Diversity and Classification of Living Organisms

Biodiversity and why India is a hotspot

Biodiversity means the variety of living things in a place: plants, animals, fungi and tiny microbes. About 1.7 to 2 million species have been named so far. Many more are still unknown.

A species is a group of organisms that look alike and can breed with each other to have young ones that can also breed.

India is a megadiverse country. It has only about 2.4% of the world's land but about 7–8% of known species. Why? It has every kind of home: hot deserts, snowy mountains, rain forests, grasslands, mangroves, coral reefs and long coasts.

A biodiversity hotspot is a region that (1) has very many species found nowhere else, and (2) has already lost most of its natural cover, so it is under threat. Four hotspots touch India: the Himalaya, the Western Ghats and Sri Lanka, Indo-Burma (the North-East), and Sundaland (the Nicobar Islands). Examples found almost only here: the lion-tailed macaque of the Western Ghats and many frogs and orchids of the North-East.

Why do we classify living things?

Classification means putting living things into groups based on features they share. It helps us to:

• study millions of organisms by studying a few groups;
• know a lot about an organism just from its group (every mammal feeds milk to its young);
• see how different groups are related and how they may have evolved from common ancestors.

Basis of classification

We use body features (characteristics). The most basic questions come first, because they decide many other features:

1. Cell type: is there a real nucleus with a membrane? No = prokaryote. Yes = eukaryote.
2. Body: one cell (unicellular) or many cells (multicellular)?
3. Cell wall: present or absent? Made of what?
4. Food: makes its own (autotroph) or eats others (heterotroph)?
5. Body plan: in plants and animals, how organised is the body?

Features that appear early in the history of life are called primitive (lower). Features that came later are advanced (higher).

The five kingdoms

Robert Whittaker (1959) sorted all cellular life into five kingdoms:

• Monera: prokaryotes, no real nucleus, one cell. Some make food, some eat. Examples: bacteria, blue-green algae (cyanobacteria).
• Protista: eukaryotes, one cell. Many move with hair-like cilia or whip-like flagella. Examples: Amoeba, Paramecium, Euglena, diatoms.
• Fungi: eukaryotes, cannot make food. They absorb food from dead and rotting matter (saprophytes). Cell wall of chitin. Examples: yeast, mushroom, bread mould (Rhizopus). Some live with algae as lichens.
• Plantae: many cells with a cellulose wall; make their own food by photosynthesis. Examples: algae like Spirogyra, mosses, ferns, pine, mango.
• Animalia: many cells, no cell wall, eat other organisms. Most can move. Examples: sponge, earthworm, frog, tiger.

The plant kingdom in short

Plants are split by three questions: Is the body divided into root, stem and leaf? Does it have tubes (vascular tissue) to carry water? Does it make seeds, and are the seeds covered by fruit?

• Thallophyta (algae): body not divided; Spirogyra, Ulva.
• Bryophyta (mosses): divided body but no water tubes; the "amphibians of the plant world".
• Pteridophyta (ferns): root, stem, leaf and water tubes, but no seeds.
• Gymnosperms: naked seeds, e.g. pine, deodar.
• Angiosperms: seeds inside fruits; flowering plants. Monocots (one seed leaf, e.g. maize) and dicots (two seed leaves, e.g. pea).

Major animal divisions

The first big question is: is there a backbone (vertebral column)?

Invertebrates (no backbone)

• Porifera (sponges): body full of holes; fixed in one place.
• Cnidaria (Hydra, jellyfish, sea anemone): a hollow body with stinging tentacles.
• Platyhelminthes (flatworms, e.g. tapeworm, planaria) and Nematoda (roundworms, e.g. Ascaris).
• Annelida (earthworm, leech): body in ring-like segments.
• Arthropoda (insects, spiders, crabs, prawns): jointed legs and a hard outer cover. The biggest group of all animals.
• Mollusca (snail, octopus): soft body, often inside a shell.
• Echinodermata (starfish, sea urchin): spiny skin; live only in the sea.

Vertebrates (backbone)

They belong to phylum Chordata. Five classes:

• Pisces (fish): gills, fins, scales; cold-blooded.
• Amphibia (frog, toad): moist skin, live in water and on land; cold-blooded.
• Reptilia (lizard, snake, turtle): dry scaly skin, lay eggs with shells; cold-blooded.
• Aves (birds): feathers, beak, lay eggs; warm-blooded.
• Mammalia (cow, bat, whale, humans): hair, feed milk to young; warm-blooded.

Cold-blooded means body temperature changes with the surroundings. Warm-blooded means it stays about the same.

Hierarchy and binomial nomenclature

Groups sit inside bigger groups. This order is called the hierarchy of classification: Kingdom → Phylum (Division for plants) → Class → Order → Family → Genus → Species. Going down, each group has fewer organisms, and they share more features.

Local names change from place to place: the same plant is aam, mamidi and mango. So scientists use one name everywhere. Carl Linnaeus gave the binomial nomenclature (two-name system):

• First word = genus, starts with a capital letter.
• Second word = species, starts with a small letter.
• Printed in italics; handwritten names are underlined separately.

Examples: tiger Panthera tigris, lion Panthera leo (same genus, so close relatives), human Homo sapiens, mango Mangifera indica, peacock Pavo cristatus.

Viruses: not a cell

A virus is much smaller than a bacterium. It has no cell: no cell membrane, no cytoplasm, no ribosomes. It is just genetic material (DNA or RNA) packed inside a protein coat. Because it is acellular (not made of cells), it fits in none of the five kingdoms.

Outside a living cell, a virus does nothing; it can even form crystals, like salt. Inside a host cell it takes over the cell's machinery and makes hundreds of copies of itself. So viruses sit on the border of living and non-living.

Diseases caused by viruses: common cold, flu, measles, chickenpox, polio, dengue, COVID-19, HIV-AIDS. Antibiotics do not work on viruses; vaccines help prevent many viral diseases.

Try it: sort your kitchen

Collect 8 things from the kitchen or garden: a leaf, a mushroom (or dry yeast), curd (it has bacteria), a moss patch from a wall, an ant, a grain of rice, a feather, a fish scale. Ask the same questions as the 3D: cells with a nucleus? makes own food? backbone? Put each on the right paper "shelf". Then open step 6 of the 3D and check your answers.

Key formulas and definitions

• Five kingdoms: Monera → Protista → Fungi → Plantae → Animalia
• Hierarchy: Kingdom → Phylum → Class → Order → Family → Genus → Species
• Scientific name = Genus (Capital) + species (small), in italics
• Prokaryote = no membrane-bound nucleus; Eukaryote = true nucleus
• Virus = DNA or RNA + protein coat, no cell

Worked examples

Common mistakes

• Calling blue-green algae 'plants'. They have no true nucleus, so they are Monera (cyanobacteria).
• Writing the species name with a capital, like 'Homo Sapiens'. Correct: Homo sapiens — genus capital, species small, in italics.
• Putting whales in fish and bats in birds. Classify by body features: both have hair and give milk, so both are mammals.
• Thinking antibiotics cure viral fever. Antibiotics only act on bacteria; a virus has no cell parts for them to attack.