Chapter 7 Diversity in Living Organisms

Organisms are classified into different groups in order to identify any specific organisms i,e for easier and convenient study of different organisms. At a time we can study the characteristics of a large number of organisms and the relationship between them.

Examples of range of variations observed in daily life are:

(i) Variation in size: size of organisms can vary from microscopic bacteria to giant redwood trees (about 100 metres).

(ii) Variation in lifespan: lifespan varies from organisms to organisms i.e for bacteria it is of few hours whereas in case of large trees it can be thousands of years .

(iii) Variation in body colour: most of the organisms (insects, flowers, birds)  show a vast range of colours to attract and protect from predators.

The more basic characteristic for classifying organisms is the kind cells they are made of because there are a number of organisms which are living in a common habitat but they belong to different taxonomic groups. Organisms can be classified into eukaryotes (protista, fungi, plants and animals) and prokaryotes (monera) on the basis of cell type.

The primary characteristic on which the first division of organisms is made is the nature / kind  of the cell - prokaryotic and eukaryotic cells. Nature of the cell includes the presence or absence of membrane-bound organelles. Therefore, on the basis of this fundamental characteristic, we can classify all living organisms into two broad categories of eukaryotes (protista, fungi, plantae, animalia) and prokaryotes (monera). Then, further classification is made on the basis of cellularity or modes of nutrition, sexual reproduction and phylogenetic relationship.

Plants and animals are put in different categories on the basis of whether the organisms produce their own food through the process of photosynthesis (plants). But, locomotion is considered as the characteristic feature that separates animals from plants. This is because the absence of locomotion in plants gave rise to many structural changes such as the presence of a cell wall (for protection), the presence of chloroplasts (for photosynthesis) etc.

Primitive organisms are those  which have a simple body structure (lower organisms) and ancient body design  that have not changed much. The organisms that have acquired more complex structure and body design are known as advanced organisms or higher organisms. For example, an Amoeba is more primitive as compared to a starfish. Amoeba has a simple body structure and primitive features as compared to a starfish. Hence, an Amoeba is considered more primitive than a starfish.

The advanced organisms will be the same as complex organisms . It is not always true that an advanced organism will have a complex body structure. This is because there is a possibility that over the evolutionary time, complexity in body design will increase.

The criterion for the classification of organisms belonging to kingdom Monera or Protista is the presence or absence of a well-defined nucleus. Kingdom Monera has no nuclear membrane and these are known as prokaryotes while kingdom. Protista shows well defined nucleus and these organisms are called eukaryotes.

Protista includes single celled, eukaryotic, and photosynthetic organisms.

In the hierarchy of classification, the grouping that includes the smallest number of organisms with a maximum of characteristics in common is species, whereas the kingdom will have the largest number of organisms.

Thallophyta or algae is the division of plants that has the simplest organisms. This group includes plants, which do not contain a well differentiated plant body. Their body is not differentiated into roots, stems, and leaves.

Therefore, classification serves the following advantages:

(i) It determines the evolutionary relationship by establishing the gradually increasing complexity of form and structure in different groups of organisms .

(ii) It helps in understanding millions of life forms in detail.

(iii) It also helps in the development of other life sciences.

For classifying an organism in a hierarchical model, we choose the fundamental characteristic among several other characteristics. For example, plants differ from animals in having chloroplast and lacking locomotion. But, only locomotion is considered as the basic or fundamental feature that is used to distinguish between plants and animals. This is because the absence of locomotion in plants gave rise to many structural changes such as the presence of a cell wall for protection, and the presence of chloroplast for photosynthesis.

The five kingdoms proposed by R.H. Whittaker are Monera, Protista, Fungi, Plantae, and Animalia.

The basis for grouping organisms into five kingdoms is as follows:

(i) On the basis of Cell type, all living organisms are divided into two broad categories of eukaryotes and prokaryotes . This division led to the formation of kingdom Monera, which includes all prokaryotes.

(ii) On the basis of body organisation, organisms can be classified into cellular tissue, organ and organ system level. Unicellular eukaryotes form kingdom Protista, and multicellular eukaryotes form kingdom Fungi, Plantae, and Animalia.

(iii) Animals are then separated on the basis of presence or absence of a cell wall.

(iv) On the basis of mode of nutrition , since fungi and plants both contain a cell wall, they are separated into different kingdoms. Fungi have heterotrophic mode of nutrition, whereas plants have autotrophic mode of nutrition. This results in the formation of the five kingdoms.

The kingdom Plantae is divided into five main divisions: Thallophyta, Bryophyta, Pteridophyta, Gymnosperms, and Angiosperms.

The classification depends on the following criteria:

• Differentiated / Undifferentiated plant body

• Presence / absence of vascular tissues

• With/without seeds

• Naked seeds/ seeds inside fruits

(i)  Thallophyta : these are simple , thalloid and undifferentiated body parts. They do not have roots, stems and leaves.

(ii)  Bryophyta : their plant body is more differentiated and has root ,stem and leaf-like body parts. They do not have vascular tissue.

(iii)  Pteridophyta : they have true root, stem and leaf. They possess well differentiated vascular tissue.

(iv)  Gymnosperm : the plants in which ovules are not enclosed by any ovary wall i.e naked - seeded plants.

(v) Angiosperm : seeds are enclosed in fruits.

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Animals in Vertebrates are classified into five classes:

(i) Class Pisces: This class includes fish such as Scoliodon, tuna, rohu, shark, etc. These animals mostly live in water. Hence, they have special adaptive features such as a streamlined body, presence of a tail for movement, gills, etc. to live in water.

(ii) Class Amphibia: The amphibians have adopted to live both on land and water. They respire with the help of gills, lungs and through skins. They are cold - blooded animals. They lay eggs and development through larval stages. e.g : Frog, Salamander etc.

(iii) Class Reptilia: The class name refers to their creeping or crawling mode of locomotion. The body of a reptile is covered with dry and cornified skin to prevent water loss . They are cold - blooded animals. They lay eggs on land. e.g : Snakes, Chameleon etc.

(iv) Class Aves: The characteristics features of aves are the presence of feathers. Most of them have feathers. Their forelimbs are modified into wings for flight, while hind limbs have scales modified for walking and clasping. They are warm blooded and lay eggs. e.g : Crow, Pigeon etc.

(v) Class Mammalia: The most unique mammalian characteristic is presence of milk producing glands by which the young ones are nourished. Their skin has hair as well as sweat glands to regulate their body temperature. e.g : Humans, Lions, Dogs etc.