Diversity in Living Organisms

1. Need for Classification

There are millions of different kinds of organisms around us. Without a system to organise them, studying, identifying and communicating about them would be impossible. Classification helps us to:

• Understand the diversity of life in a systematic way.
• Identify similarities and differences among organisms.
• Establish evolutionary relationships -- organisms grouped together share a common ancestor.
• Predict the characteristics of newly discovered organisms based on their group.
• Understand how life evolved over billions of years -- from simple unicellular forms to complex multicellular organisms.

Charles Darwin's idea of evolution (1859) provided the framework: organisms are classified according to their body design and evolutionary history. The more features two organisms share, the more closely related they are.

Early classification was done by Aristotle, who divided plants into trees, shrubs and herbs, and animals into those with red blood and those without. Modern scientific classification is based on many more criteria.

2. Basis of Classification

Modern classification uses several characteristics, moving from the most fundamental to the more specific:

• Type of cell (prokaryote vs eukaryote): Prokaryotes have no membrane-bound nucleus (e.g. bacteria). Eukaryotes have a well-defined nucleus (all other organisms). This is the most basic division.
• Number of cells: Unicellular (single cell performs all functions) vs. Multicellular (cells are specialised).
• Body organisation: Cell level, tissue level, organ level, organ-system level -- reflects increasing complexity.
• Mode of nutrition: Autotrophic (make their own food via photosynthesis or chemosynthesis) vs. Heterotrophic (depend on others -- parasitic, saprophytic, holozoic, etc.).
• Cell wall: Presence/absence and composition (cellulose in plants; chitin in fungi; absent in animals).
• Mode of reproduction: Sexual (higher genetic variation) vs. asexual (spores, budding, fission).
• Level of organisation: Whether the organism has tissues, organs and organ systems.
• Phylogenetic approach: Evolutionary relationships inferred from body design, fossil record and genetic data.

Characteristics used earliest in hierarchy are the most fundamental (e.g. presence of nucleus) and divide the largest groups. Characteristics used later narrow within those groups.

3. Taxonomic Hierarchy

Classification is arranged in levels called taxa. From broadest to most specific:

• Kingdom -- the broadest category (e.g. Animalia).
• Phylum / Division -- based on major body plan differences within a kingdom.
• Class -- organisms with similar characters within a phylum (e.g. Mammalia within Chordata).
• Order -- groups of related families (e.g. Carnivora).
• Family -- groups of related genera (e.g. Felidae for cats).
• Genus -- group of closely related species (e.g. Panthera for lions, tigers, leopards).
• Species -- most specific; organisms that can interbreed and produce fertile offspring (e.g. Panthera leo = lion).

Scientific names (binomial nomenclature, introduced by Carl Linnaeus) have two parts: Genus (capitalised) and species (lowercase), both italicised. Example: Homo sapiens.

Mnemonic: Kind People Come Over For Good Soup (Kingdom, Phylum, Class, Order, Family, Genus, Species).

4. Kingdom Monera

Key features:

• Prokaryotes -- no membrane-bound nucleus; DNA lies free in the cytoplasm.
• No membrane-bound organelles (no mitochondria, chloroplasts, etc.).
• Unicellular; some form filaments or colonies but remain unicellular at core.
• Cell wall present (peptidoglycan in most bacteria).
• Can be autotrophic (photosynthetic or chemosynthetic) or heterotrophic (decomposers, parasites).
• Reproduction mainly by binary fission; some by spores.

Examples: Bacteria (Lactobacillus, Mycobacterium tuberculosis), blue-green algae (Cyanobacteria like Nostoc, Anabaena), Mycoplasma (smallest living organisms; no cell wall).

Shapes of bacteria:

• Spherical -- Coccus (e.g. Staphylococcus)
• Rod-shaped -- Bacillus (e.g. E. coli)
• Spiral -- Spirillum
• Comma-shaped -- Vibrio (e.g. Vibrio cholerae)

5. Kingdom Protista

Key features:

• Unicellular eukaryotes -- single cell with a true membrane-bound nucleus and organelles.
• Some have cilia or flagella for movement.
• Nutrition: autotrophic (contain chloroplasts) or heterotrophic.
• Reproduction: asexual (binary fission, budding) and sexual (through cell fusion).
• Aquatic habitat (freshwater or marine); some are parasitic.

Major groups and examples:

• Amoeba -- moves by pseudopodia (false feet); engulfs food by phagocytosis.
• Paramecium -- moves by cilia; has two nuclei (macronucleus for metabolism, micronucleus for reproduction).
• Euglena -- has a flagellum and chloroplasts; autotrophic in light, heterotrophic in dark -- a "connecting link."
• Plasmodium -- parasitic protist causing malaria; belongs to Sporozoans.
• Diatoms -- photosynthetic; have cell walls made of silica; form "diatomaceous earth" used in filters.

6. Kingdom Fungi

Key features:

• Eukaryotes with cell walls made of chitin (not cellulose like plants).
• Mostly multicellular (Mucor, mushrooms); yeast is unicellular.
• Heterotrophic -- mainly saprophytic (feed on dead and decaying organic matter by secreting enzymes and absorbing digested food = absorptive nutrition). Some are parasitic.
• Do NOT perform photosynthesis; no chloroplasts.
• Reproduce by spores (asexual) and sometimes sexually.
• Body made of thread-like structures called hyphae; mass of hyphae = mycelium.

Examples:

• Rhizopus (bread mould) -- black mould on bread; reproduces by spores.
• Mucor -- pin mould; common soil fungus.
• Agaricus -- edible mushroom.
• Aspergillus -- mould; some species produce aflatoxins.
• Penicillium -- source of penicillin antibiotic; grows on citrus fruit.
• Saccharomyces cerevisiae -- baker's/brewer's yeast; unicellular; used in fermentation and bread-making.

Lichens: symbiotic association between algae (autotrophic partner) and fungi (protective partner). The fungus provides water and minerals; the alga provides food via photosynthesis. Lichens are pioneer species that colonise bare rock.

7. Kingdom Plantae -- Overview

Key features:

• Eukaryotic, multicellular organisms with chloroplasts and cell walls made of cellulose.
• Primarily autotrophic -- synthesise food by photosynthesis.
• Reproduce sexually (alternation of generations) and asexually.
• Do not move from place to place (sessile).

Plantae is divided into five major divisions based on increasing complexity:

1. Thallophyta (algae) -- no distinct body parts
2. Bryophyta (mosses, liverworts) -- no vascular tissue
3. Pteridophyta (ferns) -- vascular tissue, no seeds
4. Gymnosperms -- naked seeds
5. Angiosperms -- seeds enclosed in fruits

The first three (Thallophyta, Bryophyta, Pteridophyta) are collectively called Cryptogamae (non-flowering, no seeds visible). Gymnosperms and Angiosperms together are Phanerogamae (flowering/seed-bearing, well-differentiated plant body).

8. Thallophyta (Algae)

Key features:

• Plant body is a thallus -- undifferentiated; no true roots, stems or leaves.
• Mostly aquatic (freshwater or marine); some grow on moist surfaces, tree barks.
• Cell wall of cellulose; chloroplasts present (photosynthetic).
• Reproduce asexually (by fragmentation, spores) and sexually.
• No vascular tissue (no xylem or phloem).

Examples by colour (pigment):

• Green algae (Chlorophyceae): Spirogyra (pond scum), Ulva (sea lettuce), Chara, Chlorella. Pigment: chlorophyll a and b.
• Brown algae (Phaeophyceae): Kelp, Fucus, Sargassum. Pigment: fucoxanthin. Mostly marine.
• Red algae (Rhodophyceae): Porphyra, Gelidium (source of agar). Pigment: phycoerythrin. Found in deeper marine waters.

Economic importance: Algae produce about 50% of the world's oxygen. Used as food (Porphyra, Chlorella), in agar production, as fertilisers, and as biofuel source.

9. Bryophyta -- Mosses and Liverworts

Key features:

• Plant body is differentiated into leaf-like and stem-like parts -- but still no true roots (have rhizoids for anchorage and absorption).
• No vascular tissue (no xylem or phloem) -- water and food transported by diffusion. Hence called amphibians of the plant kingdom (need water for reproduction).
• Found in moist, shaded habitats -- hills, forests, damp walls.
• Water is essential for fertilisation (male gametes swim to female gametes).
• Show clear alternation of generations: dominant gametophyte (haploid, green leafy plant) and dependent sporophyte (diploid, produces spores).

Examples:

• Funaria -- common moss; used in studies of plant development.
• Marchantia -- liverwort; flat, ribbon-like thallus; reproduces by gemma cups.
• Riccia -- liverwort; simple thallus.
• Sphagnum (peat moss) -- forms peat bogs; used as fuel and packing material for plants.

Ecological importance: Mosses and lichens are pioneer species -- they colonise bare rocks and help form soil. They also absorb large amounts of water and prevent soil erosion.

10. Pteridophyta -- Ferns and Horsetails

Key features:

• First group of plants to have vascular tissue (xylem and phloem) -- true roots, stems and leaves present.
• Reproduce by spores (no seeds) -- spores are produced in structures called sporangia on the underside of leaves (sori = clusters of sporangia).
• Still require water for fertilisation (flagellated male gametes swim to female gametes).
• Dominant generation is the sporophyte (the large fern plant we see); gametophyte is a small, independent prothallus.
• Found in moist, shady forests, hills, and damp walls.

Examples:

• Dryopteris -- common fern; typical example with sori on leaflets.
• Adiantum -- maidenhair fern; popular ornamental plant.
• Equisetum (horsetail) -- hollow stems with whorled branches; only surviving horsetail genus.
• Selaginella -- spike moss; some species heterosporous.
• Marsilea -- water fern; looks like four-leafed clover.

Significance: Ancient pteridophytes formed massive forests during the Carboniferous period (~300 million years ago); their compressed remains form much of today's coal deposits.

11. Gymnosperms -- Naked-seed Plants

Key features:

• First seed-bearing plants; seeds are naked (not enclosed in a fruit) -- the name comes from Greek gymnos = naked, sperma = seed.
• Well-developed vascular tissue; true roots, stems and leaves (needle-like or scale-like leaves in most).
• Bear cones -- male cones produce pollen, female cones produce ovules (which become seeds after fertilisation).
• Mostly perennial, evergreen trees (do not shed all leaves at once).
• Pollination by wind (anemophily).
• No fertilisation dependent on water -- pollen tube carries male gamete to ovule.

Examples:

• Pinus (pine) -- long needle-like leaves; resin-bearing; common in hilly areas.
• Cycas -- palm-like; one of the most ancient seed plants; found in tropical regions.
• Cedrus (cedar / deodar) -- tall coniferous tree; State tree of Jammu and Kashmir.
• Ginkgo biloba -- "living fossil"; fan-shaped leaves; used in medicines.
• Abies (fir), Picea (spruce) -- common conifers of temperate forests.

Economic importance: Timber (pine, deodar), paper pulp (spruce), resins, turpentine, rosin. Pine forests form an important softwood resource worldwide.

12. Angiosperms -- Flowering Plants with Covered Seeds

Key features:

• Most evolved and most diverse group of plants (approximately 250,000 species).
• Seeds are enclosed in a fruit (the ripened ovary); "angio" = covered vessel, "sperm" = seed.
• Reproduce through flowers; pollination by wind, insects, birds, water.
• Well-developed vascular tissue; true roots, stems and leaves.
• Double fertilisation -- one male gamete fertilises egg (forms zygote); another fuses with polar nuclei (forms endosperm for seed nutrition).

Angiosperms are divided into two classes:

13. Kingdom Animalia -- Overview

Key features of all animals:

• Eukaryotic, multicellular organisms; no cell wall.
• Heterotrophic -- consume and digest food (holozoic nutrition).
• Can move (at least at some stage of life).
• Mostly sexual reproduction.

Animals are classified based on three fundamental body features:

1. Body symmetry:
   • Asymmetrical -- no line of symmetry (e.g. sponges).
   • Radially symmetrical -- any plane through the central axis gives equal halves (e.g. jellyfish, sea anemone).
   • Bilaterally symmetrical -- only one plane divides into equal halves (e.g. earthworm, insects, humans).
2. Body cavity (Coelom):
   • Acoelomate -- no body cavity (e.g. Platyhelminthes).
   • Pseudocoelomate -- false body cavity not lined by mesoderm (e.g. Nematoda).
   • Coelomate -- true body cavity lined by mesoderm on all sides (e.g. Annelida onwards).
3. Notochord:
   • Non-chordates -- no notochord (Porifera to Echinodermata).
   • Chordates -- notochord present at some stage of development (Phylum Chordata).

14. Non-chordate Phyla (Porifera to Echinodermata)

Phylum Porifera (Sponges)

• Simplest animals; no true tissues or organs -- cellular level of organisation.
• Asymmetrical; porous body with a large central cavity (spongocoel); water enters through pores (ostia) and exits through osculum.
• Non-motile (sessile) in adult stage; mostly marine, a few freshwater.
• Skeleton of spicules (silica or calcium carbonate) or spongin fibres.
• Reproduce asexually (budding, fragmentation) and sexually.
• Examples: Sycon, Spongilla (freshwater), Euspongia (bath sponge).

Phylum Coelenterata (Cnidaria)

• Aquatic; radially symmetrical; tissue level of organisation; two germ layers (diploblastic: ectoderm and endoderm) with mesoglea in between.
• Have a central gastrovascular cavity with a single opening (mouth).
• Possess cnidoblasts (stinging cells with nematocysts) used for prey capture and defence.
• Exist in two body forms: polyp (sessile, cylindrical -- e.g. Hydra) and medusa (free-swimming, umbrella-shaped -- e.g. jellyfish).
• Examples: Hydra, Aurelia (jellyfish), Adamsia (sea anemone), corals (Meandrina, Gorgonia).

Phylum Platyhelminthes (Flatworms)

• Bilaterally symmetrical; acoelomate; triploblastic (three germ layers: ectoderm, mesoderm, endoderm).
• Flattened body (dorsoventrally); organ level of organisation.
• Mostly parasitic (some free-living like Planaria); absorb nutrients through body surface.
• No circulatory or respiratory system; excretion by flame cells.
• Examples: Planaria (free-living, famous for regeneration), Taenia (tapeworm -- intestinal parasite), Fasciola (liver fluke).

Phylum Nematoda (Roundworms / Aschelminthes)

• Bilaterally symmetrical; pseudocoelomate; triploblastic.
• Cylindrical body, tapering at both ends; body wall with cuticle.
• Have a complete digestive system (mouth AND anus -- unlike Platyhelminthes).
• Mostly parasitic; cause diseases in plants and animals.
• Examples: Ascaris (roundworm -- causes ascariasis), Wuchereria (causes filariasis/elephantiasis), Ancylostoma (hookworm).

Phylum Annelida (Segmented Worms)

• Bilaterally symmetrical; true coelomate; triploblastic.
• Body is metamerically segmented -- each segment has its own set of organs; segments separated by septa.
• First phylum with a closed circulatory system.
• Excretion by nephridia; respiration through skin or gills.
• Locomotion by setae (bristles) or parapodia (paddle-like appendages in Nereis).
• Examples: Earthworm (Pheretima) -- "friend of farmers," aerates soil; Leech (Hirudinaria) -- ectoparasite; Nereis -- marine, has parapodia.

Phylum Arthropoda (Jointed-leg Animals -- Largest Animal Phylum)

• Bilaterally symmetrical; coelomate (haemocoel); triploblastic; segmented body.
• Jointed appendages (legs, antennae, mouthparts) -- the defining feature.
• Exoskeleton of chitin; periodically shed (moulting / ecdysis) for growth.
• Open circulatory system; compound eyes in most; breathe through tracheae/book lungs/gills.
• Largest animal phylum -- over a million species (about 80% of all animal species).
• Examples:
  • Insects: housefly, butterfly, mosquito, ant, bee -- 3 pairs of legs, 3 body parts (head, thorax, abdomen).
  • Crustacea: prawn, crab, lobster -- 5 pairs of legs, two pairs of antennae.
  • Arachnida: spider, scorpion -- 4 pairs of legs, no antennae.
  • Myriapoda: centipede (one pair of legs per segment), millipede (two pairs per segment).

Phylum Mollusca

• Bilaterally symmetrical; soft-bodied; often have a hard shell of calcium carbonate.
• Body divided into: muscular foot (locomotion), visceral mass (organs), mantle (secretes shell).
• Open circulatory system (except Nautilus and cephalopods); respiration by gills.
• Excretion by kidneys (nephridia).
• Examples: Pila (apple snail), Unio (freshwater mussel), Octopus and Sepia (cephalopods -- closed circulatory system; highly intelligent with well-developed brain).

Phylum Echinodermata (Spiny-skinned Animals)

• Marine; radially symmetrical as adults, bilaterally symmetrical as larvae (shows evolutionary link to bilateral ancestors).
• Endoskeleton of calcareous (calcium carbonate) spines/plates embedded in skin.
• Unique water-vascular system (hydraulic) -- used for locomotion, food capture and gas exchange via tube feet.
• Coelomate; no excretory system; remarkable regeneration ability.
• Examples: Asterias (starfish), Echinus (sea urchin), Holothuria (sea cucumber), Antedon (feather star), Ophiura (brittle star).

15. Phylum Chordata -- Vertebrates

Defining features of Chordata (at some stage of development):

• Notochord -- flexible rod providing skeletal support.
• Dorsal hollow nerve cord -- becomes the brain and spinal cord.
• Pharyngeal gill slits -- for filter feeding or gas exchange.
• Post-anal tail (present at least in embryo).

Sub-phylum Vertebrata -- the notochord is replaced by a vertebral column (backbone) in the adult.

Class Pisces (Fishes)

• Cold-blooded (poikilothermic / ectothermic) -- body temperature varies with surroundings.
• Aquatic; breathe through gills.
• Streamlined body; locomotion by fins; skin covered with scales.
• Two-chambered heart (1 auricle + 1 ventricle); single circulation.
• Cartilaginous fishes: Scoliodon (dogfish shark), Torpedo (electric ray), Trygon (stingray) -- skeleton of cartilage.
• Bony fishes: Labeo (rohu), Catla, Exocoetus (flying fish), Hippocampus (sea horse) -- skeleton of bone.

Class Amphibia (Amphibians)

• Cold-blooded; can live on both land and in water -- "double life" (Greek amphi = both, bios = life).
• Breathe through gills in larval stage, lungs and moist skin in adult stage.
• Three-chambered heart (2 auricles + 1 ventricle).
• Lay eggs in water; fertilisation mostly external.
• No scales; moist glandular skin; have four limbs (absent in caecilians).
• Examples: Rana (frog), Bufo (toad), Hyla (tree frog), Salamandra (salamander), Ichthyophis (caecilian -- limbless, burrowing).

Class Reptilia (Reptiles)

• Cold-blooded; fully terrestrial (some secondarily aquatic like sea turtles).
• Breathe through lungs; dry, scaly skin (scales of keratin) prevents water loss.
• Three-chambered heart (except crocodiles -- four-chambered).
• Amniotic eggs -- eggs with leathery shells protecting the embryo from desiccation; laid on land.
• Internal fertilisation; no metamorphosis (young resemble adults).
• Examples: Chelone (turtle), Testudo (tortoise), Chameleon, Calotes (garden lizard), Crocodylus (crocodile), snakes (Naja = cobra, Python), Hemidactylus (wall lizard).

Class Aves (Birds)

• Warm-blooded (homoiothermic / endothermic) -- maintain constant body temperature regardless of surroundings.
• Forelimbs modified into wings for flight; hindlimbs adapted for walking/perching/swimming.
• Body covered with feathers; hollow pneumatic bones reduce weight for flight.
• Four-chambered heart; high metabolic rate.
• Breathe through lungs with air sacs for highly efficient gas exchange.
• Lay hard-shelled (calcified) eggs; internal fertilisation.
• Examples: Corvus (crow), Columba (pigeon), Psittacus (parrot), Struthio (ostrich -- cannot fly; largest bird), Pavo (peacock -- national bird of India), Spheniscus (penguin -- cannot fly; aquatic).

Class Mammalia (Mammals)

• Warm-blooded; four-chambered heart.
• Presence of mammary glands to nourish young with milk -- the defining feature of the class.
• Body covered with hair / fur; external ear pinnae in most.
• Breathe through lungs; diaphragm helps in breathing.
• Most are viviparous (give birth to live young); a few are oviparous (platypus, echidna).
• Highly developed brain and complex behaviour.
• Subgroups:
  • Egg-laying mammals (Monotremes / Prototheria): Ornithorhynchus (duck-billed platypus), Echidna (spiny anteater).
  • Pouched mammals (Marsupials / Metatheria): Macropus (kangaroo), Didelphys (opossum) -- young born immature and complete development in mother's pouch.
  • Placental mammals (Eutheria): most mammals -- dog, cat, elephant, whale, bat, human (Homo sapiens) -- young develop fully in mother's uterus with placenta.

16. Five Kingdoms at a Glance

Heart chambers -- quick recall:

• Fish (Pisces): 2 chambers (1 auricle + 1 ventricle)
• Amphibia and most Reptiles: 3 chambers (2 auricles + 1 ventricle)
• Crocodiles, Birds (Aves), Mammals: 4 chambers (2 auricles + 2 ventricles)

Common mistakes to avoid:

• Blue-green algae are in Monera (prokaryotes), NOT in Thallophyta (eukaryotes).
• Fungi are NOT plants -- chitin cell wall + heterotrophic + no chlorophyll.
• Bryophytes have NO vascular tissue; Pteridophytes are the first with vascular tissue.
• In animals, use Phylum; in plants, use Division.
• Bat is a mammal (not a bird) -- hair and mammary glands are the proof.
• Crocodile has 4 chambers (exception in Reptilia).