Chapter focus: what is living?
“How wonderful is the living world!” Organisms occupy extraordinary habitats: cold mountains, deciduous forests, oceans, freshwater lakes, deserts and hot springs.
- Living diversity includes a galloping horse, migrating birds, the Valley of Flowers and an attacking shark.
- Ecological conflict and cooperation occur within a population and among populations in a community; molecular traffic also occurs inside a cell.
1.1 Diversity in the Living World
Living organisms around us include potted plants, insects, birds, pets and organisms too small to see with the naked eye. Increasing the area observed increases the range and variety seen; a dense forest generally shows many more kinds of organisms.
The number of known and described species is about 1.7-1.8 million. This number and variety of organisms on Earth is biodiversity. New organisms continue to be identified as new, and even old, areas are explored.
Why common names are not enough
We know local plants and animals by local names, but these differ from place to place, even within one country. A standard name is needed so that one organism is known by the same name globally.
| Process | Meaning / role |
|---|---|
| Identification | Correctly describing and recognising the organism to which a name is attached. |
| Nomenclature | Standardised naming of living organisms. |
| Classification | Grouping organisms into convenient categories based on easily observable characters. |
| Taxonomy | Classification based on characters; its basics are characterisation, identification, classification and nomenclature. |
Nomenclature and binomial nomenclature
Scientists established procedures to give each known organism a scientific name accepted by biologists worldwide. Plant names follow the International Code for Botanical Nomenclature (ICBN); animal names follow the International Code of Zoological Nomenclature (ICZN). These rules ensure one name per organism and prevent a name already used for another known organism.
Example: mango is Mangifera indica. Mangifera is the genus; indica is the specific epithet (a particular species).
| Rule | What to write | Example |
|---|---|---|
| Language | Names are generally Latin; Latinised or derived from Latin irrespective of origin. | Mangifera indica |
| Two parts | First word = genus; second word = specific epithet. | Mangifera + indica |
| Style | Print in italics; if handwritten, underline both words separately. | Mangifera indica |
| Capitalisation | Genus starts with capital letter; specific epithet starts with small letter. | Mangifera indica, not Mangifera Indica |
| Author citation | Author’s abbreviated name follows the specific epithet. | Mangifera indica Linn. = first described by Linnaeus |
Classification, taxa and taxonomy
Since studying every organism is nearly impossible, organisms are grouped into convenient categories on easily observable characters. “Plants”, “animals”, “dogs”, “cats”, “insects”, “mammals”, “wheat” and “rice” are examples of such categories. A dog is a mammal and mammals are animals, so they lie at different levels.
Modern taxonomic studies use external and internal structure, cell structure, developmental processes and ecological information. Characterisation, identification, classification and nomenclature are basic to taxonomy.
Systematics
Humans also seek relationships among diverse organisms. Systematics is derived from Latin systema, meaning systematic arrangement of organisms. Linnaeus used Systema Naturae as a publication title. Its scope later expanded to include identification, nomenclature and classification, and it considers evolutionary relationships.
| Taxonomy | Systematics |
|---|---|
| Classification of organisms into taxa based on characters. | Study of organismal diversity and relationships in a systematic arrangement. |
| Core processes: characterisation, identification, classification and nomenclature. | Enlarged scope includes identification, nomenclature and classification. |
| Uses external/internal structure, cells, development and ecology in modern studies. | Also takes account of evolutionary relationships. |
1.2 Taxonomic Categories
Classification is not a single step. It is a hierarchy of steps, each representing a rank/category. Together they form the taxonomic hierarchy. A category is a unit of classification, a rank, and is commonly called a taxon (plural taxa).
Taxonomic groups/categories are distinct biological entities, not merely morphological aggregates. Standard categories are kingdom, phylum (or division in plants), class, order, family, genus and species. Species is the lowest category in both plant and animal kingdoms.
Placement requires knowledge of characters, allowing identification of similarities and dissimilarities within an organism kind and among other kinds.
1.2.1 Species
A species is a group of individual organisms with fundamental similarities. Closely related species should be distinguishable by distinct morphological differences.
In Mangifera indica, Solanum tuberosum (potato) and Panthera leo (lion), indica, tuberosum and leo are specific epithets; Mangifera, Solanum and Panthera are genera. One genus may have one or more specific epithets representing organisms with morphological similarities. Panthera includes tigris; Solanum includes nigrum and melongena. Humans are species sapiens in genus Homo: Homo sapiens.
1.2.2 Genus
A genus comprises related species with more common characters than species in other genera: an aggregate of closely related species. Potato and brinjal are different species of Solanum. Lion (Panthera leo), leopard (P. pardus) and tiger (P. tigris) are in Panthera; this differs from Felis, which includes cats.
1.2.3 Family
A family contains related genera, with fewer similarities than in genus/species. In plants, families are characterised by vegetative and reproductive features. Solanum, Petunia and Datura belong to Solanaceae. In animals, Panthera (lion, tiger, leopard) and Felis (cats) are in Felidae. Cats and dogs have similarities and differences, and fall into Felidae and Canidae, respectively.
1.2.4 Order
Order and higher categories are identified by aggregates of characters. An order is an assemblage of families with a few similar characters, fewer than the similarities between genera in a family. Plant families Convolvulaceae and Solanaceae are in order Polymoniales, mainly on floral characters. Animal order Carnivora includes Felidae and Canidae.
1.2.5 Class
A class includes related orders. Order Primata (monkey, gorilla, gibbon) and order Carnivora (tiger, cat, dog) are placed in class Mammalia, which contains other orders too.
1.2.6 Phylum
Classes of fishes, amphibians, reptiles, birds and mammals form the higher category Phylum. Their common features, including a notochord and a dorsal hollow neural system, place them in phylum Chordata. In plants, classes with a few similar characters are placed into a higher category called Division.
1.2.7 Kingdom
Animal phyla are assigned to the highest animal category, Kingdom Animalia. Kingdom Plantae is distinct and contains plants from various divisions.
| Category | What it contains | NCERT examples / basis |
|---|---|---|
| Species | Individuals with fundamental similarities; lowest category. | Homo sapiens; distinguish close species by morphology. |
| Genus | Closely related species; more shared characters than across genera. | Panthera: lion, leopard, tiger. |
| Family | Related genera; fewer similarities than genus/species. | Solanaceae; Felidae; plant features are vegetative and reproductive. |
| Order | Families with a few similar characters. | Polymoniales; Carnivora. |
| Class | Related orders. | Mammalia contains Primata and Carnivora. |
| Phylum / Division | Related animal classes / plant classes respectively. | Chordata / plant division. |
| Kingdom | Highest category. | Animalia / Plantae. |
| 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 |
NCERT Summary
- The living world has immense variety; millions of plants and animals are described, while many remain unknown.
- Variation in size, colour, habitat, physiology and morphology leads to the search for defining characteristics of living organisms.
- Rules and principles for identification, nomenclature and classification facilitate study of organismal kinds and diversity; this body of knowledge is taxonomy.
- Taxonomic studies support agriculture, forestry, industry and knowledge of bio-resources and their diversity.
- International codes universally govern the basics of identification, naming and classification.
- Using resemblances and distinct differences, each organism receives a correct scientific/biological name of two words under binomial nomenclature.
- Every organism occupies a position in the system of classification. The ranks/categories are taxa, and all categories form the taxonomic hierarchy.
NCERT Exercises: revision prompts
- Why are living organisms classified?
- Why are classification systems changing every now and then?
- What criteria would you choose to classify people you meet often?
- What do we learn from identification of individuals and populations?
- Choose the correctly written scientific name of mango: Mangifera Indica or Mangifera indica.
- Define a taxon. Give examples of taxa at different hierarchical levels.
- Identify the correct taxonomic sequence: (a) Species → Order → Phylum → Kingdom; (b) Genus → Species → Order → Kingdom; (c) Species → Genus → Order → Phylum.
- Collect currently accepted meanings of “species”; discuss its meaning in higher plants/animals and in bacteria.
- Define: (i) Phylum (ii) Class (iii) Family (iv) Order (v) Genus.
- Illustrate taxonomic hierarchy using suitable examples of a plant and an animal.