1. Introduction to Biological Diversity

The variety of living beings present on the Earth is enormous. They range from invisible microscopic bacteria to 30-metre-long blue whales and redwood trees of California. Identifying, studying, and understanding this massive diversity without a proper system is impossible. That is why we classify them.

1.1 What is the Basis of Classification?

• Aristotle's Classification: The ancient Greek thinker Aristotle classified animals according to whether they lived on land, in water, or in the air. This was simple but misleading (e.g., sharks and whales both live in water, but are vastly different).
• Modern Basis: We now look for fundamental characteristics that determine a broad category, and then less fundamental characteristics for sub-categories. The major characteristics used today include:
  1. Cellular structure: Prokaryotic (without defined nucleus) vs Eukaryotic (with defined nucleus).
  2. Number of cells: Unicellular vs Multicellular.
  3. Mode of nutrition: Autotrophic (produce own food, e.g., plants) vs Heterotrophic (depend on others, e.g., animals).
  4. Level of organization: Cellular, Tissue, Organ, or Organ system level.

2. The Hierarchy of Classification

Biologists, such as Ernst Haeckel (1894), Robert Whittaker (1959), and Carl Woese (1977) tried to classify all living organisms into broad categories, called Kingdoms.

The taxonomic hierarchy introduced by Carolus Linnaeus arranges organisms in the following order (from largest to smallest):

• Kingdom (Highest category)
• Phylum (for animals) / Division (for plants)
• Class
• Order
• Family
• Genus
• Species (Lowest category, basic unit of classification)

Mnemonic to remember: Keep Pots Clean Or Family Gets Sick.

3. The Five Kingdom Classification

Proposed by R.H. Whittaker in 1959. The five kingdoms are:

3.1 Kingdom Monera

• Cell Type: Prokaryotic (no defined nucleus or membrane-bound organelles).
• Body Organization: Unicellular.
• Cell Wall: Present in some (e.g., bacteria), absent in others.
• Nutrition: Can be autotrophic or heterotrophic.
• Examples: Bacteria, blue-green algae (Cyanobacteria), and Mycoplasma.

3.2 Kingdom Protista

• Cell Type: Eukaryotic (well-defined nucleus).
• Body Organization: Unicellular.
• Movement: Many have appendages like cilia or flagella for movement.
• Nutrition: Autotrophic or heterotrophic.
• Examples: Amoeba, Paramecium, Euglena, Diatoms.

3.3 Kingdom Fungi

• Cell Type: Eukaryotic.
• Body Organization: Mostly multicellular (except Yeast).
• Cell Wall: Present, made of a tough complex sugar called chitin.
• Nutrition: Heterotrophic (Saprophytes—feed on dead and decaying organic matter).
• Examples: Mushrooms, Yeast, Rhizopus (bread mould), Penicillium.
• Lichens: Some fungi live in a mutually dependent (symbiotic) relationship with blue-green algae. Such relationships are called lichens.

3.4 Kingdom Plantae

• Cell Type: Eukaryotic.
• Body Organization: Multicellular.
• Cell Wall: Present, made of cellulose.
• Nutrition: Autotrophic (perform photosynthesis using chlorophyll).
• Examples: All plants (trees, ferns, mosses).

3.5 Kingdom Animalia

• Cell Type: Eukaryotic.
• Body Organization: Multicellular.
• Cell Wall: Absent.
• Nutrition: Heterotrophic.
• Examples: All animals (humans, insects, fishes, birds).

4. Classification of Kingdom Plantae

The plant kingdom is further classified on the basis of three main criteria:

1. Whether the plant body has well-differentiated, distinct parts.
2. Whether the differentiated plant body has special tissues for the transport of water and other substances (vascular tissue).
3. Whether the plants bear seeds, and whether the seeds are enclosed within fruits.

4.1 Thallophyta (Algae)

• Plants that do not have well-differentiated body design (roots, stem, leaves are absent). The body is called a thallus.
• They are predominantly aquatic.
• Examples: Spirogyra, Ulothrix, Cladophora, Chara.

4.2 Bryophyta

• They are called the amphibians of the plant kingdom because they live on land but require water for reproduction.
• The plant body is differentiated to form stem and leaf-like structures.
• There is no specialized vascular tissue for the conduction of water.
• Examples: Moss (Funaria), Marchantia.

4.3 Pteridophyta

• The plant body is differentiated into roots, stem, and leaves.
• They have specialized vascular tissue (xylem and phloem) for conduction of water and food.
• They do not produce seeds. They reproduce via spores.
• Examples: Ferns, Marsilea, Horsetails.

4.4 Gymnosperms

• The term is derived from Greek words: gymno means naked and sperma means seed.
• The plants bear naked seeds (seeds are not enclosed inside a fruit).
• They are usually perennial, evergreen, and woody.
• Examples: Pines (Pinus), Cycas, Deodar.

4.5 Angiosperms

• Derived from Greek: angio means covered and sperma means seed.
• These are flowering plants. The seeds develop inside an ovary which is modified to become a fruit.
• Plant embryos in seeds have structures called cotyledons (seed leaves). Angiosperms are divided into two groups:
  • Monocotyledons (Monocots): Seeds with a single cotyledon. Example: Wheat, Rice, Maize.
  • Dicotyledons (Dicots): Seeds with two cotyledons. Example: Gram, Pea, Mango.

5. Classification of Kingdom Animalia

Animals are classified on the basis of body design, symmetry, presence of a coelom (body cavity), and presence of a notochord.

5.1 Invertebrates (Without Notochord/Backbone)

1. Porifera (Sponges)

• Porifera means organisms with holes/pores.
• Non-motile animals attached to a solid support.
• Pores all over the body lead to a canal system that helps in circulating water to bring in food and oxygen.
• Cellular level of organization. Minimal differentiation.
• Examples: Sycon, Spongilla, Euplectella.

2. Coelenterata (Cnidaria)

• Aquatic animals with a body cavity (coelenteron).
• Body is made of two layers of cells (diploblastic).
• They show radial symmetry and have tentacles with stinging cells (cnidoblasts).
• Examples: Hydra, Jellyfish, Sea Anemone, Corals.

3. Platyhelminthes (Flatworms)

• Bilaterally symmetrical body (left and right halves are identical).
• Triploblastic (three layers of cells).
• No true internal body cavity (acoelomate). The body is flattened top-to-bottom.
• Mostly parasitic.
• Examples: Planaria (free-living), Tapeworm, Liver fluke (parasitic).

4. Nematoda (Roundworms)

• Bilaterally symmetrical and triploblastic.
• Body is cylindrical rather than flattened.
• They have a false body cavity (pseudocoelom).
• Mostly parasitic, causing diseases (e.g., elephantiasis, intestinal infections).
• Examples: Ascaris (roundworm), Wuchereria (filarial worm).

5. Annelida

• Bilaterally symmetrical, triploblastic, and have a true body cavity (coelom).
• Extensive organ differentiation. The body is segmented extensively from head to tail.
• Found in various habitats: freshwater, marine water, and land.
• Examples: Earthworm, Leech, Nereis.

6. Arthropoda (Jointed legs)

• The largest phylum of the animal kingdom.
• Bilaterally symmetrical, triploblastic, and segmented.
• They have an open circulatory system (blood does not flow in well-defined blood vessels).
• They have jointed legs and an exoskeleton.
• Examples: Insects (butterfly, cockroach), Spiders, Scorpions, Crabs, Prawns.

7. Mollusca

• Bilaterally symmetrical, triploblastic. Reduced coelomic cavity.
• Little segmentation. They have an open circulatory system and kidney-like organs for excretion.
• They have a muscular foot for moving around, and most have a hard calcareous shell.
• Examples: Snails, Octopus, Mussels, Chiton.

8. Echinodermata (Spiny-skinned)

• In Greek, echinos means hedgehog/spiny, and derma means skin.
• Exclusively free-living marine animals.
• Triploblastic, coelomate, and have a unique water-driven tube system used for moving.
• Hard calcium carbonate skeleton.
• Examples: Starfish, Sea urchin, Sea cucumber.

9. Hemichordata

• Worm-like marine animals.
• They were previously considered a sub-phylum under Chordata but are now a separate phylum.
• Examples: Balanoglossus.

5.2 Phylum Chordata

Animals belonging to phylum Chordata are characterized by the presence of:

1. A Notochord (a long rod-like support structure that runs along the back).
2. A dorsal nerve cord.
3. Triploblastic, coelomate, bilateral symmetry.
4. Paired pharyngeal gill slits.

Sub-phylum Vertebrata

Vertebrates have a true vertebral column (backbone) and internal skeleton. Vertebrates are grouped into five classes:

Class	Habitat / Skin	Heart Chambers	Respiration / Reproduction	Examples
Pisces (Fishes)	Exclusively aquatic. Exoskeleton of scales/plates. Cold-blooded.	2-chambered heart	Breathe through gills. Lay eggs (oviparous).	Shark (cartilaginous), Rohu, Tuna (bony).
Amphibia	Land and water. No scales. Mucus glands in skin. Cold-blooded.	3-chambered heart	Breathe through gills (larva) or lungs/skin (adult). Lay eggs.	Frogs, Toads, Salamanders.
Reptilia	Mostly terrestrial. Skin covered with scales. Cold-blooded.	3-chambered (except Crocodile: 4-chambered)	Breathe through lungs. Lay eggs with tough coverings.	Snakes, Turtles, Lizards, Crocodiles.
Aves (Birds)	Terrestrial. Body covered with feathers. Forelimbs modified into wings. Warm-blooded.	4-chambered heart	Breathe through lungs. Lay eggs.	Pigeon, Ostrich, Crow, Sparrow.
Mammalia	Diverse habitats. Skin has hair, sweat/oil glands. Warm-blooded. Mammary glands present to nourish young.	4-chambered heart	Breathe through lungs. Give birth to live young (viviparous) - mostly.	Humans, Whales, Bats, Rats, Cats.

6. Nomenclature (Naming Organisms)

Local names of organisms vary from place to place and language to language. To avoid confusion, a standardized scientific naming system is used globally, introduced by Carolus Linnaeus.

Conventions for writing scientific names:

1. The name of the genus begins with a capital letter (e.g., Homo).
2. The name of the species begins with a small letter (e.g., sapiens).
3. When printed, the scientific name is given in italics.
4. When written by hand, the genus name and the species name have to be underlined separately.