DIVERSITY IN LIVING ORGANISMS

Diversity in Living OrganismsBy Tagore House

Classification And EvolutionClassification refers to the identification, naming, and grouping of organisms into a formal system based on similarities in their internal and external structure or evolutionary history. It determines the methods of organizing diversity of life on earth. Therefore, classification helps in understanding millions of life forms in detail.Who started the classification of organisms? Let us explore the history of classification.

History of classificationOne of the earliest schemes of classification was given by the Greek thinker, Aristotle, around 300 BC. He classified animals according to their habitat – land, air, or water.However, this classification of Aristotle was misleading because animals that live on land include earthworms, mosquitoes, butterflies, rats, elephants, tigers etc. These animals do not resemble each other except that they share a common habitat.Similarly, all aquatic animals do not resemble each other.Therefore, a new system of classification was developed to classify the vast diversity of organisms present on earth.Principles of classification followed today include:Nature of cell: Nature of the cell is considered to be the fundamental feature, as it gives rise to another feature called cellularity. It includes the presence or absence of membrane-bound organelles. Therefore, on the basis of this fundamental characteristic, we can classify living organisms into two broad categories of eukaryotes and prokaryotes.Cellularity: Unicellular organisms are those organisms whose body is made up of a single cell, whereas multicellular organisms are those organisms whose body is made up of many cells. Multicellular organisms use the principle of division of labour to perform specialized functions. This results in a specific body design that distinguishes multicellular organisms from unicellular organisms.Mode of nutrition: The mode of nutrition also distinguishes different organisms. The ability to manufacture their own food makes the body design of plants different from that of animals.

Five Kingdoms Of ClassificationAll organisms present on earth are classified into five major groups. This is known as the five-kingdom classification. Who proposed this classification and what are the five kingdoms? Kingdom is the highest level of classification as proposed by Linnaeus. Based on Linnaeus’ system of classification, biologist R.H. Whittaker (in 1969) proposed a five-kingdom classification of living organisms.The five kingdoms proposed by Whittaker are Monera, Protista, Fungi, Plantae, and Animalia.Carl Woese further divided Monera into Archaebacteria and Eubacteria depending on the environment they are able to inhabit.Fundamental characteristics used for classification of living organisms:Based on the presence or absence of membrane-bound organelles, all living organisms are divided into two broad categories of eukaryotes and prokaryotes.

Five Kingdom Classification

Kingdom AnimaliaKingdom Animalia is divided into chordates and non-chordates on the basis of the presence or absence of notochord. Non-chordates do not possess a notochord, whereas all members of phylum Chordata possess notochord.Therefore, can all animals be classified as chordates and non-chordates? The answer is NO.Fishes, birds, crocodiles, frogs, and monkeys possess a vertebral column, and thus are vertebrates. However, do they show the same features or characteristics? How are they classified? Let us explore the classification of higher animals.The sub-phylum Vertebrata is further divided into five classes.

ChordataChordates are defined as organisms that possess a structure called a notochord, at least during some part of their development. The notochord is a rod that extends most of the length of the body when it is fully developed. Lying dorsal to the gut but ventral to the central nervous system, it stiffens the body and acts as support during locomotion. Other characteristics shared by chordates include the following bilateral symmetry segmented body, including segmented muscles three germ layers and a well-developed coelom. single, dorsal, hollow nerve cord, usually with an enlarged anterior end (brain) tail projecting beyond (posterior to) the anus at some stage of development pharyngeal pouches present at some stage of development ventral heart, with dorsal and ventral blood vessels and a closed blood system complete digestive system bony or cartilaginous endoskeleton usually present.

InvertebrataThe organisms of this biological group posses the following features: (1) They are multicellular organisms.(2) They don't have  vertebrae. (3) Invertebrates don't have cell walls.(4) Most of them have tissues (not sponges) that are specific organizations of cells. Most of them reproduce sexually .

VertebrataVertebrates, which include fishes, reptiles, amphibians, birds, and mammals, all share a vertebral column, or a chain of bony elements (vertebrae) that run along the dorsal surface from head to tail and form the main skeletal axis of the body. The vertebral column surrounds and more or less replaces the notochord as the chief "stiffener" of the body in locomotion.

Class PiscesA typical fish is ectothermic, has a streamlined body for rapid swimming, extracts oxygen from water using gills or uses an accessory breathing organ to breathe atmospheric oxygen, has two sets of paired fins, usually one or two (rarely three) dorsal fins, an anal fin, and a tail fin, has jaws, has skin that is usually covered with scales, and lays eggs.Each criterion has exceptions. Tuna, swordfish, and some species of sharks show some warm-blooded adaptations—they can heat their bodies significantly above ambient water temperature. Streamlining and swimming performance varies from fish such as tuna, salmon, and jacks that can cover 10–20 body-lengths per second to species such as eels and rays that swim no more than 0.5 body-lengths per second. Many groups of freshwater fish extract oxygen from the air as well as from the water using a variety of different structures. Lungfish have paired lungs similar to those of tetrapod, gourami have a structure called the labyrinth organ that performs a similar function, while many catfish, such as Corydoras extract oxygen via the intestine or stomach. Body shape and the arrangement of the fins is highly variable, covering such seemingly un-fishlike forms as seahorses, puffer fish, anglerfish, and gulpers. Similarly, the surface of the skin may be naked (as in moray eels), or covered with scales of a variety of different types usually defined as placoid, cosmoid, ganoid, cycloid, and ctenoid.

Class PiscesFish range in size from the huge 16-metre (52 ft.) whale shark to the tiny 8-millimetre (0.3 in) stout infantfish.Fish species diversity is roughly divided equally between marine (oceanic) and freshwater ecosystems. Coral reefs in the Indo-Pacific constitute the centre of diversity for marine fishes, whereas continental freshwater fishes are most diverse in large river basins of tropical rainforests, especially the Amazon, Congo, and Mekong basins. More than 5,600 fish species inhabit Neotropical freshwaters alone, such that Neotropical fishes represent about 10% of all vertebrate species on the Earth.

Bony FishBony fish, any member of the superclass Osteichthyes, a group made up of the classes Sarcopterygii and Actinopterygii in the subphylum Vertebrata, including the great majority of living fishes and virtually all the world’s sport and commercial fishes. Osteichthyes excludes the jawless fishes of the class Agnatha (hagfishes and lampreys) and the cartilaginous fishes constituting the class Chondrichthyes (sharks, skates, and rays) but includes the 20,000 species and more than 400 families of modern bony fishes of the world, as well as a few primitive forms. The primary characteristic of bony fishes is a skeleton at least partly composed of true bone Other features include, in most forms, the presence of a swim bladder (an air-filled sac to give buoyancy), gill covers over the gill chamber, bony platelike scales, a skull with sutures, and external fertilization of eggs.Bony fishes occur in all freshwater and ocean environments, including caves, deep-sea habitats, and thermal springs and vents. The variety of shapes and behavioural habits is remarkable.

Cartilaginous fishesCartilaginous fishes (Chondrichthyes) are an ancient group of animals, having changed little in 100 million years. However ancient does not mean obsolete, but rather that they hit on a very successful body plan early on. The two modern groups of cartilaginous fishes are chimaerans (holocephali) and elasmobranchs (elasmobranchii). The later group is further split into sharks and dogfishes (selachimorpha), and rays (rajiformes).

AmphibiaAmphibians comprise a large and diverse class of animals. Amphibians, although thought to be soft and squishy, do have a mostly-bone skeleton (the rest being made of cartilage). The skin is almost always moist and is water permeable. It lacks scales, and can be smooth (frogs) or bumpy (toads). Amphibians come in a wide variety of shapes, sizes and colours, and some of the most poisonous vertebrates are amphibians (arrow-point frogs). Unlike reptiles, amphibians have many different options on how to breathe. Most species have lungs, so they can breathe through their mouths. They also have gills, either internal or external, for breathing underwater. Finally, the water-permeable skin allows oxygen to diffuse through it, so they can "breathe" through their skin and the lining of their mouths. Most amphibians are oviparous and will lay several hundred small, round eggs covered in a gelatinous mass. Most species have four limbs with webbed feet, although one order lacks limbs entirely.

ReptiliaReptiles are found in a small class of cold-blooded animals that are divided into four living orders (approx. 16 other orders are extinct). Reptiles are found throughout the entire world, from the steaming deserts to the inner city to hundreds of feet below the ocean. They are absent from the polar regions and mountain peaks. Reptiles share many common traits: they are all cold-blooded (meaning they can't regulate their body temperature)

ReptiliaThey have skin covered in scales or scutes (patches of bony or horny skin); the legs are short or entirely absent; and most are oviparous (they lay eggs), although some are ovoviviparous (eggs are kept in the mother's belly until they hatch). The egg yolk is rick and the shell is strong. Incubation is caused by the warmth of the ground, whether the eggs are laid in a nest (alligators) or buried (sea turtles). There is no larval stage with reptiles, unlike with amphibians. Also, the eggs are leathery instead of jelly-encased. There are 5000-6000 species of reptiles

AvesBirds (class Aves) are feathered, winged, bipedal, endothermic (warm-blooded), egg-laying, vertebrate animals. With around 10,000 living species, they are the most speciose class of tetrapod vertebrates. All present species belong to the subclass Neornithes, and inhabit ecosystems across the globe, from the Arctic to the Antarctic. Extant birds range in size from the 5 cm (2 in) Bee Hummingbird to the 2.75 m (9 ft) Ostrich. The fossil record indicates that birds emerged within theropod dinosaurs during the Jurassic period, around 160 million years (Ma) ago. Paleontologists regard birds as the only clade of dinosaurs to have survived the Cretaceous–Paleogene extinction event 65.5 Ma (million years) ago.

AvesModern birds are characterised by feathers, a beak with no teeth, the laying of hard-shelled eggs, a high metabolic rate, a four-chambered heart, and a lightweight but strong skeleton. All living species of birds have wings- the most recent species without wings was the moa, which is generally considered to have become extinct in the 1500s. Wings are evolved forelimbs, and most bird species can fly. Flightless birds include ratites, penguins, and a number of diverse endemic island species. Birds also have unique digestive and respiratory systems that are highly adapted for flight. Some birds, especially corvids and parrots, are among the most intelligent animal species; a number of bird species have been observed manufacturing and using tools, and many social species exhibit cultural transmission of knowledge across generations.

AvesMany species undertake long distance annual migrations, and many more perform shorter irregular movements. Birds are social; they communicate using visual signals and through calls and songs, and participate in social behaviours, including cooperative breeding and hunting, flocking, and mobbing of predators. The vast majority of bird species are socially monogamous, usually for one breeding season at a time, sometimes for years, but rarely for life. Other species have polygynous ("many females") or, rarely, polyandrous ("many males") breeding systems. Eggs are usually laid in a nest and incubated by the parents. Most birds have an extended period of parental care after hatching.

MammaliaMammals (class Mammalia) are warm-blooded amniotes. Among the features that distinguish them from the other amniotes, the reptiles and the birds, are hair, three middle ear bones, mammary glands in females, and a neocortex (a region of the brain). The mammalian brain regulates body temperature and the circulatory system, including the four-chambered heart. The mammals include the largest animals on the planet, the rorqual whales, as well as the most intelligent, the apes. The basic body type is a four-legged land-borne animal, but some mammals are adapted for life at sea, in the air, in the trees, or on two legs. Their body is covered with hairs. They possess a muscular diaphragm. Oil and sweat glands are present on their skin.

MammaliaExcept for the five species of monotremes (which lay eggs), all living mammals give birth to live young. Most mammals, including the six most species-rich orders, belong to the placental group. The three largest orders, in descending order, are Rodentia (mice, rats, porcupines, beavers, capybaras, and other gnawing mammals), Chiroptera (bats), and Soricomorpha (shrews, moles and solenodons). The next three largest orders, depending on the classification scheme used, are the primates, to which the human species belongs, the Cetartiodactyla (including the even-toed hoofed mammals and the whales), and the Carnivora (cats, dogs, weasels, bears, seals, and their relatives). While the classification of mammals at the family level has been relatively stable, different treatments at higher levels—subclass, infraclass, and order—appear in contemporaneous literature, especially for the marsupials.

SPARE A MINUTEWe hope that you enjoyed learning about the rich biodiversity through the windows of this little chapter. But what about the colourful creatures of those innocent kingdoms? Are they secure?Lets have a glimpse of some of the threatened species [from The Red List, IUCN.]

THE BATTLE FOR SURVIVAL

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