Chapter 1: The Living World (Class 11 NCERT Biology Study Notes) | EKADEMY | Online Courses, Live Classes, Lessons & Quizzes

Detailed Self-Study Notes for Class 11 Students and NEET Aspirants, based on the NCERT book.

1. Introduction to Biology and the Living World

Biology is defined as the science of life forms and living processes. The living world is characterized by an amazing diversity of organisms found in extraordinary habitats, from cold mountains and oceans to deserts and hot springs.
Early humans distinguished between inanimate matter (like wind, sea, fire) and living organisms (animals, plants), often deifying them out of a sense of awe or fear.
The systematic description of life forms began later in human history. An anthropocentric (human-centered) view of biology limited progress.
Detailed systems of identification, nomenclature, and classification became necessary to manage the study of vast numbers of life forms.
A major revelation from these studies was the recognition that all present-day living organisms are related to each other and to all organisms that have ever lived on Earth. This understanding has fostered cultural movements for the conservation of biodiversity.

2. Ernst Mayr: “The Darwin of the 20th Century”

Ernst Mayr (1904-2004) was a Harvard University evolutionary biologist considered one of the 100 greatest scientists of all time.
His nearly 80-year career spanned ornithology, taxonomy, zoogeography, evolution, systematics, and the history and philosophy of biology.
Key Contributions:
- He made the origin of species diversity a central question in evolutionary biology.
- He pioneered the currently accepted definition of a biological species.
Awards: Mayr was awarded the “triple crown of biology”: the Balzan Prize (1983), the International Prize for Biology (1994), and the Crafoord Prize (1999).

3. Biodiversity and the Need for Classification

Biodiversity refers to the number and types of organisms present on Earth. The number of species known and described is between 1.7 to 1.8 million.
The vast number of organisms and the confusion caused by local names, which vary from place to place, create a need for a standardized naming system.
Nomenclature is the process of standardizing the naming of living organisms so that a particular organism is known by the same name all over the world.
Identification is the process of correctly describing an organism to know which organism the name is attached to. Nomenclature is only possible after correct identification.

4. Binomial Nomenclature

This is the universally accepted system for providing scientific names to organisms, established by Carolus Linnaeus.
Each scientific name has two components: the Generic name (genus) and the specific epithet (species).
Example: The scientific name for mango is Mangifera indica.
- Mangifera represents the genus.
- indica represents the specific epithet.

Universal Rules of Binomial Nomenclature:

Language and Format: Biological names are generally in Latin and written in italics. They are Latinised or derived from Latin, regardless of their origin.
Components: The first word represents the genus, and the second word denotes the specific epithet.
Writing Convention: When handwritten, both words are separately underlined. When printed, they are in italics to indicate their Latin origin.
Capitalization: The genus name starts with a capital letter, while the specific epithet starts with a small letter (e.g., Mangifera indica).
Author Citation: The name of the author who first described the species appears after the specific epithet in an abbreviated form (e.g., Mangifera indica Linn.).

5. Taxonomy and Systematics

Classification: The process by which anything is grouped into convenient categories based on easily observable characters.
Taxa (singular: taxon): The scientific term for these categories. Taxa can represent categories at different levels (e.g., dogs, mammals, and animals are all taxa at different hierarchical levels).
Taxonomy: The process of classification of all living organisms into different taxa based on their characteristics.
- The basis of modern taxonomy includes external and internal structure, cell structure, development process, and ecological information.
- The processes basic to taxonomy are characterisation, identification, classification, and nomenclature.
Systematics: A branch of study that deals with the different kinds of organisms, their diversities, and the relationships among them.
- Derived from the Latin word ‘systema’, meaning systematic arrangement.
- Linnaeus used Systema Naturae as the title of his publication.
- Modern systematics includes identification, nomenclature, classification, and crucially, evolutionary relationships between organisms.

6. The Taxonomic Hierarchy

Classification involves a hierarchy of steps, where each step represents a rank or category. All categories together constitute the taxonomic hierarchy. As one moves up the hierarchy from species to kingdom, the number of common characteristics decreases.

Species: The lowest category. It is a group of individual organisms with fundamental similarities. Members of one species can be distinguished from other closely related species based on distinct morphological differences.
Genus: Comprises a group of related species that share more common characters compared to species of other genera. It is an aggregate of closely related species. (e.g., the genus Panthera includes species like leo (lion), pardus (leopard), and tigris (tiger)).
Family: A group of related genera with fewer similarities than a genus or species. Families are characterized by both vegetative and reproductive features in plants. (e.g., the family Felidae includes the genus Panthera and the genus Felis (cats)).
Order: An assemblage of families exhibiting a few similar characters. The similarities are less than in a family. (e.g., Order Carnivora includes families like Felidae and Canidae).
Class: Includes related orders. (e.g., Class Mammalia includes Order Primata (monkey, gorilla) and Order Carnivora (tiger, cat, dog)).
Phylum / Division: A higher category comprising related classes. In animals, it is called Phylum (e.g., Phylum Chordata includes classes like fishes, amphibians, reptiles, birds, and mammals). In plants, it is called Division.
Kingdom: The highest category. All animals are placed in Kingdom Animalia, and all plants are placed in Kingdom Plantae.

Organisms with their Taxonomic Categories

Common Name	Biological Name	Genus	Family	Order	Class	Phylum/Division
Man	Homo sapiens	Homo	Hominidae	Primata	Mammalia	Chordata
Housefly	Musca domestica	Musca	Muscidae	Diptera	Insecta	Arthropoda
Mango	Mangifera indica	Mangifera	Anacardiaceae	Sapindales	Dicotyledonae	Angiospermae
Wheat	Triticum aestivum	Triticum	Poaceae	Poales	Monocotyledonae	Angiospermae

Quiz Section

Short-Answer Questions

Instructions: Answer the following questions in 2-3 sentences .

What is biodiversity, and what is the estimated number of known and described species?
Explain the purpose of nomenclature in biology.
Who is credited with developing the system of binomial nomenclature, and what are its two main components?
List the four universal rules for writing scientific names under the binomial system.
What is the difference between taxonomy and systematics?
Define the term “taxon” and provide examples of taxa at three different hierarchical levels.
Describe the relationship between a genus and a species, using an example from the text.
As we move up the taxonomic hierarchy from species to kingdom, what happens to the number of common characteristics?
What are the four processes that are considered basic to the study of taxonomy?
Who was Ernst Mayr and what were two of his major contributions to evolutionary biology?

Multiple-Choice Questions (MCQs)

Instructions: Choose the single best answer for each question.

Biology is defined as the science of:
- a) Animals and plants only
- b) Life forms and living processes
- c) Classification and nomenclature
- d) Earth and its inhabitants
According to the text, the known and described number of species ranges between: a) 1.7-1.8 thousand b) 1.7-1.8 billion c) 1.7-1.8 million d) 17-18 million
The standardized naming of living organisms is known as: a) Identification b) Classification c) Systematics d) Nomenclature
The two components of a binomial name are: a) The family name and the species name b) The generic name and the specific epithet c) The kingdom name and the genus name d) The specific epithet and the author’s name
The system of binomial nomenclature was given by: a) Ernst Mayr b) Alexander Agassiz c) Charles Darwin d) Carolus Linnaeus
In the scientific name Mangifera indica Linn., what does “Linn.” stand for? a) The species was discovered in Latin America. b) The name is derived from Latin. c) Linnaeus was the first to describe the species. d) It indicates the family of the plant.
Which of the following is a correct rule for writing a biological name? a) The genus name starts with a small letter. b) The specific epithet starts with a capital letter. c) Both words are underlined separately when handwritten. d) The name must be derived from Greek.
The scientific term for the categories used in classification is: a) Systema b) Taxa c) Hierarchy d) Epithet
Which branch of study takes into account evolutionary relationships between organisms? a) Taxonomy b) Nomenclature c) Identification d) Systematics
Which of the following represents the correct ascending order of taxonomic categories? a) Species -> Genus -> Family -> Order b) Genus -> Family -> Order -> Species c) Order -> Family -> Genus -> Species d) Species -> Family -> Genus -> Order
What is the lowest category in the taxonomic hierarchy? a) Genus b) Family c) Species d) Kingdom
In the classification of plants, the category equivalent to Phylum in animals is: a) Class b) Division c) Order d) Kingdom
The genera Solanum, Petunia, and Datura are all placed in which family? a) Felidae b) Poaceae c) Solanaceae d) Anacardiaceae
The family Felidae and the family Canidae are both included in which order? a) Primata b) Sapindales c) Poales d) Carnivora
According to the provided table, what is the family of Man (Homo sapiens)? a) Hominidae b) Muscidae c) Poaceae d) Homo
What is the class of the Housefly (Musca domestica)? a) Mammalia b) Dicotyledonae c) Insecta d) Arthropoda
Ernst Mayr was known as: a) The Father of Taxonomy b) The Darwin of the 20th Century c) The Pioneer of Systematics d) The Founder of ICZN
What is the genus of wheat? a) aestivum b) Poaceae c) Triticum d) Poales
Which of the following is NOT listed as a basis for modern taxonomic studies? a) External and internal structure b) Structure of cell c) Organism’s habitat preference d) Ecological information of organisms
The title of Carolus Linnaeus’s major publication mentioned in the text was: a) On the Origin of Species b) International Code for Botanical Nomenclature c) Systema Naturae d) The Living World

Answer Keys

Answer Key: Short-Answer Questions

Biodiversity refers to the number and types of organisms present on Earth. The number of species that are known and have been described is estimated to be between 1.7 and 1.8 million.
The purpose of nomenclature is to standardize the naming of living organisms. This ensures that a particular organism is known by the same name all over the world, eliminating the confusion created by local names that vary between places.
Carolus Linnaeus is credited with developing the system of binomial nomenclature. Its two main components are the Generic name (the genus) and the specific epithet (the species).
The four rules are: 1) Names are in Latin and written in italics. 2) The first word is the genus and the second is the specific epithet. 3) When handwritten, they are separately underlined. 4) The genus name starts with a capital letter and the specific epithet with a small letter.
Taxonomy is the process of classifying organisms into different taxa based on their characteristics. Systematics is a broader field that includes taxonomy (identification, classification, nomenclature) but also takes into account the evolutionary relationships between organisms.
A taxon (plural: taxa) is a scientific term for a category at any level of the classification hierarchy. For example, ‘dogs’ is a taxon, ‘mammals’ is a taxon at a higher level, and ‘animals’ is a taxon at an even higher level.
A genus is a group of closely related species. For example, the species leo (lion) and tigris (tiger) are different species, but they share enough common features to be placed together in the genus Panthera.
As we go higher up the taxonomic hierarchy from species to kingdom, the number of common characteristics shared by the members of a taxon goes on decreasing. This makes determining relationships at higher levels more difficult.
The four processes basic to taxonomy are characterisation, identification, classification, and nomenclature.
Ernst Mayr was a Harvard evolutionary biologist known as “The Darwin of the 20th Century.” His two major contributions were making the origin of species diversity a central question of evolutionary biology and pioneering the currently accepted definition of a biological species.

Answer Key: Multiple-Choice Questions (MCQs)

b) Life forms and living processes
c) 1.7-1.8 million
d) Nomenclature
b) The generic name and the specific epithet
d) Carolus Linnaeus
c) Linnaeus was the first to describe the species.
c) Both words are underlined separately when handwritten.
b) Taxa
d) Systematics
a) Species -> Genus -> Family -> Order
c) Species
b) Division
c) Solanaceae
d) Carnivora
a) Hominidae
c) Insecta
b) The Darwin of the 20th Century
c) Triticum
c) Organism’s habitat preference
c) Systema Naturae

Essay Questions

Instructions: Formulate comprehensive answers to the following questions.

Discuss the historical progression of biological classification, from early human attempts based on use to the modern principles of taxonomy and systematics. Why was this progression necessary?
Explain the concept of the taxonomic hierarchy in detail. Describe each major rank from Species to Kingdom, illustrating the relationships between them and explaining why the number of common characteristics changes at each level.
Elaborate on the universal rules of binomial nomenclature. Using the examples of Mango (Mangifera indica) and Man (Homo sapiens), demonstrate how each rule is applied and explain the importance of having such a standardized system.
Compare and contrast the fields of Taxonomy and Systematics. Explain how Systematics expands upon the goals of Taxonomy and why understanding evolutionary relationships is crucial for modern biology.
Based on the table of “Organisms with their Taxonomic Categories,” analyze and compare the classification of Man and Housefly. What does their placement in different taxa at the Phylum, Class, and Order levels tell us about their biological similarities and differences?

Glossary of Key Terms

Biology: The science of life forms and living processes.
Biodiversity: The number and types of organisms present on earth.
Binomial nomenclature: A system of providing a name with two components (Generic name and specific epithet) to an organism, given by Carolus Linnaeus.
Class: A taxonomic category that includes related orders.
Classification: The process by which anything is grouped into convenient categories based on some easily observable characters.
Division: A taxonomic category in plant classification, equivalent to Phylum in animals, that includes classes with a few similar characters.
Family: A taxonomic category containing a group of related genera with still fewer similarities compared to genus and species.
Generic name (Genus): The first word in a biological name. A genus comprises a group of related species with more characters in common in comparison to species of other genera.
Identification: The process of correctly describing an organism to determine what organism a particular name is attached to.
ICBN (International Code for Botanical Nomenclature): The agreed principles and criteria for the scientific naming of plants.
ICZN (International Code of Zoological Nomenclature): The set of rules evolved by animal taxonomists for naming animals.
Kingdom: The highest taxonomic category, into which all organisms are placed (e.g., Kingdom Animalia, Kingdom Plantae).
Nomenclature: The process of standardizing the naming of living organisms such that a particular organism is known by the same name all over the world.
Order: A taxonomic category that is an assemblage of families which exhibit a few similar characters.
Phylum: A taxonomic category comprising classes of animals that share common features like the presence of a notochord and dorsal hollow neural system.
Species: The lowest taxonomic category, representing a group of individual organisms with fundamental similarities and distinct morphological differences from other closely related species.
Specific epithet: The second component in a biological name, denoting the particular species within a genus.
Systematics: A branch of study concerned with different kinds of organisms, their diversities, and also the evolutionary relationships among them.
Taxon (plural: taxa): The scientific term for a unit of classification or a taxonomic category at any rank (e.g., species, family, class are all taxa).
Taxonomy: The branch of study based on characteristics of living organisms, involving the processes of characterisation, identification, classification, and nomenclature.
Taxonomic hierarchy: The arrangement of all taxonomic categories in a sequence of steps, from lowest (species) to highest (kingdom).