slime moulds, and protozoan protists.
3. Kingdom Fungi: This kingdom comprises multicellular (a few are unicellular) organisms that are saprophytes (a type of heterotrophic nutrition).
4. Kingdom Plantae: This kingdom includes all plants that have tissue or organ level organisation and exhibits an autotrophic mode of nutrition. However, it also includes certain algae that are unicellular.
5. Kingdom Animalia: The kingdom includes all animals that have cellular, tissue, organ or organ system level organisation. Advanced animals like mammals have a highly organised body to perform several functions. They exhibit a heterotrophic mode of nutrition.
   Phylogenetically, kingdom Protista links the prokaryotic Monera with complex, eukaryotic multicellular kingdoms Plantae, Fungi, and Animalia.

Fig: Basis of the Five-Kingdom System of Classification

Basis of Classification of Animals

Animals are more complex organisms than plants. Hence along with the basic characteristics, the following characteristics also play a significant role in the further classification of animals:

1. Body Plan: There are three basic body plans into which all animals fit.
   i. The cell aggregate plan refers to the simplest organisms that exhibit clusters of cells as the whole body. There is no coordination between the cells of clusters. Examples are poriferans.
   ii. The body sac plan is exhibited by coelenterates and flatworms. The animals that fit into this plan have a digestive cavity with only one opening to the outside. The cells are more specialised in the division of labour.
   iii. Tube-within-a-tube-plan is exhibited by more complex organisms that have a digestive system with two openings, one for ingestion and another for egestion. Annelids, arthropods exhibit this type of body plan.
2. Body symmetry: Animals either show an asymmetrical body or symmetrical body. Asymmetrical animals like sponges cannot be divided into two equal halves by any plane of dissection. Symmetrical animals can be divided into two equal planes. These are further categorised into two types:
   i. Radially symmetrical animals: The body can be divided into two equal halves by cutting through any plane. For example, sea anemones, jellyfishes, comb jellies, etc., have radial symmetry.
   ii. Bilaterally symmetrical animals: The body can be divided into two equal halves in only one plane. For example, flatworms, insects, chordates, etc., have bilateral symmetry.
3. Diploblastic and triploblastic organisation: The body of diploblastic animals originates from ectoderm and endoderm. These germ layers remain separated by gelatinous mesoglea. Coelenterates are diploblastic animals. Whereas ectoderm, endoderm, and mesoderm give rise to the body of triploblastic animals.
   Coelenterates and Ctenophores are diploblastic animals. Flatworms, roundworms, Annelids, arthropods, echinoderms, molluscs, and chordates are triploblastic animals.
4. Segmentation: In certain groups of animals, the body is divided into segments that show the serial repetition of parts. It is called metamerism. Annelids, arthropods, and vertebrates (in the embryonic stage) exhibit true metamerism.
5. Body cavity or Coelom: The body cavity formed by the lining of mesoderm in animals with a tube-within-a-tube type of body plan is called coelom.
   i. The animals that lack coelom are called acoelomate (flatworms).
   ii. The animals that have a body cavity that is not lined by mesoderm are called pseudocoelomate (roundworms).
   iii. The animals having a true coelom that arises as a cavity in embryonic mesoderm are called coelomates (annelids, arthropods, molluscs, echinoderms, and chordates).
6. Heterotrophic mode of nutrition: Animals cannot synthesise their food and are called heterotrophs. The heterotrophs may be herbivores, carnivores, omnivores, parasites, saprophytes based on their feeding habits.
7. Reproduction is also the basis of the classification of animals. Primitive organisms reproduce through asexual methods such as fission, budding. Advanced invertebrates and vertebrates reproduce through sexual means.

Fig: Outline of Classification of Animals

Summary

Biodiversity refers to the huge variety of living organisms on the earth. More than one million species of organisms have been identified so far. It is almost impossible to study each individual species in detail. Hence, there is a need for the classification of organisms into different groups.

The organisms are classified into different kingdoms based on the similarities and dissimilarities of various characteristics. Cell structure, number of cells, mode of nutrition, body organisation, mode of reproduction and evolutionary history are the few characteristics that constitute the basis of the classification of living organisms.

FAQs on Basis of Classification of Living Organisms

Q.1. What is the basis of the classification of living organisms?
Ans: Cell complexity, number of cells, level of organisation, mode of nutrition, mode of reproduction and phylogenetic relationship are the basis of the classification of living organisms.

Q.2. What is called biological classification?
Ans: The method of arranging organisms into groups based on similarities and dissimilarities among the organisms is called biological classification.

Q.3. What are the advantages of classification?
Ans: 1. It makes the systematic study of a wide variety of organisms easier.
2. It helps to understand the interrelationship among the organisms.
3. It provides the foundation for the advancement in the other branches of science such as medicine, agriculture, etc.

Q.4. Who is the father of biological classification?
Ans: Carolus Linnaeus is the father of biological classification.

Q.5. Who is the father of the five-kingdom classifications?
Ans: Robert Harding Whittaker is the father of the five-kingdom system of classification.