Diversity in the Living World- Introduction, Systems of Classification

Diversity in the Living World can be experienced everywhere on the earth. Each different kind of plant, animal or organism that you see represents a species. The number of species that are known and described ranges between \(1.7\) to \(1.8\) million, which is referred to as biodiversity. Every individual is unique with respect to structure, body functions, genetic makeup, etc. Living organisms inhabiting different habitats have different structural organs or functions developed according to the conditions of the habitat. The warm and humid regions of the earth have diverse organisms and thus are known as the region of mega biodiversity.

What is Classification?

Since it is nearly impossible to study all living organisms, it is necessary to classify all the organisms into different categories according to their specific features. Thus, classification is defined as the process by which things are grouped into different categories based on some easily observable characteristics.
Biological Classification refers to the scientific arrangement of organisms in a hierarchy of groups or subgroups on the basis of similarities and differences in their traits.

What is the Need for Classification?

Classification of organisms is necessary for the following reasons:

1. Convenience of study
2. Knowledge of adaptations
3. Knowledge of phylogenetic relationships
4. Learning about the sequence of evolution
5. Discovery of new organisms

What is the Basis of Classification?

Classification of organisms is based on the following characteristics:

1. Presence of nucleus- Prokaryotic or eukaryotic
2. Body structure- unicellular or multicellular
3. Mode of nutrition- Autotrophic or Heterotrophic
4. Reproduction- Sexual or Asexual Reproduction
5. Phylogenetic Relationship

Fig: Diversity of the Living World

System of Classification

Biological classification refers to the process by which all living organisms are categorized in a hierarchical structure of groups and subgroups based on their features and environment. Scientists have been trying to classify all living organisms in various ways for centuries and have proposed their own model of classifying organisms. Some of these are given below:

1. Two Kingdom Classification: This classification system was proposed by Carolus Linnaeus, who divided the whole living world into two kingdoms, i.e., Plants and Animals.
2. Five Kingdom Classification: This classification system was proposed by R.H. Whittaker, who divided the whole living world into five kingdoms, i.e., Monera, Protista, Fungi, Plantae and Animalia.

Hierarchy of Classification

a) Classification is not a single-step process but involves a hierarchy of steps in which each step represents a rank or category.
b) Since the category is a part of the overall taxonomic arrangement, it is called the taxonomic category, and all categories together constitute a taxonomic hierarchy.
c) Each category, referred to as a unit of classification, represents a rank and is commonly known as a taxon.
d) Taxonomic study of all known organisms has led to the development of common categories such as Kingdom, Phylum or Division, Class, Order, Family, Genus and Species.
e) Of all organisms, including those in the plant and animal kingdom, species occupy the lowest category.

1. Species
   Taxonomic studies consider a group of individual organisms with fundamental similarities as a species. One species is distinguished from another species based on distinct morphological differences. Human beings belong to the species sapiens, which is grouped in the genus Homo. The scientific name thus for human beings is written as Homo sapiens.

2. Genus
   Genus consists of a group of related species which has more characters in common in comparison to species of other genera. We can say that genera are aggregates of closely related species. For example, potatoes and brinjal are two different species, but both belong to the genus Solanum.

3. Family
   The next category, the family, has a group of related genera with less number of similarities as compared to genus and species. Families are characterized on the basis of both vegetative and reproductive features of plant species. Among plants, for example, three different genera Solanum, Petunia and Datura are placed in the family Solanaceae.

4. Order
   The categories like species, genus and families are based on a number of similar characters. Generally, order and other higher taxonomic categories are identified based on the aggregates of characters. Order being a higher category is the assemblage of families which exhibit a few similar characters. The similar characters are less in number as compared to different genera, including in a family. Plant families like Convolvulaceae, Solanaceae are included in the family Polymoniales on the basis of floral characters.

5. Class
   This category includes related orders. For example, Primates consisting of monkeys, gorillas and gibbons are placed in Class Mammalia along with order Carnivora that include animals like tigers, cats and dogs. Class Mammalia has other orders also.

6. Phylum
   Classes composed of animals like fishes, amphibians, reptiles, birds, along with mammals, constitute the next higher category called Phylum. All these, based on the common features like the presence of notochord and dorsal hollow neural system, are included in Phylum Chordata. In the case of plants, classes with a few similar characters are assigned to a higher category called Division.

7. Kingdom
   All animals belonging to various phyla are assigned to the highest category called Kingdom. Kingdom Animalia is the classification system of animals. The Kingdom Plantae comprises all plants from various divisions.

Lower the taxon, more are the characteristics the members share within the taxon. The higher the category, the greater is the difficulty of determining the relationship to other taxa at the same level. Hence, the problem of classification becomes more complex.

Five Kingdom Classification

The five-kingdom classification was proposed by R.H. Whittaker \(\left( {1969} \right)\), who divided the whole living world into five kingdoms, namely Monera, Protista, Fungi, Plantae and Animalia.

The characteristics of the five kingdoms are as follows:

1. Kingdom Monera
   a) They are the most abundant microorganisms found almost everywhere.
   b) They are prokaryotic (lack a true nucleus) and unicellular.
   c) They also live in extreme habitats such as hot springs, deserts, snow and deep oceans where very few other life forms can survive.
   d) They may be heterotrophic or autotrophic.
   e) For example- Cyanobacteria, Bacteria, Archaebacteria, etc.
2. Kingdom Protista
   a) They are eukaryotic and unicellular.
   b) They are primarily aquatic organisms.
   c) They have a well-defined nucleus and other membrane-bound organelles.
   d) Some have pseudopodia, flagella or cilia for locomotion.
   e) They reproduce asexually and sexually by a process involving cell fusion and zygote formation.
   f) For example- Amoeba, Paramecium, etc.
3. Kingdom Fungi
   a) They show a great diversity in morphology and habitat.
   b) They prefer to grow in warm and humid places.
   c) They are multicellular and eukaryotic organisms.
   d) Most fungi are heterotrophic and absorb soluble organic matter from dead substrates and hence are called saprophytes. 
   e) Some of them depend on living plants and animals, called parasites, whereas someliveas symbionts.
   f) They have a cell wall made up of chitin.
   g) For example- Yeast, Aspergillus, etc.
4. Kingdom Plantae
   a) They include all multicellular, eukaryotic and chlorophyll-containing organisms commonly called plants.
   b) Few members are partially heterotrophic such as insectivorous plants or parasites.
   c) They have a cell wall mainly made up of cellulose.
   d) They include algae, bryophytes, pteridophytes, gymnosperms and angiosperms.
   e) The life cycle of plants has two distinct phases, i.e., the diploid sporophytic and the haploid gametophytic phases that alternate with each other which is called alternation of generation.
   f) For example-Pines, Mango, etc.
5. Kingdom Animalia
   a) They include multicellular, eukaryotic organisms that do not have chlorophyll of their own.
   b) They lack cell walls and directly or indirectly depend on plants for food.
   c) They digest their food in an internal cavity and store the reserve food material as glycogen or fat.
   d) Their mode of nutrition is holozoic- by ingestion of food.
   e) They follow a definite growth pattern and grow into adults.
   f) They show elaborate sensory and neuromotor mechanisms.
   g) For example- Earthworms, Frog, etc.

Summary

The living world is rich in variety. Millions of plants and animals have been identified and described, but a large number is still unknown. They vary in terms of size, colours, habitats, psychological and morphological features. In order to facilitate the study of kinds and diversity of organisms, biologists have laid down certain rules and principles for the identification, nomenclature and classification of organisms. The study of various species of plants and animals are useful in agriculture, forestry, industry and in general for knowing our bio-resources. 

The biological classification of plants and animals was first proposed by Aristotle on the basis of simple morphological characters. Linnaeus later classified all living organisms into two Kingdoms- Plantae and Animalia. Then later, Whittaker proposed an elaborate five-kingdom classification- Monera, Protista, Fungi, Plantae and Animalia. The main criteria of the five-kingdom classification were cell structure, body organization, mode of nutrition and reproduction and phylogenetic relationship.

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