Classification is a very important aspect of Biology. It has greatly eased the study of various organisms and their evolution. All living organisms are widely classified into kingdoms based on their particular characteristics. In this chapter, we will learn about the biological classification and various kingdoms.
Types of Biological Classification
There are three main types of classification:
1. Artificial System of Classification:It is a system of classification which uses one or two morphological characters for grouping of organisms. Some artificial system has used habit and habitat for this purpose.
Aristotle (c 350 BC) divided animals into two categories, Enigma (with red blood) and Anaima (without red blood).
Aristotle also classified animals on the basis of their habitat-aquatic (e.g, fish, whale), terrestrial (e.g, reptiles, cattle) and aerial (e.g. birds, bat).
Pliny the Elder (23-79 A.D.) used an artificial system of classification for both plants and animals dividing them into land, air and water. Pliny distinguished animals into flight and inflight ones. Flight animals included bats, birds and insects.
The four classes of extant tetrapods, (clockwise from upper left), Rana (amphibian), Opisthocomus (bird), Eumeces (reptil) and Mus (mammal)
2. Natural System of Classification
It is a system of classification which takes into consideration comparable study of a number of characters so as to bring out the nature of similarities and dissimilarities and hence the nature of the relationship among the organisms.
The system employs those characters which are relatively constant. They include morphological characters, anatomical characters, cytological characters, physiology, ontogeny or development, reproduction, cytochemistry and biochemistry, experimental taxonomy, etc.
The characteristics are helpful in bringing out a maximum number of similarities in a group and comparable differences with another group of organisms. For example, mammals are characterized by the presence of mammal, birds possess wings, feathers, pneumatic bones, ovipary, 4- chambered. They are cold-blooded.
3. Phylogenetic System of Classification
Classification based on the evolutionary relationship of organisms is called a phylogenetic system of classification. It is based on the evolutionary concept from Darwin’s book- "On the origin of species" by means of natural selection. The preservation of favoured races in the struggle for life (1859). It reflects the true relationships among the organisms.
The first phylogenetic system was proposed by Engler and Prantl (1887-99). Geologists believe that since similarity in structure represents a close evolutionary relationship, their natural classification represents evolutionary and phylogenetic classification.
HISTORY OF CLASSIFICATION
1. Aristotle: Father of biology & father of zoology.
Statue of Theophrastus, Palermo Botanical Garden
Few of them are as follows:
(a) Historia Plantarum
(b) Causes of plants
(c) Enquiry into plants
(iii) Under shrubs
3. Carolus Linnaeus [1707 – 1778]:
Some important books are:
Hortus Uplandicus - First book
Systema Naturae (1737)
Species Plantarum - last book (1753)
4. A.P. De Candolle:
He wrote the book - "Theories Elementaire de la Botanique”. He was the first to propose the significance of vascular tissue in taxonomy.
On this basis of vascular tissue he classified plants into two groups:
(a) Cellular plants (Non-vascular plants)- This group includes Thallophyta and Bryophyta.
(b) Vascular plants- This group includes Pteridophyta, Gymnosperm and Angiosperms.
5. George Bentham (1800 -1884) and Joseph Dalton Hooker (1817 -1911):
6. A. W. Eichler:
7. Engler (1844 - 1930) & Pranti (1849 - 1893):
8. Oswald Tippo:
Cyanophyta: B.G. Algae
Chlorophyta: Green algae
Chrysophyta: Yellow-green algae
Pyrrophyta: Dinoflagellates & Diatoms
Phaeophyta: Brown algae
Rhodophyta: Red algae
Myxomycophyta: Slime moulds (False fungi)
Eumycophyta: True fungi
9. Karl Menz:
FIVE KINGDOM CLASSIFICATION (from 1969 to 1990)
In order to develop phylogenetic classification, R.H. Whittaker (1969), an American taxonomist, divided all the organisms into five kingdoms. As the viruses are on the border line of living and non-living, they have been left out. Whittaker has used five criteria for delimiting the different kingdoms.
Table: Characteristics of five kingdom
THREE DOMAINS OF LIFE (Six Kingdom Classification) - 1990
The three-domain system is a biological classification which was introduced by Carl Woese, a professor in the department of microbiology, University of Illinois, Urbana- Champaign in 1990 that divides cellular life forms into archaea, bacteria and Eukarya domains.
The three-domain tree
It is actually a six-kingdom classification:
(1) Archaea domain
The domain contains prokaryotic organisms which have a monolayer core of lipids in the cell membrane and distinct nucleotides in their 16S rRNA. It contains a single kingdom.
Kingdom archaebacteria: The kingdom contains early prokaryotes which live in extreme environments.
(2) Bacteria domain
The domain contains prokaryotes which lack membrane covered cell organelles but do have a sort of micro chambers for separating various activities. There is a single kingdom.
Kingdom eubacteria: The domain contains a diverse type of bacteria having a peptidoglycan cell wall, glycogen as a food reserve, naked DNA coiled to form nucleoid, the absence of sap vacuoles and presence of 70S ribosomes. Some common groups are bacteria, mycoplasma, actinomycetes, rickettsiae, spirochaetes, Firmicutes, cyanobacteria.
(3) Eukarya domain
The domain contains eukaryotic organisms which originated by the endosymbiotic association between some archaebacteria and eubacteria. It has four kingdoms- protista, fungi, plantae and animalia.
Viroids (L. Virus- Point, EIos – Diminutive):
Structure of viroids
Prions (Prusiner, 1983):