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Chapter: 11th 12th standard bio zoology Human Body higher secondary school

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Taxonomy and Phylogeny

It is not possible for any one to study all the organisms. But if they are grouped in some convenient way the study would become easier as the characters of a particular group or a family would apply to all the individuals of that group. Classification allows us to understand diversity better.

Diversity in living organisms

 

There is a great diversity among living organisms found on the planet earth. They differ in their structure, habit, habitat, mode of nutrition, and physiology. The Biodiversity of the earth is enormous. Current estimates suggest that the earth may have anywhere from 10 to over 40 million species of organisms, but only about 1.7 million have actually been described including over 7,50,000 insects, about 2,50,000 flowering plants and 47,000 vertebrate animals. We call such a diversity among living organisms asBiodiversity. Even though there is such a variety and diversity among them, the living organisms show a lot of similarities and common features so that they can be arranged into many groups. In order to understand them and study them systematically, these living organisms, mainly the plants and animals are grouped under different categories.

 

The branch of biology dealing with identification, naming and classifying the living organisms is known as Taxonomy. Taxonomy in Greek means rendering of order. The word Systematics means to put together. It was Carolus Linnaeus who used this word first in his book 'Systema Naturae' . Systematics may be defined as the systematic placing of organisms into groups or taxa on the basis of certain relationships between organisms.

 

Need for Classification

 

It is not possible for any one to study all the organisms. But if they are grouped in some convenient way the study would become easier as the characters of a particular group or a family would apply to all the individuals of that group. Classification allows us to understand diversity better.

 

History of Classification

 

In the 3rd and 4th century BC Aristotle and others categorized organisms into plants and animals. They even identified a few thousands or more of living organisms.

 

Hippocrates (460-377 BC), the Father of Medicine listed organisms with medicinal value. Aristotle and his studentTheophrastus (370-282 BC) made

the first attempt to classify organisms without stressing their medicinal value. They tried to classify the plants and animals on the basis of their form and habitat. It was followed by Pliny the Elder (23-79 AD) who introduced the first artificial system of classification in his book ' Historia Naturalis' . John Ray an English naturalist introduced the term species for the first time for any kind of living things. It was then Carolus Linnaeus the Swedish naturalist of 18th century now known as Father of Taxonomy developed the Binomial System of nomenclature which is the current scientific system of naming the species. In his famous book 'Species Plantarum'(1753) he described 5,900 species of plants and in ' systema Naturae'(1758) he described 4200 species of animals.

 

Taxonomy and Phylogeny

 

Taxonomy is the branch of biology that deals with identification and nomenclature (naming) of living organisms and their classification on the basis of their similarities and differences. It was the Swiss-French botanist Augustin-Pyramus de Candolle(1778-1841) who coined the word Taxonomy, the science of naming and classifying of organisms.


Species

 

Species is the basic unit of Classification. It is defined as the group of individuals which resemble in their morphological and reproductive characters and interbreed among themselves and produce fertile offsprings.

 

Species are then grouped into more inclusive taxa, which are grouped into larger taxa so that the classification is a hierarchy of a system of units that increase in inclusiveness from each level to the next higher level. The seven main categories used in any plan of classification are given below.


1. Kingdom 

2.Phylum or Division 3.Class

4.Order

5.Family

6.Genus

7.Species

Phylogeny

 

The evolutionary history of a particular taxon like species is called phylogeny. The classification based on the basis of evolution is called phylogenetic classification. Phylogenetic classification is not always possible since there are several gaps in

the fossil records which form the basis of phylogenetic studies and also evolution is never unidirectional. Classification not explicitly based on evolutionary relationships is called artificial, for example, organisms are grouped according to usefulness (economic plants) size (herbs, shrubs) colour (flowers) ecological role (ground cover) and so-forth. Nevertheless many biologists make use of this non-systematic classification.


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