UPSC Biology Biological Classification And System Of Classification / जैविक वर्गीकरण और वर्गीकरण की प्रणाली Biological Classification

Biological Classification

1.           Classification Technique

• In Linnaeus' time a Two Kingdom system of classification with Plantae and Animalia kingdoms was developed that included all plants and animals respectively.
• H. Whittaker (1969) proposed a Five Kingdom Classification. The kingdoms defined by him were named Monera, Protista, Fungi, Plantae and Animalia. Earlier classification systems included bacteria, blue green algae, fungi, mosses, ferns, gymnosperms and the angiosperms under 'Plants'. The character that unified this whole kingdom was that all the organisms included had a cell wall in their cells. This placed together groups which widely differed in other characteristics.
• It brought together the prokaryotic bacteria and the blue green algae with other groups which were eukaryotic. It also grouped together the unicellular organisms and the multicellular ones, say, for example, Chlamydomonas and Spirogyra were placed together under algae.
• The classification did not differentiate between the heterotrophic group - fungi, and the autotrophic green plants, though they also showed a characteristic difference in their walls composition - the fungi had chitin in their walls while the green plants had a cellulosic cell wall.
• When such characteristics were considered, the fungi were placed in a separate kingdom - Kingdom Fungi. All prokaryotic organisms were grouped together under Kingdom Monera and the unicellular eukaryotic organisms were placed in Kingdom Protista.
• Kingdom Protista has brought together Chlamydomonas, Chlorella (earlier placed in Algae within Plants and both having cell walls) with Paramoecium and Amoeba (which were earlier placed in the animal kingdom which lack cell wall). It has put together organisms which, in earlier classifications, were placed in different kingdoms. This happened because the criteria for classification changed.

2.           Kingdom Monera

• Bacteria are the sole members of the Kingdom Monera. They are the most abundant micro-organisms.
• Though the bacterial structure is very simple, they are very complex in behaviour. Compared to many other organisms, bacteria as a group show the most extensive metabolic diversity. Some of the bacteria are autotrophic, i.e., they synthesise their own food from inorganic substrates. They may be photosynthetic autotrophic or chemosynthetic autotrophic. The vast majority of bacteria are heterotrophs, i.e., they do not synthesise their own food but depend on other organisms or on dead organic matter for food.
• Archaebacteria are special since they live in some of the most harsh habitats such as extreme salty areas (halophiles), hot springs (thermoacidophiles) and marshy areas (methanogens).
• Archaebacteria differ from other bacteria in having a different cell wall structure and this feature is responsible for their survival in extreme conditions.
• Methanogens are present in the gut of several ruminant animals such as cows and buffaloes and they are responsible for the production of methane (biogas) from the dung of these animals.
• The cyanobacteria (also referred to as blue-green algae) have chlorophyll a similar to green plants and are photosynthetic autotrophs. The cyanobacteria are unicellular, colonial or filamentous, freshwater/marine or terrestrial algae. The colonies are generally surrounded by gelatinous sheath. They often form blooms in polluted water bodies.
• Some of these organisms can fix atmospheric nitrogen in specialised cells called hetero- cysts, e.g., Nostoc and Anabaena.
• Chemosynthetic autotrophic bacteria oxidise various inorganic substances such as nitrates, nitrites and ammonia and use the released energy for their ATP production. They play a great role in recycling nutrients like nitrogen, phosphorous, iron and sulphur.
• Heterotrophic bacteria are the most abundant in nature. The majority are important decomposers. Many of them have a significant impact on human affairs. They are helpful in making curd from milk, production of antibiotics, fixing nitrogen in legume roots, etc.
• Some are pathogens causing damage to human beings, crops, farm animals and pets. Cholera, typhoid, tetanus, citrus canker are well known diseases caused by different bacteria.
• Bacteria reproduce mainly by fission. Sometimes, under unfavourable conditions, they produce spores. They also reproduce by a sort of sexual reproduction by adopting a primitive type of DNA transfer from one bacterium to the other.
• The Mycoplasma are organisms that completely lack a cell wall. They are the smallest living cells known and can survive without oxygen. Many mycoplasma are pathogenic in animals and plants.

3.           Kingdom Protista

• All single-celled eukaryotes are placed under Protista, but the boundaries of this kingdom are not well defined. What may be 'a photosynthetic protistan' to one biologist may be 'a plant' to another.
• Chrysophytes, Dinoflagellates, Euglenoids, Slime moulds and Protozoans under Protista.
• Members of Protista are primarily aquatic. This kingdom forms a link with the others dealing with plants, animals and fungi. Being eukaryotes, the protistan cell body contains a well-defined nucleus and other membrane-bound organelles.
• Chrysophytes includes diatoms and golden algae (desmids). They are found in fresh water as well as in marine environments. They are microscopic and float passively in water currents (plankton). Most of them are photosynthetic.
• In diatoms the cell walls form two thin overlapping shells, which fit together as in a soap box. The walls are embedded with silica and thus the walls are indestructible. Thus, diatoms have left behind large amount of cell wall deposits in their habitat; this accumulation over billions of years is referred to as 'diatomaceous earth5. Being gritty this soil is used in polishing, filtration of oils and syrups. Diatoms are the chief 'producers' in the oceans.
• Dinoflagellates are mostly marine and photosynthetic. They appear yellow, green, brown, blue or red depending on the main pigments present in their cells. The cell wall has stiff cellulose plates on the outer surface. Most of them have two flagella; one lies longitudinally and the other transversely in a furrow between the wall plates. Very often, red dinoflagellates (Example: Gonyaulax) undergo such rapid multiplication that they make the sea appear red (red tides). Toxins released by such large numbers may even kill other marine animals such as fishes.
• Euglenoids are fresh water organisms found in stagnant water. Instead of a cell wall, they have a protein rich layer called pellicle which makes their body flexible. They have two flagella, a short and a long one. Though they are photosynthetic in the presence of sunlight, when deprived of sunlight they behave like heterotrophs by predating on other smaller organisms. Interestingly, the pigments of euglenoids are identical to those present in higher plants. (Example: Euglena).
• Slime moulds are saprophytic protists. The body moves along decaying twigs and leaves engulfing organic material. Under suitable conditions, they form an aggregation called plasmodium which may grow and spread over several feet. During unfavourable conditions, the plasmodium differentiates and forms fruiting bodies bearing spores at their tips. The spores possess true walls. They are extremely resistant and survive for many years, even under adverse conditions. The spores are dispersed by air currents.
• All protozoans are heterotrophs and live as predators or parasites. They are believed to be primitive relatives of animals. There are four major groups of protozoans.
• Amoeboid protozoans: These organisms live in fresh water, sea water or moist soil. They move and capture their prey by putting out pseudopodia (false feet) as in Amoeba. Marine forms have silica shells on their surface. Some of them such as Entamoeba are parasites.
• Flagellated protozoans: The members of this group are either free-living or parasitic. They have flagella. The parasitic forms cause diaseases such as sleeping sickness. (Example: Trypanosoma).
• Ciliated protozoans: These are aquatic, actively moving organisms because of the presence of thousands of cilia. They have a cavity (gullet) that opens to the outside of the cell surface. The coordinated movement of rows of cilia causes the water laden with food to be steered into the gullet. (Example: Paramoecium).
• Sporozoans: This includes diverse organisms that have an infectious spore-like stage in their life cycle. The most notorious is Plasmodium (malarial parasite) which causes malaria, a disease which has a staggering effect on human population.

4.           Kingdom Fungi

• The fungi constitute a unique kingdom of heterotrophic organisms. They show a great diversity in morphology and habitat. When your bread develops a mould or orange rots it is because of fungi.
• The common mushroom we eat and toadstools are also fungi. White spots seen on mustard leaves are due to a parasitic fungus.
• Some unicellular fungi, e.g., yeast are used to make bread and beer. Other fungi cause diseases in plants and animals; wheat rust-causing Puccinia is an important example.
• Some are the source of antibiotics, e.g., Penicillium.
• Fungi are cosmopolitan and occur in air, water, soil and on animals and plants. They prefer to grow in warm and humid places.
• The cell walls of fungi are composed of chitin and polysaccharides. Most fungi are heterotrophic and absorb soluble organic matter from dead substrates and hence are called saprophytes. Those that depend on living plants and animals are called parasites. They can also live as symbionts - in association with algae as lichens and with roots of higher plants as mycorrhiza.
• Reproduction in fungi can take place by vegetative means - fragmentation, fission and budding.
• Members of phycomycetes are found in aquatic habitats and on decaying wood in moist and damp places or as obligate parasites on plants.
• Some common examples are Mucor, Rhizopus (the bread mould mentioned earlier) and Albugo (the parasitic fungi on mustard).
• Ascomycetes are commonly known as sac-fungi, the as comycetes are mostly multicellular, e.g., Penicillium, or rarely unicellular, e.g., yeast (Saccharomyces). They are saprophytic, decomposers, parasitic or coprophilous (growing on dung). Mycelium is branched and septate.
• Some examples are Aspergillus, Claviceps and Neurospora. Neurospora is used extensively in biochemical and genetic work. Many members like morels and truffles are edible and are considered delicacies.
• Basidiomycetes are mushrooms, bracket fungi or puffballs. They grow in soil, on logs and tree stumps and in living plant bodies as parasites, e.g., rusts and smuts.
• Deuteromycetes are commonly known as imperfect fungi because only the asexual or vegetative phases of these fungi are known.
• Some members are saprophytes or parasites while a large number of them are decomposers of litter and help in mineral cycling. Some examples are Altemaria, Colletotrichum and Trichoderma.

5.           Plant Kingdom

• Kingdom Plantae includes all eukaryotic chlorophyll-containing organisms commonly called plants. A few members are partially heterotrophic such as the insectivorous plants or parasites. Bladderwort and Venus fly trap are examples of insectivorous plants and Cuscuta is a parasite.
• The plant cells have an eukaryotic structure with prominent chloroplasts and cell wall mainly made of cellulose.
• Plantae includes algae, bryophytes, pteridophytes, gymnosperms and angiosperms.

• Algae
• Algae are chlorophyll-bearing, simple, thalloid, autotrophic and largely aquatic (both fresh water and marine) organisms. They occur in a variety of other habitats: moist stones, soils and wood. Some of them also occur in association with fungi (lichen) and animals (e.g., on sloth bear).
• The form and size of algae is highly variable. The size ranges from the microscopic unicellular forms like Chlamydomonas, to colonial forms like Volvox and to the filamentous forms like Ulothrix and Spirogyra. A few of the marine forms such as kelps, form massive plant bodies.
• The algae reproduce by vegetative, asexual and sexual methods.
• Algae are useful to man in a variety of ways. At least a half of the total carbon dioxide fixation on earth is carried out by algae through photosynthesis.
• Being photosynthetic they increase the level of dissolved oxygen in their immediate environment. They are of paramount importance as primary producers of energy-rich compounds which form the basis of the food cycles of all aquatic animals.
• Many species of Porphyra, Laminaria and Sargassum are among the 70 species of marine algae used as food.
• Certain marine brown and red algae produce large amounts of hydrocolloids (water holding substances), e.g., algin (brown algae) and carrageen (red algae) which are used commercially.
• Agar, one of the commercial products obtained from Gelidium and Gracilaria are used to grow microbes and in preparations of ice-creams and jellies. Chlorella a unicellular alga, rich in proteins is used as food supplement even by space travellers.
• The algae are divided into three main classes: Chlorophyceae, Phaeophyceae and Rhodophyceae.
• The members of chlorophyceae are commonly called green algae. The plant body may be unicellular, colonial or filamentous. They are usually grass green due to the dominance of pigments chlorophyll a and b.
• Some commonly found green algae are: Chlamydomonas, Volvox, Ulothrix, Spirogyra and Chara.
• The members of phaeophyceae or brown algae are found primarily in marine habitats. They possess chlorophyll a, c, carotenoids and xanthophylls. They vary in colour from olive green to various shades of brown depending upon the amount of the xanthophyll pigment, fucoxanthin present in them.
• The common forms are Ectocarpus, Dictyota, Laminaria, Sargassum and Fucus.
• The members of rhodophyceae are commonly called red algae because of the predominance of the red pigment, r-phycoerythrin in their body. Majority of the red algae are marine with greater concentrations found in the warmer areas. They occur in both well-lighted regions close to the surface of water and also at great depths in oceans where relatively little light penetrates.

• Bryophytes
• Bryophytes include the various mosses and liverworts that are found commonly growing in moist shaded areas in the hills. Bryophytes are also called amphibians of the plant kingdom because these plants can live in soil but are dependent on water for sexual reproduction. They lack true roots, stem or leaves.
• Bryophytes in general are of little economic importance but some mosses provide food for herbaceous mammals, birds and other animals. Species of Sphagnum, a moss, provide peat that have long been used as fuel, and as packing material for trans-shipment of living material because of their capacity to hold water.
• Mosses along with lichens are the first organisms to colonise rocks and hence, are of great ecological importance. They decompose rocks making the substrate suitable for the growth of higher plants. Since mosses form dense mats on the soil, they reduce the impact of falling rain and prevent soil erosion. The bryophytes are divided into liverworts and mosses.

• Pteridophytes
• Pteridophytes are used as ornamentals. Evolutionarily, they are the first terrestrial plants to possess vascular tissues - xylem and phloem.
• Pteridophytes is differentiated into true root, stem and leaves. These organs possess well-differentiated vascular tissues.

• Gymnosperms
• The gymnosperms (gymnos: naked, sperma: seeds) are plants in which the ovules are not enclosed by any ovary wall and remain exposed, both before and after fertilisation. Gymnosperms include medium-sized trees or tall trees and shrubs.
• The giant redwood tree Sequoia is one of the tallest tree species.
• Roots in some genera have fungal association in the form of mycorrhiza (Pinus), while in some others (Cycas) small specialised roots called coralloid roots are associated with \[{{N}_{2}}\] - fixing cyanobacteria.

• Angiosperms
• Unlike the gymnosperms where the ovules are naked, in the angiosperms or flowering plants, the pollen grains and ovules are developed in specialised structures called flowers.
• In angiosperms, the seeds are enclosed by fruits. The angiosperms are an exceptionally large group of plants occurring in wide range of habitats. They range in size from tiny, almost microscopic Wolfia to tall trees of Eucalyptus (over 100 metres).
• They are divided into two classes: the dicotyledons and the monocotyledons. The dicotyledons are characterised by having two cotyledons in their seeds while the monocolyledons have only one.

6.           Animal Kingdom

• Phylum-Porifera
• The word Porifera means organisms with holes. These are non-motile animals attached to some solid support. There are holes or 'pores', all over the body.
• These lead to a canal system that helps in circulating water throughout the body to bring in food and oxygen. These animals are covered with a hard outside layer or skeleton.
• The body design involves very minimal differentiation and division into tissues. They are commonly called sponges, and are mainly found in marine habitats. Some examples are Euplectelea, Sycon, Spongilla etc.

• Phylum-Coelenterata (Cnidaria)
• These are animals living in water. They show more body design differentiation. There is a cavity in the body. The body is made of two layers of cells: one makes up cells on the outside of the body, and the other makes the inner lining of the body.
• Some of these species live in colonies (corals), while others have a solitary like-span (Hydra). Jellyfish and sea anemones are common examples.

• Phylum-Ctenophora
• Ctenophores, commonly known as sea walnuts or comb jellies are exclusively marine, radially symmetrical, diploblastic organisms with tissue level of organisation. Bioluminescence (the property of a living organism to emit light) is well-marked in Ctenophores. Examples: Pleurobrachia and Ctenoplana.

• Phylum-Platyhelminthes
• They have dorso-ventrally flattened body, hence are called flatworms.
• These are mostly endoparasites found in animals including human beings.
• Have high regeneration capacity. Examples: Taenia (Tapeworm), Fasciola (Liver fluke).

• Phylum-Aschelminthes
• The body of the aschelminthes is circular in cross-section, hence, the name roundworms. They may be freeliving, aquatic and terrestrial or parasitic in plants and animals. Examples: Ascaris (Round Worm), Wuchereria (Filaria worm), Ancylostoma (Hookworm).

• Phylum-Annelida
• They may be aquatic (marine and fresh water) or terrestrial; free-living, and sometimes parasitic. Their body surface is distinctly marked out into segments.
• Nereis, an aquatic form, is dioecious, but earthworms and leeches are monoecious. Reproduction is sexual. Examples: Nereis, Pheretima (Earthworm) and Hirudinaria (Blood sucking leech).

• Phylum-Arthropoda
• This is the largest phylum of Animalia which includes insects. Over two-thirds of all named species on earth are arthropods. The body of arthropods is covered by chitinous exoskeleton. The body consists of head, thorax and abdomen.
• They have jointed appendages (arthros-joint, poda-appendages). Examples: Economically important insects - Apis (Honey bee), Bombyx (Silkworm), Laccifer (Lac insect) Vectors - Anopheles, Culex and Aedes (Mosquitoes) Gregarious pest - Locusta (Locust) Living fossil - Limulus (King crab).
• Phylum-Mollusca
• This is the second largest animal phylum. Molluscs are terrestrial or aquatic (marine or fresh water) having an organ-system level of organisation.
• Body is covered by a calcareous shell and is unsegmented with a distinct head, muscular foot and visceral hump. A soft and spongy layer of skin forms a mantle over the visceral hump. Examples: Pila (Apple snail), Pinctada (Pearl oyster), Sepia (Cuttlefish), Loligo (Squid), Octopus (Devil fish), Aplysia (Seahare), Dentalium (Tusk shell) and Chaetopleura (Chiton).

• Phylum-Echinodermata
• These animals have an endoskeleton of calcareous ossicles and, hence, the name Echinodermata (Spiny bodied). All are marine with organ-system level of organisation. Examples: Asterias (Star fish). Echinus (Sea urchin), Antedon (Sea lily), Cucumaria (Sea cucumber) and Ophiura (Brittle star).
• Phylum-Hemichordata
• This phylum consists of a small group of worm-like marine animals.

• Phylum-Chordata
• Animals belonging to phylum Chordata are fundamentally characterised by the presence of a notochord, a dorsal hollow nerve cord and paired pharyngeal gill slits.
• Phylum Chordata is divided into three subphyla: Urochordata or Tunicata, Cephalochordata and Vertebrata.
• Subphyla Urochordata and Cephalochordata are often referred to as protochordates and are exclusively marine. In Urochordata, notochord is present only in larval tail, while in Cephalochordata, it extends from head to tail region and is persistent throughout their life. Examples: Urochordata - Ascidia, Salpa, Doliolum; Cephalochordata - Branchiostoma (Amphioxus or Lancelet).
• Class Chondrichthyes are marine animals with streamlined body and have cartilaginous endoskeleton. Due to the absence of air bladder, they have to swim constantly to avoid sinking.
• Heart is two-chambered (one auricle and one ventricle). Some of them have electric organs (e.g., Torpedo) and some possess poison sting (e.g., Trygon).
• They are cold-blooded (poikilothermous) animals, i.e., they lack the capacity to regulate their body temperature. Examples: Scoliodon (Dog fish), Pristis (Saw fish), Carcharodon (Great white shark), Trygon (Sting ray).
• Class Osteichthyes includes both marine and fresh water fishes with bony endoskeleton. Examples: Marine - Exocoetus (Flying fish), Hippocampus (Sea horse); Freshwater - Labeo (Rohu), Catia (Katia), Clarias (Magur); Aquarium - Betta (Fighting fish), Pterophyllum (Angel fish).

• Class-Reptilia
• The class name refers to their creeping or crawling mode of locomotion (Latin, repere or reptum, to creep or crawl). They are mostly terrestrial animals and their body is covered by dry and comified skin, epidermal scales or scutes.
• They do not have external ear openings. Tympanum represents ear. Limbs, when present, are two pairs. Heart is usually three-chambered, but four-chambered in crocodiles.
• Reptiles are poikilotherms. Snakes and lizards shed their scales as skin cast. Sexes are separate. Fertilisation is internal. They are oviparous and development is direct. Examples: Chelone (Turtle), Testudo (Tortoise), Chameleon (Tree lizard), Calotes (Garden lizard), Crocodilus (Crocodile), Alligator (Alligator). Hemidactylus (Wall lizard), Poisonous snakes-Naja (Cobra), Bangams (Krait), Vipera (Viper).

• Class-Aves
• The clw acteristic features of Aves (birds) are the presence of feathers and most of them can fly except flightless birds (e.g., Ostrich). They possess beak. The forelimbs are modified into wings. The hind limbs generally have scales and are modified for walking, swimming or clasping the tree branches.
• Skin is dry without glands except the oil gland at the base of the tail. Endoskeleton is fully ossified (bony) and the long bones are hollow with air cavities (pneumatic).
• Heart is completely four chambered. They are warm-blooded (homoiothermous) animals, i.e., they are able to maintain a constant body temperature. Respiration is by lungs.

• Class-Mammalia
• They are found in a variety of habitats - polar ice caps, deserts, mountains, forests, grasslands and dark caves. Some of them have adapted to fly or live in water.
• The most unique mammalian characteristic is the presence of milk producing glands (mammary glands) by which the young ones are nourished.
• The skin of mammals is unique in possessing hair. External ears or pinnae are present. Different types of teeth are present in the jaw. Heart is fourchambered. They are viviparous with few exceptions and development is direct. Examples: Oviparous-Omithorhynchus (Platypus); Viviparous-Macropus (Kangaroo), Pteropus (Flying fox), Camelus (Camel).
• Most mammals familiar to us produce live young ones. However, a few of them, like the platypus and the echidna lay eggs, and some, like kangaroos give birth to very poorly developed young ones.

Note - These animals differ from the fish in the lack of scales, in having mucus glands in the skin, and a three-chambered heart. Respiration is through either gills or lungs. They lay eggs. These animals are found both in water and on land. Frogs, toads and salamanders are some examples.

7.           Viruses, Viroids and Lichens

• In the five kingdom classification of Whittaker there is no mention of some a cellular organisms like viruses and viroids, and lichens.
• The viruses are non-cellular organisms that are characterised by having an inert crystalline structure outside the living cell. Once they infect a cell they take over the machinery of the host cell to replicate themselves, killing the host.
• The name vims that means venom or poisonous fluid was given by Pasteur. DJ. Ivanowsky (1892) recognised certain microbes as causal organism of the mosaic disease of tobacco.
• These were found to be smaller than bacteria because 'they passed through bacteria- proof filters. M.W. Beijerinek (1898) demonstrated that the extract of the infected plants of tobacco could cause infection in healthy plants and called the fluid as Contagium vivum fluidum (infectious living fluid). W.M. Stanley (1935) showed that viruses could be crystallised and crystals consist largely of proteins.
• They are inert outside their specific host cell. Viruses are obligate parasites.
• In addition to proteins, viruses also contain genetic material that could be either RNA or DNA. No virus contains both RNA and DNA. A virus is a nucleoprotein and the genetic material is infectious.
• In general, viruses that infect plants have single stranded RNA and viruses that infect animals have either single or double stranded RNA or double stranded DNA. Bacterial viruses or bacteriophages (viruses that infect the bacteria) are usually double stranded DNA viruses.
• The protein coat called capsid made of small subunits called capsomeres, protects the nucleic acid. These capsomeres are arranged in helical or polyhedral geometric forms.
• Viruses cause diseases like mumps, small pox, herpes and influenza. AIDS in humans is also caused by a virus. In plants, the symptoms can be mosaic formation, leaf rolling and curling, yellowing and vein clearing, dwarfing and stunted growth.
• In 1971, T.O. Diener discovered a new infectious agent that was smaller than viruses and caused potato spindle tuber disease. It was found to be a free RNA; it lacked the protein coat that is found in viruses, hence the name viroid. The RNA of the viroid was of low molecular weight.
• Lichens are symbiotic associations i.e. mutually useful associations, between algae and fungi. The algal component is known as phycobiont and fungal component as mycobiont, which are autotrophic and heterotrophic, respectively. Algae prepare food for fungi and fungi provide shelter and absorb mineral nutrients and water for its partner. So close is their association that if one saw a lichen in nature one would never imagine that they had two different organisms within them.
• Lichens are very good pollution indicators - they do not grow in polluted areas.