Early classification of organisms were based on only two criteria, beneficial or harmful animals. An ancient classification system recognized 5 animal groups – domestic, wild, creeping, flying and
sea animals.
Initially the classification was based on organism’s fundamental characteristics such as the habitat and morphology only.
Aristotle (384 to 322 BC), was the first to classify all animals in his History of Animals (Historia Animalium in Latin). He attempted a basic classification of all living organisms into Plants and Animals. Animals were classified based on locomotion; walking (terrestrial), flying (aerial) and swimming (aquatic).
Based on the presence or absence of red blood he classified the animals into two as Enaima with blood and those without blood as Anaima.
Aristotle’s classification system had limitations and many organisms were not fitting into his classification. For example, the tadpoles of frogs are born in water and have gills but when they metamorphosed into adult frogs they have lungs and can live both in water and on land.
How to classify frogs and where to place them? Aristotle classified organisms based on locomotion, hence, birds, bats, and flying insects were grouped together just by observing one single characteristic feature, the flying ability.
On the contrary to the above said example, the ostrich, emu and penguin are all birds but cannot fly. So Aristotle would not have classified them as birds. In spite of these limitations Aristotle’s classification system was followed for more than 2000 years upto 1700.
After Aristotle, his student Theophrastus (372-287 BC) continued his research on the classification of plants, and he was known as the “Father of Botany.”
There was a huge gap till 16th century, then the English naturalist John Ray (1627–1705) wrote several important works through his life. His most important contribution was the establishment of species as the ultimate unit of taxonomy.
In 1682 he published the Methodus Plantarum Nova, which contained about 18,000 plant species, a result of a relatively narrow species concept.
His complicated classification was based on many combined characters, as opposed to earlier taxonomists. John Ray also aimed at publishing a complete system of nature, which included works
on mammals, reptiles, birds, fishes and insects.
The Swedish biologist Carolus Linnaeus (1707 – 1788) father of modern taxonomy and founder of modern systematics developed a scientific system of taxonomy and binomial nomenclature, which is still (with modifications) in use.
Aristotle to Linnaeus employed easily observable single to few traits for classification of organisms. With increased knowledge of the several biological domains, many characters were considered for classifying organisms.
This represented the phase of classical taxonomy which was based on overall similarities or affinities derived from morphology, anatomy and embryology of organisms. A modification of this system is the numerical taxonomy, which evolved in the 1950s.
This system evaluates the resemblances and differences through statistical methods followed by computer analyses to establish the numerical degree of relationship among individuals.
Later on biologists initiated studies on the evolutionary and genetic relationships among organisms, which led to the emerge of phylogenetic classification or cladistics. It is an evolutionary classification based on how a common ancestry was shared. Cladistic classification summarizes the genetic differences between all species in the ‘phylogenetic tree’.
Ernst Haeckal introduced the method of representing evolutionary relationships with the help of a tree diagram known as cladogram.
This system of classification takes into account ancestral characters (traits of basic body design which would be in the entire group) and derived characters (traits whose structure and functions differs from those of ancestral characters).
One or more derived characters which appeared during evolution resulted in the formation of new subspecies. In a cladogram each evolutionary step produces a branching and all the members of the branch would possess the derived character which will not be seen in organisms below the particular branch point.
Arranging organisms on the basis of their similar or derived characters which differ from the ancestral characters .produced a phylogenetic tree or cladogram.
Depending on the system of classification, organisms were classified into two or three kingdoms. Later into four, five, six and now into seven kingdoms.
R.H.Whittaker (1969) proposed the Five kingdom Classification, the Kingdoms defined by him were Monera, Protista, Fungi, Plantae, and Animalia based on the cell structure, mode of nutrition, mode of reproduction and phylogenetic relationships.
Below table gives a comparative account of different characteristics of the five kingdoms.
Classification has come a long way and now takes into an account even molecular level DNA and RNA identification. The advancement in molecular techniques and biochemical assays has led to a new classification – The “Three Domain” classification.
Three Domains of Life
Three domain classification was proposed by Carl Woese (1977) and his co- workers. They classified organisms based on the difference in 16S rRNA genes. The three domain system adds the taxon ‘domain’ higher than the kingdom.
This system emphasizes the separation of Prokaryotes into two domains, Bacteria and Arachaea, and all the eukaryotes are placed into the domain Eukarya.
Archaea appears to have more in common with the Eukarya than the Bacteria. Archaea differ from bacteria in cell wall composition and differs from bacteria and eukaryotes in membrane composition and rRNA types.
1.Domain Archaea
Th is domain includes single celled organisms, the prokaryotes which have the ability to grow in extreme conditions like volcano vents, hot springs and polar ice caps, hence are also called extremophiles.
Th ey are capable of synthesizing their food without sunlight and oxygen by utilizing hydrogen sulphide and other chemicals from the volcanic vents.
Some of the them produced methane (methanogens), few live in salty environments (Halophiles) and are thermoacidophiles which thrive in acidic environments and at high temperatures.
2.Domain Bacteria
Bacteria are prokaryotic, their cells have no definite nucleus and DNA exists as a circular chromosomes and do not have histones associated with it. They do not possess membrane bound organelles except for ribosome (70S type).
Their cell wall contains peptidoglycans. Many are decomposers, some are photo-synthesizers and few cause diseases. There are benefi cial probiotic bacteria and harmful pathogenic bacteria which are diversely populated.
Cyanobacteria are photosynthetic blue green algae which produce oxygen. Th ese had played a key role in the changes of atmospheric oxygen levels from anaerobic to aerobic during the early geologic periods.
3.Domain Eukarya (Eukaryotes)
Eukaryotes are animals which have true nucleus and membrane bound organelles. DNA in the nucleus is arranged as a linear chromosome with histone proteins, ribsosomes of 80S type in the cytosol and 70S type in the chloroplast and mitochondria.
Animals in this domain are classifi ed under kingdoms, namely, Protista, Fungi, Plantae and Animalia.
In 1987, Cavalier-Smith revised the six kingdom system to Seven Kingdom system. The concept of super kingdom was introduced and revised to seven kingdom classification.
The classification is divided into two Super Kingdoms (Prokaryota and Eukaryota) and seven kingdoms, two Prokaryotic Kingdoms (Eubacteria and Archaebacteria) and five Eukaryotic Kingdoms (Protozoa, Chromista, Fungi, Plantae and Animalia).
Taxonomic Hierarchy
In biological classification, the taxonomical hierarchy includes seven major categories namely kingdom, phylum, class, order, family, genus and species and other intermediate categories such as subkingdom, grade, division, subdivision, subphylum, superclass, subclass, superorder, suborder, superfamily, subfamily and subspecies.
Species
Species is the basic unit of classification in the taxonomic hierarchial system. It is a group of animals having similar morphological features (traits) and is reproductively isolated to produce fertile offspring.
There are some exceptional animals which can produce sterile offspring because of mating with closely related species.
Genus
It is a group of closely related species which have evolved from a common ancestor. It some genus there is only one species which is called as monotypic genus such as Red panda is the only species in the genus Ailurus: Ailurus fulgens.
It there are more than one species in the genus it is known as polytypic genus, for example ‘cats’ come under the Genus Felis, which has a number of closely related species, Felis domestica (domestic cat) ,Felis silvestris (wild cat).
Family
It is a taxonomic category which includes a group of related genera with less similarity as compared to genus and species. For example, the family Felidae includes the genus Felis (cats) and the genus Panthera (lions, tigers, leopards).
Order
This category includes an assemblage of one or more related families which show few common features. One or more similar families are grouped together to form an order. For example, family Canidae and Felidae are placed in the order Carnivora.
Class
This category includes one or more related orders with some common characters. For example order Primata comprising monkeys, apes and man is placed in the Class Mammalia, along with the order Carnivora which includes dogs and cats.
Phylum
The group of classes with similar distinctive characteristics constitute a phylum. The classes Pisces, Amphibia, Reptilia, Aves and Mammalia constitute the next higher category, phylum Chordata.
These classes share some common features like presence of a notochord and a dorsal tubular nerve cord hence included in the phylum Chordata.
Kingdom
The group of classes with similar distinctive characteristics constitute a phylum. The classes Pisces, Amphibia, Reptilia, Aves and Mammalia constitute the next higher category, phylum Chordata.
These classes share some common features like presence of a notochord and a dorsal tubular nerve cord hence included in the phylum Chordata.
Kingdom
All living animals belonging to various phyla are included in the Kingdom Animalia and it is the top most of the taxonomic hierarchy.
Downloads
1.Classification of living organisms tnpsc pdf (Samacheer Kalvi)