Classification And Evolution
Classification 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 classification
One 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 Classification
All 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
Kingdom 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.
Chordates 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
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.
The 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 .
Vertebrates, 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
A 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.
Fish range in size from the huge 16-metre (52 ft.) whale shark to the tiny 8-millimetre (0.3 in)
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 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 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).
Amphibians 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.
Reptiles 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)
They 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
Birds (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.
Modern 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.
Many 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.
Mammals (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.
Except 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 MINUTE
We 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.]