Biological classification 

 "Biological classification is a method by which biologists group and categorize species of organisms.

" Aims of Classification: 

There are two main aims of classification 
1- To determine similarities and differences among organisms so that they can be studied easily. 
 2- To find the evolutionary relationships among organisms. 


"The branch of biology which deals with classification is"called taxonomy" 


' The branch of biology which deals with classification and also traces the evolutionary history of organisms is known as systematics. "

 Basis of classification 

Evolutionry history

Classification is based on relationship amongst organisms and such relationship is got through similarity in form or structure. These similarities suggest that all organisms are related to one another at some point in their evolutionary histories. However, some organisms are more closely related than others. For example sparrows are more closely related to pigeons than to the insects. It means that the former two have common evolutionary histories.

Similarities in structure,biochemistry

 Biologists have classified all the known organisms into groups and subgroups on the basis of similarities. These similarities are seen in structures, biochemistry, cytology and genetics. Modern genetics has provided another type of information to taxonomists. The genetic differences between two studied organisms can be determined and can be used about similarities and differences in their structures and functions. 

Taxonomic Hierarchy

 "Taxa form a ladder, called taxonomic hierarchy.

" Taxa

 "The groups into which organisms are classified are known as taxonomic categories or taxa (singular “taxon”)" taxa form a ladder, called taxonomic hierarchy. All the organisms are divided into five Kingdoms. Then, on the basis of similarities, each K'ngdom is further divided. 


 A Phylum is a group of related classes 


A class is a group of related orders 


 A order is a group of related genera 


A genus is a group of related species 


A species is a group of similar organisms 

Species – The Basic Unit of Classification:

 Species is the basic unit of classification. “A species is a group of organisms which can interbreed freely among them and produce fertile offspring, but are reproductively isolated from all other such groups in nature.” Each species possesses its own distinct structural, ecological and behavioural characteristics. In the definition of species we must emphasize “in nature” because two organisms related to two different but closely related species can cross-breed under artificial conditions. In such unnatural crosses they produce infertile offspring.


Across between a male donkey and a female horse produces an infertile offspring, the mule. The criteria of interbreeding apply primarily to those that regularly interbreed among themselves. However, it cannot be used as a criterion for species recognition in the group of organisms where asexual reproduction predominates and they do not interbreed with one another.

 Early system of classification 

The earliest known system of classification of organises comes from the Greek philosopher Aristotle, who classified all living organisms known at that time as either in the group ‘plants’ or in ‘animals’. In 700s Abu-Usman Umer Aljahiz described the characteristics of 350 species of animals in his book. He wrote a lot about the life of ants. In 1172 Ibn Rushd (Averroes), who was a judge (Qazi) in Seville, translated and abridged Aristotle's book “de Anima (On the Soul)” into Arabic. His original commentary is now lost, but its translation into Latin is available. The last part of the 16th century and the beginning of the 17th, careful study of animals commenced, which 
served as an anatomical basis for classification.
 Scientists concerned with classification Since late in the 15th century, a number of science authors had become concerned with e method of classification. 
Some of them are: 

 Andrea Caesalpino

An Italian botanist Andrea Caesalpino (1519-1603) proposed the first methodical arrangement of plants. He divided plants into fifteen groups called “genera”.

 John Ray

 An English naturalist John Ray (1627-1705) published important works on plants’classification. 

  Augustus Quirinus Rivinus

 Augustus Quirinus Rivinus (1652-1723; German botanist) and Pitton de Tournefort (1656-1708; French botanist) introduced the practice of naming the P'ants according to their genera.
 Rivinus introduced the category of order and Tournefort introduced even more sophisticated categories of class, section, genus, and species. 

Modern classification 

 Modern classification has its root in the work of a Swedish biologist, Carolus Linnaeus, who grouped species according to similar physical characteristics. 

Preference of biologists 

 Biologists prefer the system that can provide maximum information about the basic differences and similarities among different organisms.

 Earlier classification system 

According to earlier classification system, organisms were classified into two kingdoms but now taxonomists agree on five-kingdom classification system. We will see the basis of these classification systems and the drawbacks in the two-kingdom and three-kingdom classification systems. 

Two-kingdom classification system

 Two kingdom classification system It classifies all organisms into two kingdoms i.e. 
2- Animalia. 


According to it, "All organisms that can prepare food from simple inorganic materials and thus can preserve energy are autotrophs" These are included in kingdom plantae. 


 "The organisms that cannot synthesize their food and depend on autotrophs or others are heterotrophs". These are included in kingdom animalia like Euglena have both plant-like (presence of chlorophyll) and animal-like (heterotrophic mode of nutrition in darkness and lack of cell wall) characters. So there should be a separate kingdom for such organisms. This system also ignores the difference between organisms having prokaryotic and those having eukaryotic cells. 

Three-kingdom classification system

 In 1866. Ernst Hackel solved the first objection and proposed a third kingdom; Protista to accommodate euglena-like organisms. He also included bacteria in kingdom protista. In this system, fungi were still included in the kingdom plantae. This'system did not clear the difference between prokaryotes and eukaryotes.Some biologists disagreed about the position of fungi in kingdom plantae. Fungi resemble plants in many ways but are not autotrophs. 
There are special form of heterotrophs that get their food by absorption. They do not have cellulose in their cell walls rather possess chitin. 

Five-kingdom classification system 

 In 1937, E-Chatton suggested the terms of, “Procariotique” to describe bacteria and “Eucariotique” .to describe animal and plant cells. In 1967, Robert Wbittaker introducec the five-kingdom classification system. 

Basis of five -kingdom classification system 

1-The levels of cellular organization i.e. prokaryotic, unicellular eukaryotic and multicellular eukaryotic.
 2-The principal modes of nutrition i.e. photosynthesis, absorption, and ingestion. On this basis, organisms are classified into five Kingdoms 1-Monera 2-Protista 3-Fungi 4-Plantae 5-Animalia. 

Modification of five kingdom classification system

 Tin Margulis and Karlene Schwartz modified the five-kingdom classification organelles of symbiotic origin (mitochondria and chloroplast). They classified the organisms into the same five kingdoms as proposed by Whittaker. 

Kingdom monera: 

 It includes prokaryotic' organisms i.e. they are made of prokaryotic cells. Monerans are unicellular, although some types form chains, clusters, or colonies of cells. Prokaryotic cells are radically different from eukaryotic cells. Most are heterotrophic but some perform photosynthesis because they have chlorophyll in their cytoplasm. Within this kingdom there are two vastly different kinds of organisms i.e. archaebacteria and eubacteria. 
Many biologists believe that these two groups should be considered separate kingdoms. 

Kingdom protista:

 It includes eukaryotic unicellular and simple multicellular organisms. There are three main types of protists. Algae are unicellular, colonial or sometime simple multicellular. They resemble plant cells with cell walls and chlorophyll in chloroplasts. Simple multicellular means that they do not have multicellular sex organs and do not form embryos during their life cycles. Protozoans resemble animals whose cells lack chlorophyll and cell walls. Some protists are like the fungi. 

Kingdom fungi:

 It includes eukaryotic multicellular reducers for example mushrooms. Fungi are heterotrophic organisms that are absorptive in their nutritional mode. Most fungi are decomposers. They live on organic material, secrete digestive enzymes and absorb small organic molecules formed by the digestion by enzymes.

 Kingdom plantae: 

 It includes eukaryotic multicellular autotrophs. Plants are autotrophic in nutritional mode, making their own food by photosynthesis. They have multicellular reproductive organs and form embryos during their life cycles. Mosses, ferns and flowering plants are included in this kingdom.

 Kingdom animalia: 

 It includes eukaryotic multicellular consumers. Animals live mostly by ingesting food and digesting it within specialized cavities. They lack cell wall and show movements. Division of five kingdoms Into groups on the basis of types of cells Biologists believe that kingdom protista evolved from monera and then it gave rise to the other 3 eukaryotic kingdoms .
 1-Fungi 2-plantae 3-Animalia. Viruses are acellular .They do not possess cellular organization yet show some characters of only and act as infectious particles livings. Viruses are infectious entities which contain either RNA or DNA, normally encased in circular RNA only and act as protein coat, and reproduce only in living cells, and cause a number of diseases. 
Thus they are not included in the five-kingdom classification system. Prions and viroids are also acellular organisms and are not included in the five-kingdom classification system.

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