NCERT Solutions: The Living World

Q1: Why are living organisms classified?
Ans: Living organisms are classified for the following reasons:

• The vast diversity of organisms, with millions of species, makes studying them individually impractical. Classification groups organisms into taxa based on similarities and dissimilarities, simplifying their study by focusing on shared characteristics within a taxon.

• It facilitates the identification of organisms, ensuring each is recognized by a standardized scientific name worldwide.

• Classification reveals relationships (phylogenetic or ancestral) among organisms, highlighting similarities and differences within and across taxa, which aids in understanding their systematic arrangement.
  

• It supports the study of organisms from different regions by organizing them into recognizable categories, enhancing knowledge of global biodiversity.

Q2: Why are the classification systems changing every now and then?
Ans: 

• Evolution is the major factor responsible for the change in classification systems. Since evolution still continues, so many different species of plants and animals are added to the already existed biodiversity. These newly discovered plant and animal specimens are then identified, classified, and named according to the already existing classification systems.
• Since, evolution still continues, so many different species of plants and animals are added in the already existed biodiversity.
• From very early days till now biologists use several characters for classification system. These are morphology, anatomy, cytology, physiology, ontogeny, phylogeny, reproduction, biochemistry, etc. But day by day biologists are learning something new about organisms from their fossil records and using" advanced study techniques such as molecular phylogeny, etc. So their point of view about classification keeps changing. Thus the system of classification is modified every now and then.

Q3: What different criteria would you choose to classify people that you meet often?
Ans: The people we come across in our daily life can be classified on the basis of:

• Their gender (male or female)
• Morphological characteristics such as height, weight, the colour of their eyes or hair
• Areas they live in, or states they belong to.
• Level of education or favourite subjects or sport, etc.

• The identification of individuals based on their characteristics helps us to know how an individual differs from the other members of the population and at what points it is similar to the population.
• It also helps us in deriving relationships between individuals of different populations. Such relationships help in classifying organisms in their right groups. 
• Learning the characteristics of the populations aid in the study of that population as well as how it can be related to the other populations.
• Through this practice, one can establish evolutionary relationships between different populations.
• Also, By identifying individuals and populations, we learn the following things
• (i) Skin colour
  (ii) Sex of the individual
  (iii) Native place
  (iv) Mother tongue
  (v) Food habit
  (vi) Religion
  (vii) Caste

Ans:  (ii) Mangifera indica 
In the binomial system of nomenclature, the generic name of a species always starts with a capital letter whereas the specific name starts with a small letter. Both the names are typed or printed in italics or underlined separately when hand-written. Therefore, the correct scientific name of Mango is Mangifera indica.
Mangifera indica

Following are the hierarchical levels:
(i) Kingdom
(ii) Phylum
(iii) Class
(iv) Order
(v) Family
(vi) Genus
(vii) Species
Example: Insects represent a class of phylum-Arthropoda. All the insects possess common characters of three pairs of jointed legs. 
Example of taxa for human: Phylum - Chordata; Class - Mammalia; Order - Primates; Family - Hominidae; Genus - Homo; Species - sapiens  

Q7: Can you identify the correct sequence of taxonomical categories?
(a) Species → Order → Phylum → Kingdom
(b) Genus → Species → Order → Kingdom
(c) Species → Genus → Order → Phylum
Ans: (a) and (c)
The correct hierarchical arrangement of taxonomic categories in ascending order is:
Species → Genus → Family → Order → Class → Phylum → Kingdom
Therefore, both (a) and (c) represent correct sequences of taxonomic categories. In sequence (b), species should be followed by genus. Therefore, it does not represent the correct sequence.

Q8: Try to collect all the currently accepted meanings for the word 'species'. Discuss with your teacher the meaning of species in case of higher plants and animals on one hand, and bacteria on the other hand.
Ans: In biological terms, species is the basic taxonomical rank. It can be defined as a group of similar organisms that are capable of interbreeding under natural conditions to produce fertile offspring. Therefore, a group of similar individuals are respectively isolated form a species. Species can also be defined as a group of individuals that share the same gene pool.

• A group of individual organisms with fundamental similarities is called species. 
• It can be distinguished from other closely related species on the basis of distinct morphological differences. 
• In case of higher plants and animals, one genus may have one or more than one species, e.g., Panthera leo (lion) and Panthera tigris (tiger). In this example, Panthera is a genus, which includes leo (lion) and tigris (tiger) as species. 

Genus Panthera

• In the case of bacteria, different categories are present on the basis of shape. These are spherical, coccus, rod-shaped, comma and spiral-shaped. Thus, the meaning of species in case of higher organisms and bacteria are different.
  
  Shapes of Bacteria

Q9: Define and understand the following terms:
(i) Phylum
(ii) Class
(iii) Family
(iv) Order
(v) Genus
Ans:

(i) Phylum - Phylum is a category higher than that of Class. The term Phylum is used for animals. A Phylum is formed of one or more classes, e.g., the Phylum Chordata of animals contains not only the class Mammalia but also Aves (birds), Reptilia (reptiles), Amphibia (amphibians), etc. In plants the term Division is used in place of Phylum.
(ii) Class - A Class is made of one or more related Orders. For example, the Class Dicotyledoneae of flowering plants contains all dicots which are grouped into several orders (e.g., Rosales, Sapindales, Ranales, etc.).
(iii) Family - It is a taxonomic category which contains one or more related genera. All the genera of a family have some common features or correlated characters. They are separable from genera of a related family by important and characteristic differences in both vegetative and reproductive features. E.g., the genera of cats (Fells) and leopard (Panthera) are included in the Family Felidae. The members of Family Felidae are quite distinct from those of Family Canidae (dogs, foxes, wolves).
Similarly, the family Solanaceae contains a number of genera like Solanum, Datura, Petunia and Nicotiana. They are distinguishable from the genera of the related family Convolvulaceae (Convolvulus, Ipomoea).
(iv) Order - The category includes one or more related families. E.g., the plant Family Solanaceae is placed in the Order Polemoniales alongwith four other related families (Convolvulaceae, Boraginaceae, Hydrophyllaceae and Polemoniaceae). Similarly, the animal families Felidae and Canidae are included under the Order Carnivora alongwith Hyaenidae (hyaenas) and Ursidae (bears).
(v) Genus - It is a group or assemblage of related species which resemble one another in certain correlated characters. Correlated characters are those similar or common features which are used in delimitation of a taxon above the rank of species. All the species of genus are presumed to have evolved from a common ancestor. A genus may have a single living species e.g., Genus Homo. Its species is Homo sapiens - the living or modem man. The Genus Felis has many species, e.g., F. domestica - common cat, F. chaus (jungle cat) etc.

Q10: Illustrate the Taxonomical hierarchy with suitable examples of a plant and an animal.
Ans: The arrangement of various taxa in a hierarchical order is called taxonomic hierarchy. The hierarchy indicates the various levels of kinship. The number of similar characters of categories decreases from lowest rank to highest rank. 

The hierarchy of major categories is:
Species -►Genus-►Family -► Order-► Class
Kingdom -4- Phylum or Division
Increasing specificity - ► Decreasing specificity
Classification of a plant (Wheat):
Kingdom  -  Plantae
Division   -  Angiospermae
Class         -  Monocotyledonae
Order        -  Poales
Family      -  Poaceae
Genus       - Triticum
Species     -  aestivum
Classification of an animal (Housefly):
Kingdom  -   Animalia
Phylum    -   Chordata
Class        -   Insecta
Order       -   Diptera
Family     -  Muscidae
Genus      -   Musca
Species    -   domestica