Patterns in Life: Diversity and Classification

Class 09 Science

The Earth is home to an enormous variety of life forms. This immense variety of living organisms is known as biodiversity. Biodiversity is essential for life on the Earth. Every organism plays a role in keeping nature stable and functioning.

To study diversity systematically, scientists group and classify organisms based on their shared characteristics and evolutionary relationships. Classification helps us understand how organisms are related, how they function and how we can use this knowledge in activities, such as ecosystem management, biodiversity conservation, sustainable farming, and so on.

Five Kingdom Classification

In the five kingdom classification, all life forms are grouped based on certain common features.

1. Kingdom Monera - Unicellular prokaryotes

Bacteria and cyanobacteria are single-celled prokaryotes that are grouped under Monera.

2. Kingdom Protista - Unicellular eukaryotes

All single-celled eukaryotes without cell wall or with cell wall made up of cellulose are grouped under the kingdom protista.

3. Kingdom Fungi - Multicellular, heterotrophic eukaryotes with a cell wall

Fungi are mostly multicellular eukaryotes with cell walls made of chitin. They do not make their own food. They either absorb nutrients from dead or decaying matter through fine filaments (which together form a network known as mycelium). Most of the fungi are saprophytes and therefore, they play a very important ecological role as decomposers, that means, they feed on dead organic matter, such as fallen leaves, twigs or dead organisms. They break down the complex organic matter into simpler substances and make them readily available in soil as minerals.

Some of the fungi establish a mutualistic (symbiotic) relationship with other organisms while others live as parasites, and cause diseases in plants and animals. They reproduce both sexually and asexually, often by forming spores, and grow best in warm and moist conditions.

4. Kingdom Plantae - Multicellular, autotrophic eukaryotes with a cell wall

Plants are multicellular, autotrophic organisms that perform photosynthesis. Their cells have a rigid cell wall primarily made up of cellulose, which provides support and protection. All plants belong to the kingdom Plantae. Plants form the base of most of the food chains and release oxygen which is essential for life on the Earth.

5. Kingdom Animalia - Multicellular, heterotrophic eukaryotes

Animals are multicellular and heterotrophic organisms. They depend on other organisms for their food. Most animals exhibit the characteristics of locomotion, rapid response to stimuli and coordinated behaviour. These abilities allow animals to actively search for food, avoid predators and interact dynamically with their surroundings.

Kingdom Plantae

Kingdom Plantae is further divided into five classes. From algae to angiosperms, plant groups show a sequence of structural changes that help plants meet the challenges of life on land. Early plants relied on water for support and reproduction. Gradually, plant groups evolved transport tissues, seeds and flowers.

1. Thallophyta (algae) - Primitive plants

Thallophytes (thallos means undifferentiated body and phyton means plant) are among the simplest plant forms and are mostly found in water or moist environments. They form a thallus, a simple body that allows direct exchange of gases, nutrients and water with the surroundings. They are well-adapted to aquatic habitats. An example of Thallophyta is Spirogyra.

2. Bryophyta - First steps on land, still need water

Bryophytes (bryon means moss and phyton means plant), such as mosses and liverworts, which represents an important shift from water to land.

Bryophytes have a more differentiated body as compared to thallophytes. They have root-like structures called rhizoids, and may possess stem-like and leaf-like simple structures. Bryophytes survive in moist and shady places, growing as green mats on the soil, especially during the monsoons. Examples of Bryophytes include Marchantia and moss.

Bryophytes require water for reproduction. Therefore, they are called the ‘amphibians’ of the plant kingdom. They show how plants began to colonise land, while remaining dependent on moisture.

3. Pteridophyta - Adaptation to land and having structural transport system

Pteridophytes (pteris derived from pteron means feather and phyton means plant), such as ferns, possess true roots, stems and leaves.

4. Gymnosperms - Reproduction without water

Gymnosperms (gymnos means naked and spermos means seed), such as pines and cycads are well-adapted to cold and dry regions. Their needle-like or scale-like leaves reduce water loss and withstand harsh conditions. They produce seeds, which protect the developing embryo and contain stored food. Gymnosperms do not require aquatic conditions for fertilisation. Their seeds are not enclosed in fruits and are often exposed on cones.

5. Angiosperms - Efficient reproduction and seed dispersal

Angiosperms (angeion means vessel and spermos means seeds) or flowering plants, represent plants with the most complex body organisation. They produce flowers and fruits. Flowers attract pollinators, increasing the efficiency of reproduction, while fruits help spread seeds to new locations. These features allow angiosperms to occupy a wide range of environments. Therefore, they are the most diverse plant group on the Earth.

Kingdom Animalia

One of the major criteria for classifying animals is the presence or absence of a notochord, which is a flexible rod-shaped structure. Based on the absence or presence of the notochord, animals are classified into two groups - non-chordata (invertebrata) and chordata. In some chordates, notochord acts as a precursor for the development of the vertebral column. The chordata is further classified as protochordata and vertebrata.

1. Invertebrates - Animals without a notochord

Invertebrates lack a notochord, yet they show a wide range of body organisation, from a simple structure to complex organ systems.

From sponges to echinoderms, animal body structure shows a pattern of increasing complexity in body organisation. New structural features improve feeding, movement and protection but often introduce new challenges. These help explain survival in diverse environments.

1.1 Porifera (pore-bearers) - Multicellularity without tissues

Sponges represent one of the simplest animal body plans. They are multicellular but lack organisation of tissues and organs. Numerous pores in their body allow water to continuously flow through it, bringing food particles and oxygen directly to individual cells, carrying the waste away. Sponges remain fixed in one place and are found in aquatic environments.

1.2 Cnidaria - True tissues and active feeding

Cnidarians, such as Hydra, jellyfish and corals show a major change in body organisation. Tissue-level organisation is present in them which allows specialised cells to perform specific functions, such as tentacles, which are used for capturing prey. Unlike poriferans, cnidarians can catch their prey using tentacles instead of depending on water currents to bring food particles inside through pores. However, cnidarians possess a single opening that functions for both food intake and waste removal.

1.3 Platyhelminthes (flatworms) - Bilateral symmetry and directional movement

Flatworms show organisational advancement through bilateral symmetry (organism’s body can be divided into two halves along one plane), in which the body has distinct head-tail and front-back regions. This body organisation allows better coordination of movement and this is the beginning of directional activity. Their flattened bodies permit efficient diffusion of gases without specialised respiratory organs but there is still a single opening for food intake and waste elimination.

Many flatworms have adopted a parasitic lifestyle. Hooks and suckers are present in the parasitic forms which helps them to attach firmly to the host tissues for obtaining nutrients.

1.4 Nematoda (roundworms) - Efficient body design with two openings

Roundworms have elongated, cylindrical bodies. This body structure allows their efficient movement through soil, water, or host tissues. This body form increases the range of environment in which roundworms can survive, despite relatively simple internal organisation. The body has two openings (mouth and anus). The organ system level of body organisation is distinct in male and female worms.

1.5 Annelida (segmented worms) - segmentation and body cavities

Annelids, such as earthworms, represent a notable development in body organisation. Their bodies are cylindrical and are divided into segments. They possess organ system level of organisation. Presence of muscles help them in locomotion, and nerve cord helps in control and coordination.

1.6 Arthropoda - Jointed appendages and an external skeleton

Arthropods (arthro means limbs and poda means appendages) includes insects, crabs and spiders. They have segmented bodies with different segments specialised for different functions. A defining structural feature of arthropods is the development of a hard external skeleton (rigid external covering). This outer covering provides protection, reduces water loss and supports powerful muscles, allowing arthropods to survive in dry and exposed environments.

1.7 Mollusca - Organ system level organisation with soft bodies

Molluscs, such as snails, squids and octopuses show organ system level organisation with soft bodies. In many molluscs, the development of a shell provides protection to these soft bodies. Their body is segmented with a distinct head, a muscular foot and a hump. 

This group shows how a basic body plan can be modified in different directions depending on environmental demands.

1.8 Echinodermata - Internal support without a notochord

Echinoderms (echinos means spiny and derma means skin), such as starfish and sea urchins possess a hard internal skeleton made of calcium carbonate. Although they lack a notochord, this internal skeleton provides them protection and controlled movement. Their body organisation is quite like that of more complex animals, showing a gradual shift towards internal skeletal support.

2. Protochordates - The appearance of the notochord

Protochordates (like Amphioxus) possess a notochord at least once during their life. This structure provides internal support without restricting movement and represents a crucial change in body organisation. Protochordates are a primitive type of chordates which help us understand how animals with a notochord may have arisen from simpler forms.

3. Vertebrates - Animals with a backbone

Vertebrates possess a vertebral column (backbone), an internal skeletal structure that supports the body and protects vital organs, such as the brain and the spinal cord. This internal framework allows a larger body size, efficient movement and the development of complex organ systems. Vertebrates show advanced sensory abilities and coordinated behaviour.

Vertebrates are classified into five groups, namely fish, amphibians, reptiles, birds and mammals based on the broad patterns of their habitat use, body covering and reproduction.