What Is The Basis Of Classification?

Throughout history, humans have attempted to classify living things. Early attempts were often simple and based on superficial similarities or habitat, like Aristotle classifying animals based on whether they lived on land, in water, or in the air. However, such simple methods can be misleading, as diverse organisms can share a habitat (e.g., whales, corals, and sharks all live in the sea but are vastly different).

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A more appropriate approach requires identifying characteristics that represent fundamental differences in body design and function. These characteristics are used hierarchically: the most fundamental characteristics form the broadest groups, and less fundamental characteristics are used to form subgroups within those broad groups.

What exactly is meant by a 'characteristic' in this context? It refers to a specific feature or function of an organism. For example, having five fingers is a characteristic for humans; the ability to photosynthesise is a characteristic of plants.

To understand the hierarchical nature of characteristics, consider building a stone wall. The shape and size of the base stones influence the layers above. Similarly, some characteristics are more basic, independent of others, and affect many subsequent aspects of an organism's design. Other characteristics are dependent on these earlier ones and contribute to variations within groups.

Modern classification uses a hierarchy of interrelated characteristics, often starting with fundamental features at the cellular level.

Examples of Characteristics Used for Classification:

• Nature of the Cell: Is the cell prokaryotic (lacking a defined nucleus and membrane-bound organelles) or eukaryotic (having a defined nucleus and membrane-bound organelles)? This is a very basic difference that impacts how cellular processes are organised and affects all aspects of cell design. Eukaryotic cells, with their internal compartmentalisation, have the potential for complex multicellular organisation.
• Number of Cells: Is the organism single-celled (unicellular) or made up of many cells (multicellular)? Multicellular organisms typically show division of labour, with specialised cells grouped into tissues, leading to complex body designs significantly different from unicellular organisms like Amoeba.
• Mode of Nutrition: Does the organism produce its own food through photosynthesis (autotrophic), or does it obtain food from external sources (heterotrophic)? This fundamental difference in how organisms obtain energy directly affects their body design and interaction with the environment.
• Level of Organisation (in multicellular organisms): How are cells organised? Do they form tissues? Are tissues organised into organs and organ systems? The complexity of body organisation is a key characteristic for classification.

The characteristics used for classifying plants (which are primarily autotrophic) will differ from those used for classifying animals (which are primarily heterotrophic), as their basic life strategies and body designs are different.

Classification And Evolution

The classification of living organisms is closely intertwined with their evolutionary history.

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Evolution is the process by which life forms change over successive generations through the accumulation of variations in body design. These changes allow organisms to survive and reproduce more successfully in their environment.

The concept of evolution was notably described by Charles Darwin in his book 'The Origin of Species' (1859).

Characteristics that appeared earlier in evolutionary history are generally considered more basic and are used to create broader classification groups. Characteristics that appeared later are considered less basic and define smaller subgroups.

Based on evolution, some organisms have relatively ancient body designs that have changed little over vast periods. These are sometimes called 'primitive' or 'lower' organisms. Other organisms have evolved more recently and have more complex body designs. These are sometimes called 'advanced' or 'higher' organisms.

While the terms 'primitive' and 'advanced' can be misleading, it is generally true that older organisms tend to be simpler in design, while younger organisms (in an evolutionary sense) tend to be more complex. Classification systems reflect these evolutionary relationships.

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Biodiversity refers to the variety of life forms found in a particular region or on Earth as a whole. This diversity results from millions of years of evolution. Regions with warm, humid climates, particularly between the tropics (e.g., parts of Brazil, India, Australia), are known for exceptionally high biodiversity and are called megadiversity regions.

The Hierarchy Of Classification- Groups

To classify the vast diversity of life systematically, biologists have proposed broad categories called kingdoms.

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A widely accepted system is the Five Kingdom Classification proposed by Robert Whittaker in 1969. This system divides all living organisms into five kingdoms based on key characteristics like cell structure (prokaryotic/eukaryotic), number of cells (unicellular/multicellular), mode of nutrition (autotrophic, heterotrophic, saprotrophic), and body organisation.

The five kingdoms are:

1. Monera
2. Protista
3. Fungi
4. Plantae
5. Animalia

Carl Woese later proposed a modification to this system by dividing the Kingdom Monera into Archaebacteria (Archaea) and Eubacteria (Bacteria), based on differences in cell structure.

Within these kingdoms, organisms are further classified into smaller and smaller subgroups based on a hierarchy of increasingly specific characteristics. The standard hierarchical levels of classification are:

Kingdom

↓

Phylum (for animals) / Division (for plants)

↓

Class

↓

Order

↓

Family

↓

Genus

↓

Species

As we move down the hierarchy from Kingdom to Species, the number of organisms in each group decreases, but the common characteristics shared by the organisms in that group increase. The basic unit of classification is the species, which broadly includes organisms that are similar enough to interbreed and produce fertile offspring.

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Important Characteristics of the Five Kingdoms:

Monera

Characteristics:

• These are prokaryotic organisms (lack a defined nucleus and membrane-bound organelles).
• They are typically unicellular; they do not show multicellular body designs.
• Some have cell walls, while others do not.
• Mode of nutrition can be either autotrophic (synthesise their own food, e.g., some bacteria, cyanobacteria) or heterotrophic (obtain food from the environment).

Examples: Bacteria, blue-green algae (cyanobacteria), mycoplasma.

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Protista

Characteristics:

• These are eukaryotic organisms (have a defined nucleus and membrane-bound organelles).
• They are predominantly unicellular.
• Some have structures for movement like cilia (hair-like) or flagella (whip-like).
• Mode of nutrition can be autotrophic (e.g., unicellular algae) or heterotrophic (e.g., protozoans).

Examples: Unicellular algae, diatoms, protozoans (Amoeba, Paramoecium, Euglena).

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Fungi

Characteristics:

• These are eukaryotic organisms.
• They are heterotrophic.
• They have cell walls made of a tough, complex sugar called chitin.
• Mode of nutrition:
  • Mostly saprotrophs, using decaying organic matter as food.
  • Some are parasites, living on or inside other living organisms and obtaining nutrients from them.
• Some fungi can be multicellular at certain stages of life.

Examples: Yeasts, molds, mushrooms (Agaricus, Penicillium, Saccharomyces).

Some fungi also live in symbiotic (mutually beneficial) relationships with blue-green algae (cyanobacteria). These symbiotic life forms are called lichens.

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Plantae

Characteristics:

• These are eukaryotic organisms.
• They are multicellular.
• They have cell walls (composed mainly of cellulose).
• Mode of nutrition is autotrophic, as they contain chlorophyll and perform photosynthesis.

Examples: All plants.

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Animalia

Characteristics:

• These are eukaryotic organisms.
• They are multicellular.
• They do not have cell walls.
• Mode of nutrition is heterotrophic (they cannot make their own food).
• Most animals are mobile (can move from place to place).

Examples: All animals.

Plantae

The Kingdom Plantae (plants) is a large and diverse group. Plants are classified into subgroups based on several criteria:

1. Whether the plant body is well-differentiated into distinct parts (roots, stem, leaves).
2. Whether specialised tissues are present for the transport of water and other substances (vascular tissue).
3. Whether the plant produces seeds.
4. If seeds are produced, whether they are naked or enclosed within a fruit.

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Thallophyta

Characteristics:

• Plant body is not well-differentiated into roots, stem, and leaves. The body is typically a simple structure called a thallus.
• Commonly known as algae.
• Predominantly aquatic.
• Lack specialised vascular tissues.

Examples: Spirogyra, Ulothrix, Cladophora, Ulva, Chara.

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Bryophyta

Characteristics:

• Often called the amphibians of the plant kingdom because they live in both water and land habitats (though they require water for reproduction).
• Plant body is more differentiated than Thallophytes, typically forming stem-like and leaf-like structures.
• However, they lack specialised vascular tissue for the conduction of water and other substances over long distances.

Examples: Mosses (Funaria), Liverworts (Marchantia, Riccia).

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Pteridophyta

Characteristics:

• Plant body is well-differentiated into true roots, stem, and leaves.
• Possess specialised vascular tissue (xylem and phloem) for transport.

Examples: Ferns, Marsilea, Horse-tails.

The reproductive organs of plants in Thallophyta, Bryophyta, and Pteridophyta are relatively inconspicuous or 'hidden'. These groups are collectively called Cryptogams.

Plants with well-differentiated reproductive parts that produce seeds are called Phanerogams. Seeds contain the embryo and stored food for initial growth (germination). Phanerogams are further classified based on whether the seeds are naked or enclosed in fruits.

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Gymnosperms

Characteristics:

• Phanerogams that produce naked seeds (seeds are not enclosed within an ovary/fruit).
• The name comes from Greek words 'gymno' (naked) and 'sperma' (seed).
• Typically perennial, evergreen, and woody plants.

Examples: Pines, Cycas, Deodar.

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Angiosperms

Characteristics:

• Phanerogams that produce seeds enclosed within an ovary, which develops into a fruit.
• Commonly called flowering plants.
• The name comes from Greek words 'angio' (covered) and 'sperma' (seed).
• Seeds contain embryonic leaves called cotyledons.

Angiosperms are divided into two groups based on the number of cotyledons in the seed:

• Monocotyledonous (Monocots): Seeds have a single cotyledon. Examples: wheat, maize, rice, grasses.
• Dicotyledonous (Dicots): Seeds have two cotyledons. Examples: peas, gram, beans, mango, neem.

Differences between monocots and dicots extend to their root systems (fibrous vs. taproot), leaf venation (parallel vs. reticulate), and flower parts (usually in multiples of 3 vs. multiples of 4 or 5).

Animalia

The Kingdom Animalia (animals) includes eukaryotic, multicellular, heterotrophic organisms that typically lack cell walls and are mobile. Animals are classified into subgroups based on increasing complexity in their body design differentiation.

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Porifera

Characteristics:

• Means 'organisms with holes' (pores).
• These are non-motile, attached animals.
• Body is covered with a hard outside layer or skeleton.
• Body design shows very minimal differentiation and lacks true tissues and organs.
• Have a canal system where water circulates through pores, bringing in food and oxygen.
• Predominantly marine.

Examples: Sponges (Sycon, Euplectella, Spongilla).

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Coelenterata (Cnidaria)

Characteristics:

• Aquatic animals.
• Show more body design differentiation than Porifera.
• Body has a cavity.
• Body is made of two layers of cells (diploblastic): an outer layer (epidermis) and an inner lining (gastrodermis).
• Some live in colonies (e.g., corals), others are solitary (e.g., Hydra).

Examples: Hydra, Jellyfish, Sea anemones.

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Platyhelminthes

Characteristics:

• Body design is more complex than Coelenterates.
• Body is bilaterally symmetrical (left and right sides are mirror images).
• Body is made of three layers of cells (triploblastic), allowing for differentiation of tissues and some organs.
• Body is flattened dorsoventrally (from top to bottom), hence called flatworms.
• Lack a true internal body cavity (coelom).
• Can be free-living or parasitic.

Examples: Free-living: Planarians. Parasitic: Liverfluke, Tapeworm.

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Nematoda

Characteristics:

• Body is bilaterally symmetrical and triploblastic.
• Body is cylindrical (roundworms) rather than flattened.
• Possess tissues, but lack true organs.
• Have a body cavity, but it is not a true coelom; it is a pseudocoelom.
• Many are parasitic, causing diseases.

Examples: Roundworm (Ascaris), Filarial worms (Wuchereria - causes elephantiasis), Pinworms.

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Annelida

Characteristics:

• Body is bilaterally symmetrical and triploblastic.
• Possess a true body cavity (coelom), allowing for accommodation of well-developed organs.
• Show extensive organ differentiation.
• Body is segmented, with segments lined up sequentially.
• Found in freshwater, marine, and terrestrial habitats.

Examples: Earthworms, Leeches, Nereis.

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Arthropoda

Characteristics:

• The largest group in the animal kingdom.
• Body is bilaterally symmetrical and segmented.
• Have jointed legs (the name 'arthropod' means jointed legs).
• Possess an open circulatory system (blood does not flow in defined vessels).
• The coelomic cavity is filled with blood.

Examples: Prawns, Butterflies, Houseflies, Spiders, Scorpions, Crabs, Centipedes, Cockroaches.

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Mollusca

Characteristics:

• Body is bilaterally symmetrical.
• Possess a coelomic cavity, but it is reduced.
• Show little segmentation.
• Have an open circulatory system.
• Have kidney-like organs for excretion.
• Possess a muscular 'foot' used for movement.
• Often have a hard shell.

Examples: Snails, Mussels, Octopus, Chiton, Pila, Unio.

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Echinodermata

Characteristics:

• Means 'spiny skinned organisms' (Greek: echinos = hedgehog/spiny, derma = skin).
• Exclusively free-living marine animals.
• Are triploblastic and have a coelomic cavity.
• Show radial symmetry in adult stage (larvae are bilaterally symmetrical).
• Possess a peculiar water-driven tube system used for movement and capturing food.
• Have hard calcium carbonate structures that form a skeleton.

Examples: Sea-stars (Starfish), Sea urchins (Echinus), Feather stars (Antedon), Sea cucumbers (Holothuria).

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Protochordata

Characteristics:

• Body is bilaterally symmetrical, triploblastic, and has a coelom.
• Show a new feature: the presence of a notochord at least at some stage of their life. The notochord is a long rod-like support structure along the back, separating the nervous tissue from the gut. It provides muscle attachment points.
• Notochord may not be present for the entire life or length of the animal.
• Marine animals.

Examples: Balanoglossus, Herdmania, Amphioxus.

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Vertebrata

Characteristics:

• Possess a true vertebral column (backbone) and an internal skeleton. This allows for a wide range of movement.
• Are bilaterally symmetrical, triploblastic, coelomic, and segmented.
• Show complex differentiation of body tissues and organs.
• Possess the following features (shared with Chordates):
  • Have a notochord (present during embryonic development).
  • Have a dorsal nerve cord.
  • Are triploblastic.
  • Have paired gill pouches (present at some stage).
  • Are coelomate.

Vertebrates are further classified into six classes.

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Cyclostomata

Characteristics:

• These are jawless vertebrates.
• Have an elongated, eel-like body.
• Possess a circular mouth.
• Skin is slimy and lacks scales.
• Are ectoparasites (live on the surface of) or borers of other vertebrates.

Examples: Lamprey (Petromyzon), Hagfish (Myxine).

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Pisces

Characteristics:

• Commonly known as fish.
• Exclusively aquatic animals.
• Body is typically streamlined for efficient movement in water.
• Obtain oxygen from water using gills.
• Skin is covered with scales or plates.
• Have a muscular tail for movement.
• Are cold-blooded (body temperature varies with environment).
• Have a two-chambered heart.
• Lay eggs (oviparous).
• Skeletons can be entirely cartilage (cartilaginous fish like sharks) or bone and cartilage (bony fish like tuna, rohu).

Examples: Sharks, Tuna, Rohu (Labeo rohita), Sea horse (Hippocampus), Flying fish (Exocoetus), Climbing perch (Anabas), Electric ray (Torpedo), Sting ray.

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Amphibia

Characteristics:

• Can live both in water and on land (though tied to water for reproduction).
• Skin lacks scales and has mucus glands.
• Are cold-blooded.
• Have a three-chambered heart.
• Respiration can be through gills (in larvae), lungs, or skin.
• Lay eggs in water.

Examples: Frogs (Rana tigrina, Hyla), Toads, Salamanders.

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Reptilia

Characteristics:

• Live primarily on land, though some inhabit water.
• Skin is covered with scales.
• Breathe through lungs.
• Are cold-blooded.
• Most have a three-chambered heart, but crocodiles have a four-chambered heart.
• Lay eggs with tough coverings, typically on land (do not require water to lay eggs).

Examples: Snakes (King Cobra), Turtles, Lizards (House wall lizard, Flying lizard - Draco, Chameleon), Crocodiles.

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Aves

Characteristics:

• Commonly known as birds.
• Are warm-blooded (maintain a constant internal body temperature).
• Have a four-chambered heart.
• Body is covered with feathers.
• Forelimbs are modified into wings for flight (though some birds are flightless).
• Breathe through lungs.
• Lay eggs.

Examples: Pigeons, Sparrows, Crows, White Stork, Ostrich, Tufted Duck.

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Mammalia

Characteristics:

• Are warm-blooded.
• Have a four-chambered heart.
• Possess mammary glands for producing milk to nourish their young.
• Skin has hairs, as well as sweat and oil glands.
• Most give birth to live young ones (viviparous).
• Some exceptions lay eggs (e.g., platypus, echidna).
• Some give birth to underdeveloped young (e.g., kangaroos - marsupials).

Examples: Humans, Cats, Rats, Bats, Whales, Kangaroos, Platypus, Echidna.

Nomenclature

Given the vast diversity of living organisms and the numerous local names for the same organism, there is a need for a standardised system of naming that is universally understood and accepted. This system is called scientific nomenclature.

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The system of giving scientific names to organisms, known as Binomial Nomenclature, was introduced by Carolus Linnaeus in the 18th century. 'Binomial' means 'two-name' system.

In this system, every organism is given a scientific name consisting of two parts:

1. The first part is the name of the Genus.
2. The second part is the name of the species.

These names are derived from Latin forms or Latinised from other languages to ensure universality.

Conventions for Writing Scientific Names:

1. The name of the Genus begins with a capital letter (Uppercase).
2. The name of the species begins with a small letter (Lowercase).
3. When printed, the scientific name is usually written in italics.
4. When written by hand, both the Genus name and the species name are underlined separately.

Example: The scientific name for human beings is Homo sapiens. When handwritten, it would be Homo sapiens.

Scientific names are unique for each organism and reflect its classification, particularly its genus and species, allowing clear identification globally.

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