Diversity In Living Organisms

We classify an organism because of the following reason:-

▪ There are a wide variety of living organisms on this earth, and they have great diversity in their physical appearance.

▪ The vast number of living organisms makes it very difficult to study them one by one.

Therefore, we look for similarities among the organisms, which will allow us to put them into different classes and then study different classes or groups as a whole. Thus, classification makes the study of an organism very easy.

Various kinds of variations in life forms such as:

1) Variation in terms of size: microscopic bacteria are few micrometre in size while, on the other hand blue whale and red wood trees of california are approximate 30 metres and 100 metres in size repectively.

2) Variation in terms of age: Pine trees live for thousands of years while insects like mosquitoes die within a few days.

3) Variation in terms of colours: Worm are colourless or even transparent, on the other hand birds and flowers are brightly coloured.

The primary characteristic on which the first division of organism is made is the nature of cell. It is a fundamental characteristic of classification. On the basis of nature of cell, all living organisms are classified into two categories- eukaryotic and prokaryotic. Eukaryotic cell has membrane-bound organelles and defined nucleus while prokaryotic cell does not have membrane-bound organelles and have primitive type of nucleus called nucleoid.

Plants and animals put into different categories on the basis of:

(i) Mode of nutrition: Plants can synthesise their own food, i.e., they are autotrophic while animals are heterotropic.

(ii) Movement: Plants are static, i.e., they can not move while animals can move.

(iii) Cell wall: Plants have cell wall made up of cellulose while animals do not have cell wall.

Primitive organism is one which has primitive or ancient body designs that have not changed very much over period of time. Whereas advanced organism has complex body structure and organisation. For example, Unicellular bacteria are primitive while multicellular eukaryotic mammalia are advanced organisms.

• There is a high possibility that advanced organisms would be similar to complex organisms.
• The reason for this is that with time, as the organism evolves, the internal cellular machinery may gradually become more complex.
• With evoltuion, beneficial traits get preserved in organisms, thus further increasing the complexity.

Kingdom Protista have unicellular, eukaryotic and photosynthetic organisms.

• The taxon "species" has the smallest number of organisms having maximum of characteristics in common.
• While taxon "kingdom" has the largest number of organisms with a maximum number of differences among its members.

Thallophyta is the division of plants that has the simplest organisms. The organisms of this division are commonly called algae.

Pteridohyta have naked embryo called spores and inconspicuous reproductive organs whereas phanerogams have well differentiated reproductive organs and covered embryo.

Differences between Porifera and Coelenterata:

Differences between Annelida and Arthropoda are:

Differences between amphibians and reptiles:

Differences between Aves and Mammalia:

The classification of organisms have the following advantages:

•Classification makes the study of wide variety of organisms easier and systematic.

•It reveals relationship among the various groups of organisms.

•Classification gives information about the organisms which do not occur in one's locality.

•It helps to establish a hierarchy of groups of organisms.

•It provides a base for the study of other branches of biology.

•Classification reveals evolution trends by showing gradually increasing complexity of structure in various groups of organisms.

To develop a hierarchy in classification, we choose the fundamental characteristics among several other characteristics are as follow:-

(1) This fundamental or basic characteristic helps to make the broad division.

(2) Then we should pick up next set of characteristics for making subgroups.For example, nature of cell and form of the body is considered to classify organisms into broad divisions (e.g., Prokaryotes and Eukaryotes).

(3)The characteristics of the next level should be dependent on the previous one that will decide the subsequent divisions of the groups.

(4) This process must continue and each time new characteristics should be used.

The basis for grouping of organisms into five kingdoms are:

1) The complexity of cell structure: It has two major cellular divisions within living things: Prokaryotic and Eukaryotic. Thus, two broad groups can be formed- one having prokaryotic cell structure and the other having eukaryotic cell structure.

2) Some cells: It divides living organisms into unicellular and multicellular.

3) Mode of nutrition: Organisms have two types of nutrition- autotrophic and heterotrophic

4) Based on the organisms that perform photosynthesis (plants), what is the level of organization of their body, that is unicellular and multicellular

5) the animals, how does the individual’s body develop and organize its different parts, and what are the specialized organs found for different functions

The major division in the Kingdom Plantae are-

1. Thallophyta (Algae); 2. Bryophyta; 3. Pteridophyta; 4. Gymnospermae; 5. Angiospermae.

The division is based on the following features:

(a) Well-differentiated plant body: The first level of classification of plants is based on the presence and absence of well-differentiated plant body. Algae (Thallophyta) have simple and less differentiated plant body. Hence, they are separated from rest of the plants.

(b) Vascular tissue: The next level of classification is based on the presence and absence of vascular tissues (i.e., xylem and phloem). Bryophtes do not have vascular tissue, hence they are separated from the rest of the plants.

(c) Ability to bear seeds: The further classification is based on the ability to bear seeds. Pteridophytes do not bear seeds.

(d) Seeds naked or enclosed: Further it is classified whether seeds are naked or enclosed within fruits. The gymnosperms have naked seeds whereas Angiosperms bear seeds enclosed within the fruits.