Diversity in the Living World
Introduction
The Earth contains an enormous variety of living organisms. They differ in their size, shape, structure, habitat, mode of nutrition, reproduction, life cycle, and other characteristics. This variety of life is known as biological diversity or biodiversity.
From microscopic organisms such as bacteria to large organisms such as elephants and trees, every organism has particular characteristics that help it survive in its environment. Because there are millions of different organisms, it is necessary to study, identify, name, classify, and organize them systematically.
The study of the diversity of living organisms is an important part of biology and provides the foundation for understanding plants, animals, microorganisms, and their relationships with one another.
1. Biodiversity
Biodiversity refers to the variety and variability of living organisms present on Earth or in a particular geographical region.
The term biodiversity includes differences among organisms at different levels, from genetic differences within a species to differences between different species and ecosystems.
Example
A forest may contain:
- Different species of trees
- Shrubs and herbs
- Birds
- Insects
- Mammals
- Reptiles
- Amphibians
- Microorganisms
All these organisms together contribute to the biodiversity of the forest.
Levels of Biodiversity
Biodiversity can be broadly studied at three major levels:
1. Genetic Diversity: Genetic diversity refers to the variation in genetic characteristics among individuals of the same species.
For example, different varieties of rice, wheat, mango, or dogs show genetic variation within the same species or closely related populations.
Genetic diversity helps organisms adapt to changing environmental conditions and diseases.
2. Species Diversity: Species diversity refers to the variety of different species present in a particular region.
For example, a forest may contain many species of plants, birds, mammals, insects, fungi, and microorganisms.
Greater species diversity generally means that a region contains a larger variety of organisms.
3. Ecosystem Diversity: Ecosystem diversity refers to the variety of ecosystems and habitats present in a geographical area.
Examples include:
- Forests
- Grasslands
- Deserts
- Rivers
- Lakes
- Wetlands
- Oceans
- Mountains
Each ecosystem has its own characteristic organisms and environmental conditions.
2. Importance of Biodiversity
Biodiversity is essential for the functioning and stability of ecosystems. It provides several ecological, economic, medicinal, and social benefits.
Ecological Importance: Different organisms perform different functions in an ecosystem. Plants act as primary producers, animals participate in food chains, decomposers recycle nutrients, and microorganisms contribute to nutrient cycling.
Food Resources: Humans obtain food from a wide variety of plants and animals. Important food crops include rice, wheat, maize, pulses, fruits, and vegetables. Animals provide products such as milk, eggs, and meat.
Medicinal Importance: Many plants and microorganisms contain biologically active compounds that are useful in medicine. Several modern drugs have originated from natural sources.
Economic Importance
Plants and animals provide raw materials for industries such as agriculture, pharmaceuticals, textiles, food processing, and cosmetics.
Environmental Stability
Biodiversity contributes to ecosystem stability and helps ecosystems respond to environmental changes.
Scientific Importance: Different organisms provide important material for research in areas such as genetics, pharmacology, biotechnology, ecology, and medicine.
3. Classification of Living Organisms
Because there are a huge number of organisms, studying each organism separately is difficult. Therefore, organisms are grouped into different categories based on their similarities and differences.
Classification is the systematic arrangement of organisms into groups and categories based on their characteristics.
The science concerned with the identification, naming, and classification of organisms is called taxonomy.
Main purposes of classification
Classification helps in:
- Identification of organisms
- Studying similarities and differences
- Understanding relationships between organisms
- Organizing biological information
- Studying evolutionary relationships
4. Basis of Classification
Living organisms can be classified using several characteristics.
Important characteristics include:
1. Cell Type
Organisms may have:
- Prokaryotic cells – cells without a true nucleus, e.g., bacteria.
- Eukaryotic cells – cells with a well-defined nucleus, e.g., plants and animals.
2. Number of Cells
Organisms may be:
- Unicellular – made up of a single cell.
- Multicellular – made up of many cells.
3. Mode of Nutrition
Organisms may obtain nutrition by:
- Autotrophic nutrition – organisms prepare their own food, mainly through photosynthesis.
- Heterotrophic nutrition – organisms obtain food from other organisms.
4. Body Organization
Organisms differ in their level of structural organization. Some are simple single-celled organisms, while others have highly organized tissues, organs, and organ systems.
5. Reproduction
Organisms may reproduce by:
- Asexual reproduction
- Sexual reproduction
6. Evolutionary Relationships
Modern classification also considers evolutionary relationships and common ancestry.
5. Major Classification of Living Organisms
One commonly used broad classification divides living organisms into five kingdoms:
- Monera
- Protista
- Fungi
- Plantae
- Animalia
However, modern biological classification also uses systems based on three domains: Bacteria, Archaea, and Eukarya. For introductory study, the five-kingdom system remains useful for understanding basic differences among major groups.
6. Kingdom Plantae
The plant kingdom includes multicellular, eukaryotic organisms that generally contain chlorophyll and perform photosynthesis.
Plants are generally autotrophic, meaning they manufacture their own food using light energy.
Salient Features of Plants
Important characteristics of plants include:
- They are generally multicellular and eukaryotic.
- Plant cells possess a cell wall, mainly composed of cellulose.
- Most plants contain chlorophyll.
- Most plants perform photosynthesis.
- Food is commonly stored in the form of starch.
- Plants generally remain fixed at one place.
- They show both sexual and asexual modes of reproduction.
- Their bodies may be differentiated into roots, stems, leaves, flowers, and other structures depending on the group.
7. Major Groups of Plants
The plant kingdom can be broadly studied through the following groups:
Algae → Bryophytes → Pteridophytes → Gymnosperms → Angiosperms
A. Algae
Algae are mostly aquatic, photosynthetic organisms. They may be unicellular or multicellular.
Salient features
- Mostly found in freshwater or marine environments.
- They contain photosynthetic pigments.
- Their body is generally called a thallus.
- They do not have true roots, stems, and leaves.
- Reproduction may be vegetative, asexual, or sexual.
Examples: Chlamydomonas, Spirogyra, Ulothrix, Volvox
Importance
Algae contribute significantly to oxygen production and form an important part of aquatic food chains. Some algae are also used as food and industrial raw materials.
8. Bryophytes
Bryophytes are small, non-vascular land plants that generally grow in moist and shady places.
They are sometimes called the amphibians of the plant kingdom because they live on land but require water for sexual reproduction.
Salient features
- They lack true vascular tissues.
- They do not have well-developed true roots, stems, and leaves.
- They generally grow in moist habitats.
- The gametophyte is the dominant generation.
- Water is required for fertilization.
Examples: Marchantia, Riccia, Funaria
9. Pteridophytes
Pteridophytes are vascular plants that reproduce mainly through spores rather than seeds.
Salient features
- They possess vascular tissues: xylem and phloem.
- They have true roots, stems, and leaves.
- They do not produce flowers or seeds.
- They reproduce by spores.
- The sporophyte is the dominant generation.
- Water is generally required for fertilization.
Examples: Pteris, Dryopteris, Equisetum, Selaginella
10. Gymnosperms
Gymnosperms are seed-producing plants in which the seeds are not enclosed within fruits.
The word gymnosperm means “naked seed.”
Salient features
- They are vascular plants.
- They produce seeds.
- Seeds are not enclosed by fruits.
- Most are woody and perennial.
- They commonly bear cones.
- Many have needle-like or scale-like leaves.
- They are generally adapted to terrestrial environments.
Examples: Pinus, Cycas, Cedrus, Gnetum
11. Angiosperms
Angiosperms are flowering plants. They are the most diverse group of land plants.
Their seeds are enclosed within fruits.
Salient features
- They produce flowers.
- They produce seeds enclosed within fruits.
- They possess well-developed vascular tissues.
- They show sexual reproduction involving flowers.
- They have highly developed roots, stems, and leaves.
Angiosperms are broadly divided into:
Monocotyledons
They generally have one cotyledon in the seed.
Examples:
- Rice
- Wheat
- Maize
- Grass
- Onion
Dicotyledons
They generally have two cotyledons in the seed.
Examples:
- Pea
- Gram
- Bean
- Mango
- Sunflower
12. Kingdom Animalia
The animal kingdom includes multicellular, eukaryotic organisms that obtain nutrition from other organisms.
Animals are generally heterotrophic and cannot prepare their own food through photosynthesis.
Salient Features of Animals
- They are multicellular and eukaryotic.
- Their cells do not have a cell wall.
- They are heterotrophic.
- Most animals are capable of movement.
- They generally possess specialized tissues and organs.
- Most reproduce sexually.
- Nervous and sensory systems are well developed in many animals.
- They respond to external stimuli.
13. Classification of Animals
Animals can broadly be divided into:
Invertebrates and Vertebrates
Invertebrates: Animals that do not possess a vertebral column are called invertebrates.
Examples include:
- Sponges
- Jellyfish
- Worms
- Insects
- Snails
- Starfish
Vertebrates: Animals possessing a vertebral column or backbone are called vertebrates.
They belong to the phylum Chordata, with vertebrates forming the major vertebrate group.
Examples include:
- Fish
- Amphibians
- Reptiles
- Birds
- Mammals
14. Major Animal Groups
A. Porifera
Poriferans are commonly called sponges.
Salient features
- Mostly aquatic.
- Body contains numerous pores.
- Generally have a simple body organization.
- Mostly marine.
- They are generally attached to a surface.
Example: Sycon, Spongilla
B. Cnidaria
Cnidarians are aquatic animals characterized by specialized stinging cells called cnidocytes.
Salient features
- Mostly aquatic and marine.
- Radial symmetry is common.
- Have a simple body organization.
- Possess tentacles.
- Cnidocytes help in capturing prey and defense.
Examples: Hydra, Aurelia, Physalia, sea anemones
15. Platyhelminthes
These are commonly called flatworms.
Salient features
- Body is dorsoventrally flattened.
- They are bilaterally symmetrical.
- Many are parasitic.
- They are generally acoelomate.
- Organ-system level of organization is present.
Examples: Taenia, Fasciola
16. Aschelminthes / Nematoda
These include roundworms.
Salient features
- Body is cylindrical and unsegmented.
- They are bilaterally symmetrical.
- They possess a complete digestive tract.
- Many species are parasitic.
Examples: Ascaris, Wuchereria
17. Annelida
Annelids are segmented worms.
Salient features
- Body is divided into repeated segments.
- They possess bilateral symmetry.
- They have a true body cavity.
- Organ-system level of organization is present.
- Many have a well-developed circulatory system.
Examples: Earthworm, Leech, Nereis
18. Arthropoda
Arthropoda is the largest phylum in the animal kingdom.
Salient features
- Body is generally segmented.
- They have paired, jointed appendages.
- They possess a hard external covering called an exoskeleton.
- The body is commonly divided into regions.
- They have an open circulatory system.
Examples: Cockroach, housefly, mosquito, butterfly, crab, spider
19. Mollusca
Molluscs are soft-bodied animals, many of which possess a shell.
Salient features
- Body is soft and generally unsegmented.
- They generally possess a muscular foot.
- Many possess a shell.
- Most are aquatic, although some are terrestrial.
Examples: Snail, mussel, oyster, octopus, squid
20. Echinodermata
Echinoderms are marine animals with a characteristic internal skeleton and, in adults, commonly show radial symmetry.
Salient features
- Exclusively marine.
- Possess an internal skeleton.
- Adults generally show radial symmetry.
- Possess a water vascular system.
- Tube feet are associated with movement and feeding.
Examples: Starfish, sea urchin, sea cucumber
21. Chordata
Chordates are animals characterized, at some stage of their life, by structures such as a notochord, dorsal hollow nerve cord, and pharyngeal slits.
The vertebrates are the major group within Chordata.
22. Major Classes of Vertebrates
1. Pisces / Fish
Fish are primarily aquatic vertebrates.
Features
- Live mainly in water.
- Respire mainly through gills.
- Body is generally streamlined.
- Fins help in movement.
- Most are cold-blooded (ectothermic).
- Most reproduce by laying eggs.
Examples
Rohu, shark, tuna, goldfish
2. Amphibia
Amphibians can generally live both in aquatic and terrestrial environments during different stages of their life cycle.
Features
- Skin is generally moist.
- Larvae commonly live in water.
- Adults may live on land.
- Respiration may occur through skin and lungs.
- They are ectothermic.
- Fertilization is generally external in many species.
Examples
Frog, toad, salamander
3. Reptilia
Reptiles are primarily terrestrial vertebrates with relatively dry, keratinized skin.
Features
- Skin is generally dry and covered with scales or scutes.
- Respiration occurs through lungs.
- They are ectothermic.
- Internal fertilization occurs.
- Many lay eggs with protective coverings.
Examples
Snake, lizard, crocodile, turtle
4. Aves
Aves include birds.
Features
- Body is covered with feathers.
- Forelimbs are modified into wings.
- They possess a beak.
- Respiration occurs through lungs.
- They are endothermic.
- They lay eggs with hard shells.
- Most are adapted for flight.
Examples
Pigeon, crow, sparrow, eagle
5. Mammalia
Mammals are highly specialized vertebrates characterized by the presence of mammary glands, which produce milk for nourishing young ones.
Features
- Body generally has hair or fur.
- Mammary glands are present.
- They are endothermic.
- Respiration occurs through lungs.
- Most mammals give birth to young ones.
- Internal fertilization occurs.
- The heart generally has four chambers.
Examples
Human, cow, dog, cat, elephant, whale
23. Plant vs Animal – Basic Differences
| Feature | Plants | Animals |
| Cell wall | Present | Absent |
| Nutrition | Mostly autotrophic | Heterotrophic |
| Chlorophyll | Usually present in photosynthetic plants | Absent |
| Movement | Generally fixed in one place | Usually capable of movement |
| Food storage | Commonly starch | Commonly glycogen and fats |
| Nervous system | Absent | Present in most complex animals |
| Growth | Often continues throughout life in many regions | Usually more limited after maturity |
| Reproduction | Sexual and asexual | Mainly sexual in higher animals |
24. Importance of Classification in Pharmacy and Medical Sciences
Correct identification of biological organisms is essential because medicinal plants, microorganisms, and animal-derived materials may be used as sources of drugs and pharmaceutical ingredients.
For example, different plant species may have different chemical constituents and medicinal properties. Incorrect identification can therefore lead to the use of the wrong biological material.
Classification also helps students understand:
Organism → Group → Characteristics → Identification → Biological/Medicinal importance