Classification of Plants and Animals

The term Biodiversity was coined by Walter  G Rosen. It actually means the diversity of life forms found in a particular region. A need to classify this diverse life forms into groups is felt from time to time.

The method of arranging organisms into groups on the basis of their similarities and differences is called classification.

Advantages of classification are as follows:

(a)  It gives information regarding the existing diversity of plants and animals.
(b)  It makes the study of wide variety of organisms convenient.
(c)  It helps us to understand the pattern of evolution.
(d)  It forms the basis for the development of other biological sciences like Ecology, Biogeography, Biochemistry and other disciplines of Biology.

BASICS OF CLASSIFICATITON

In order to classify a broad group of organisms, we have to look for ways in which some of them are similar enough so as to be kept together. Such details of appearance, behaviour, form and function are called characteristics. Some  of the characteristics used for hierarchical classification are given below:

The Complexity of Cell Structure
The complexity of cell structure matters at first, i.e. whether the cell is prokaryotic or eukaryotic. In a eukaryotic cell, the membrane-bound organelles are present along with a nucleus. It efficiently allows isolated cellular processes to be carried out. On the contrary, in a prokaryotic cell,     the membrane-bound organelles and a membrane-bound nucleus are absent. Therefore, the biochemical pathways are organised in different ways in both cells. It facilitates different modes of cell division and the ability to form multicellular organisms. This difference in cell structure forms the basic characteristic of classification.

The Body Structure
The organisms can be found as a single cell (unicellular)  or group of cells (multicellular). The body structure changes the way the cell works, e.g. a multicellular organism like a worm shows the division of labour by forming tissues, organs and organ systems. Unicellular organisms like Amoeba carry out all its functions through its single cell.

Mode of Nutrition
According to this characteristic, organisms are of two types, i.e. autotrophic and heterotrophic. Autotrophic organisms can make their own food through the process of photosynthesis  (e.g. plants). Heterotrophic organisms depend on autotrophs and other animals for their food (e.g. animals and fungi).

Levels of Organisation of Body
The level of body organisation is different in different organisms. Like plants have different types of tissues as compared to animals. This is because plants require anchorage, while animals require mobility.

Functional Organisation
As the structure and the body design vary, the function of living organisms also shows changes, for e.g. in Amoeba, the pseudopodia are present for locomotion, while in higher animals, legs are present. Humans can run, but plants and trees cannot. This is the result of the organisation of different organs and organ systems.

HIERARCHY OF CLASSIFICATION

A system of arrangement for classification in which organisms are placed in order of logical sequence is called the hierarchy. A unit of classification of organisms which represents a  definite category at any level of classification is called taxon, e.g. phylum.

Biologists, Ernst Haeckel (1894), Robert Whittaker (1959) and  Carl Woese (1977) classified all organisms into broad categories called kingdoms. Whittaker’s classification of organisms has five kingdoms. They are Monera, Protista, Fungi, Plantae and Animalia.

Woese introduced Archaebacteria  (or Archaea) and Eubacteria (or Bacteria) by dividing kingdom-Monera.

Classification  is done by naming the sub-groups at various levels as given in the scheme that follows:

Classification Basics
Classification Basics

An idea of the taxonomic categories is given below:

Species
It is the basic unit of classification. It is a group of individuals, capable of interbreeding among themselves to give birth to fertile offspring e.g. horse (Equus cables) and ass (E.  asinus) are different species.

Genus
It consists of closely related species ranked higher than species, e.g. dog and wolf belong to the same genus, i.e. Canis.

Family
Group of related genera having several common characters to form a family, e.g. cat and lion belong to the same family-Felidae.

Order
The assemblage of a number of families having few common characters, e.g. tiger and wolf belong to the same order-Carnivora.

Class
Organisms of related order or orders having similarity are placed together in a class, e.g. rats, camels and monkeys are all included in the same Class-Mammalia.

Phylum
The different classes having some specific characters, which are common in them, are placed under the same phylum, e.g. phylum-Chordata includes animals having a notochord.

Kingdom
The highest category of taxonomic study consists of organisms distinguishing in a set of common characters. Under a kingdom, the organisms having the same fundamental characteristics are grouped together, e.g. kingdom-Plantae and Animalia.

Note   (i) The science dealing with the identification, nomenclature and classification of organisms is called taxonomy.
             (ii) As we go upwards from species to kingdom, the number of similar characters decreases.

FIVE KINGDOM CLASSIFICATION

Whittaker classified organisms  into five kingdoms on the basis of the following three levels of organisation:

(i) Cell structure (prokaryotic or eukaryotic)
(ii) Modes   and source   of nutrition  (autotrophic or heterotrophic)
(iii) Body organisation (unicellular  or multicellular)

Let us explore the five kingdoms.

1. Monera [Gk. moneres-single]

 The characteristics of  kingdom-Monera are as follows:

(a) These organisms are unicellular prokaryotes.
(b) They do not have a well-defined nucleus and membrane-bound cell organelles like mitochondria, Golgi apparatus, plastids, lysosomes, etc., as found in eukaryotes.
(c) Mode of nutrition can either be autotrophic or heterotrophic.
(d) Some of the organisms have cell-wall (i.e. bacteria and blue-green algae), while others lack cell-wall (i.e. Mycoplasma).

Example– Mycoplasma, Escherichia coli and  blue-green algae or cyanobacteria (Anabaena and Nostoc).

Monera
Monera

2. Protista [Gk. protistos– primitive or the very first]

The  characteristics of kingdom-Protista are as follows:

(a) This group consists of unicellular eukaryotes.
(b) A true nucleus is present in them. Membrane-bound cell organelles-like mitochondria, plastids and sap vacuoles are also present.
(c) Some protists use appendages, such as hair-like cilia (Paramecium),  pseudopodia (Amoeba) or whip-like flag all (Euglena) for locomotion.
(d) Mode of nutrition can be autotrophic (Euglena) or heterotrophic (Paramecium).

Example– unicellular algae  (Euglena), diatoms (Navicula), protozoans (Amoeba and   Paramecium) etc.

paramecium
Amoeba
Euglena

3. Fungi [Lt fungus– mushroom]

The  characteristics of kingdom-Fungi are as follows:

(a) These are heterotrophic and eukaryotic organisms.
(b) They may be unicellular (yeast) or filamentous (most fungi).  Many of them have a capacity to become multicellular organism at certain stages of their lives.
(c) Some are parasitic (depends on the host for their food)  such as Puccinia, Albugo and Ustilago. Some are saprophytes use a decaying organic material as food, such as Mucor and Agaricus.
(d) The cell wall is made up of a rough complex sugar called chitin or fungal cellulose e.g. yeast, Aspergillus, Penicillium and mushrooms (Agaricus).
(e) Some fungi live in permanent, mutually dependent relationship with blue-green algae (or cyanobacteria). This relationship is known as symbiosis. These symbiotic life forms are called lichens or dual organisms. These are seen as slow-growing large coloured patches on tree barks.
(f) Algae provide the fungus with food whereas fungi provide them shelter.

Fungi
Fungi

4. Plantae

The characteristics of kingdom-Plantae are as follows:

(a) These are multicellular eukaryotes with cell walls made up of cellulose.
(b) Growth is unlimited and continues till death.
(c) They are always fixed at one site and cannot move like animals.
(d) Their mode of nutrition is autotrophic. They produce their own food by a process called photosynthesis in the presence of light, with the help of chlorophyll.

5. Animalia

The characteristics of Kingdom-Animalia are as follows:

(a) These are multicellular eukaryotes without cell wall.
(b) They have definite shape and size.
(c) Most of them can move easily from one place to another.
(d) Their mode of nutrition is heterotrophic (i.e. depend on autotrophs or plants for their food).

NOMENCLATURE

Nomenclature is a system of assigning scientific names to the organisms listed in any classification

It is difficult for people speaking or writing in different languages to understand that they are talking about the same organism. The same organism is known by various names in different parts of the world. Common names cannot be used by scientists all over the world. Therefore,  a need of a system arisen through which a single name can be given to a particular organism that would be universally used.

Binomial Nomenclature
The system of scientific naming or nomenclature that we use today is known as binomial nomenclature.  It is comprised of two words. The first word represents the genus. The second word represents the species of that particular organism.

Thus,  the binomial nomenclature is represented as a generic name and a specific name. All over the world, it has been agreed that both these names will be used in Latin forms. This system of nomenclature was first introduced by Carolus  Linnaeus. A scientific name for an organism is thus, unique. It can be used to identify it anywhere in the world. It is the result of the process of classification which puts it along with the organisms, it is most related to.

Rules and Norms for Writing the Scientific Names

(i)The name of the genus begins with a capital letter.
(ii)The name of the species begins with a small letter.
(iii) When printed, the scientific name should always be given in italics.
(iv)The genus name and the species name have to be underlined separately when handwritten, e.g. scientific name of mango is Mangifera indica.

Note: Carolus  Linnaeus in his book entitled  ‘Systema Naturae’ in the year 1735 first used the system of binomial nomenclature. He is known as the Father of Taxonomy (Science of classification).

Some examples of scientific names for common Animals:

Common Name to Scientific Name
Common Name to Scientific Name_Vegetables
Common Name to Scientific Name of Vegetables and Fruits

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