Class 11-science NCERT Solutions Biology Chapter 1 - The Living World

Organisms are classified because of the following reasons:

                 (i) For easy identification.

                (ii) To study organisms of other places.

                (iii) To study fossils.

                (iv) To bring out similarities and dissimilarities between different organisms.

                 (v) To know about the evolution of various taxa.

Scientific study has been ever progressing due to continuous addition of new tools and techniques. The earliest workers relied on only habitat and habits for classifying organisms. External morphology then became an important tool for classification. Anatomy and embryology were then used in classification. After this, phylogenetic relationships among organisms were considered. Then cytology, i.e. chromosome number, cellular structure and biochemical analysis, were used for the classification of organisms. Therefore, classification systems have been changing every now and then, and have been evolving with time. 

In our day-to-day life, we categorise people based on the following criteria:

 i. Level of education

 ii. Profession

 iii. Height and skin colour

 iv. Native place

 v. Sex

 vi. Hobbies

 vii. Nature

Identification of individuals: Each individual possesses a specific combination of traits not found in other members of the population.

Identification of populations:

     i.    Each population is reproductively isolated.

    ii.    Members of a population interbreed among themselves.

   iii.     Members of a population resemble one another more than they resemble members

            of other populations.

   iv.      Karyotype is similar in all the individuals of the population.

Mangifera indica is the scientific name of Mango, where Mangifera is the genus name and indica is the species name.

A taxon is a particular level of hierarchy in the system of classification of living beings. The following figure gives taxa at different hierarchical levels:

(a) and (c) are the correct sequence of taxonomical categories showing the hierarchical arrangement in the ascending order.

Definition of species:

    i. It is a natural population or group of natural populations of individuals with similar

          morphology, anatomy, physiology and cytology.

    ii.  It is a basic unit of classification where individuals share a common genetic set up.

    iii.   Species is an assemblage of structurally similar individuals which interbreed freely

         among them but are reproductively isolated from members of other species.

Higher plants and animals: Higher plants and animals are sexually reproducing organisms. The criteria of reproductive isolation can be used for them so the third definition of species is applicable to them.

Bacteria: The criteria of free interbreeding and reproductive isolation cannot be applied in their case as they do not reproduce sexually. Only the first definition of the species is applicable to them.

(i) Phylum: In the taxonomical hierarchy, Phylum is immediately below Kingdom and above Class. It is a taxon consisting of one or more classes of organisms all of which possess some similar correlated characters. For example, all arthropods have jointed legs.

(ii) Class: It is the taxonomic grouping of organisms higher than Order and lower than Phylum. A Class includes related orders. For example, all mammals have mammary glands to secrete milk and have a hairy body. All birds or Aves have pneumatic bones which assist in flying. The common character among them is the presence of a notochord, thereby making them members of the same phylum.

(iii) Family: Family is the grouping of organisms immediately below Order and above Genus. A Family contains a group of related genera such as dog and fox. They are members of the same family.

(iv) Order: It is a grouping of organisms higher than Family and lower than Class. An Order is a taxonomic category with one or more families of organisms all of which possess some specific correlated characters not found in others. For example, humans and monkeys belong to the order Primates. Both humans and monkeys can use their hands to manipulate objects and can walk on their hind legs.

(v) Genus: It is a taxonomic category above Species and below Family. It comprises a group of related species which has more characters in common in comparison to species of other genera. For example, potato, tomato and brinjal are three different species but all belong to the genus Solanum. Lion (Panthera leo), leopard (P. pardus) and tiger (P. tigris) with several common features are all species of the genus Panthera.

Keys are pairs of two contrasting characters. Selection of one character leads to rejection of another character, while identifying a particular species, genera or family and so on. For example, the presence or absence of hair on the body can give a clue if an animal belongs to the mammalian class or not.

After knowing the family, an identification key is used to know the genus and then the name of the species. If the organism is not already recorded, efforts are made first to check and recheck its discovery before giving it a new name.

Keys are available for knowing the phylum/division, class, order and family of the organisms in each system of classification. With its help, every organism can be classified whether it is known or unknown.

Taxonomical hierarchy is the classification of organisms in a definite sequence of taxonomic categories in the descending order, starting with kingdom and ending in species.