The Living World: NEET notes

1. Diversity and the need for naming

NCERT §1.1

• Biodiversity is the number and the kinds of organisms present on earth. Each distinct kind of plant, animal or other organism is a species.
• Between 1.7 and 1.8 million species are known and described so far, and new ones keep being identified, both in unexplored places and in well-studied ones.
• The wider the area you observe, the more kinds of organisms you meet: a dense forest holds far more kinds than a school garden, and many organisms are too small to see with the naked eye.
• Local names of one organism change from region to region even within a country, which makes it hard for people in different places to know they are talking about the same organism.
• Nomenclature is the standardised naming of organisms so that one organism carries the same name everywhere in the world.
• Identification comes before naming: the organism must first be described correctly so we know exactly which organism a name belongs to.
• Plant names follow the International Code for Botanical Nomenclature (ICBN); animal names follow the International Code of Zoological Nomenclature (ICZN).
• A scientific name is unique in two ways: each organism has only one such name, and that name is not used for any other known organism. A proper description should let anyone, anywhere, reach the same name.

2. Binomial nomenclature

NCERT §1.1

• Every scientific name has two parts: the generic name (the genus) and the specific epithet. Naming with two components is binomial nomenclature, introduced by Carolus Linnaeus and used by biologists all over the world.
• In Mangifera indica, the scientific name of mango, Mangifera is the genus and indica is the specific epithet.
• Scientific names are in Latin or are Latinised, whatever language the word first came from.
• Printed names appear in italics. When written by hand, the two words are underlined separately, not with one continuous line.
• The genus begins with a capital letter and the specific epithet begins with a small letter, as in Mangifera indica, never Mangifera Indica.
• The name of the author who first described the species may follow the epithet in an abbreviated form, for example Mangifera indica Linn., which shows that Linnaeus first described it.
• Other examples used in the chapter: Solanum tuberosum (potato), Panthera leo (lion) and Homo sapiens (human being).

3. Classification, taxonomy and systematics

NCERT §1.1

• Since no one can study every organism one by one, organisms are grouped. Classification is the grouping of things into convenient categories on the basis of easily observable characters.
• Each such category is a taxon (plural taxa). 'Plants', 'animals', 'mammals', 'dogs', 'wheat' and 'rice' are all taxa, but at very different levels: dogs are mammals, and mammals are animals.
• Naming a taxon calls up its characters at once. Say 'mammals' and you picture animals with external ears and body hair.
• Taxonomy is the classification of organisms into taxa based on their characteristics. Modern taxonomy uses external and internal structure, cell structure, the course of development and ecological information.
• Characterisation, identification, classification and nomenclature are the four processes basic to taxonomy.
• Taxonomy is old. The earliest classifications grouped organisms by their uses to people, for food, clothing and shelter.
• Systematics is the study of the kinds and diversity of organisms and of the relationships among them. The word comes from the Latin 'systema', meaning systematic arrangement; Linnaeus titled his publication Systema Naturae.
• The scope of systematics was later widened to include identification, nomenclature and classification, and systematics takes evolutionary relationships between organisms into account.

4. Taxonomic categories and hierarchy

NCERT §1.2

• Classification is not a single step. It is a hierarchy of steps, and each step is a rank or category.
• Because each category is part of the overall arrangement it is called a taxonomic category, and all the categories together make up the taxonomic hierarchy.
• Each category is a unit of classification and represents a rank; it is commonly called a taxon (plural taxa).
• Insects are a concrete example: they share features such as three pairs of jointed legs, so they can be recognised and given a rank.
• Taxonomic groups are distinct biological entities, not just collections of organisms that happen to look alike.
• The common categories, from lowest to highest, are species, genus, family, order, class, phylum (called division in plants) and kingdom. Species is the lowest category for all organisms.
• To place an organism in these categories you need to know its characters, which reveal its similarities to and differences from individuals of its own kind and of other kinds.
• Taxonomists have also created sub-categories within this hierarchy so that taxa can be placed more soundly.

5. Species and genus

NCERT §1.2.1–1.2.2

• A species is a group of individual organisms with fundamental similarities. It should be distinguishable from closely related species by distinct morphological differences.
• In Mangifera indica, Solanum tuberosum and Panthera leo, the words indica, tuberosum and leo are specific epithets, while Mangifera, Solanum and Panthera are genera, a higher level of taxon.
• A genus may contain one species or several that resemble one another. Panthera has another species, tigris; Solanum includes nigrum and melongena besides tuberosum.
• Human beings belong to the species sapiens of the genus Homo, written Homo sapiens.
• A genus is a group of related species that share more characters with one another than with species of other genera; genera are aggregates of closely related species.
• Potato (Solanum tuberosum) and brinjal (Solanum melongena) are two different species of the one genus Solanum.
• The lion (Panthera leo), the tiger (P. tigris) and the leopard (P. pardus) have several features in common and all sit in the genus Panthera. Cats belong to a separate genus, Felis.
• Once a genus has been named in full, later mentions may shorten it to its capital initial, as in P. pardus.

6. Family, order and class

NCERT §1.2.3–1.2.5

• A family is a group of related genera that share fewer similarities than the members of a genus or a species do.
• In plants, families are recognised using both vegetative and reproductive characters. Solanum, Petunia and Datura, for instance, are three genera grouped in the family Solanaceae.
• Among animals, the genus Panthera (lion, tiger, leopard) and the genus Felis (cats) together form the family Felidae. Cats and dogs share some features but differ in others, so they go into two families, Felidae and Canidae.
• Species, genus and family rest on a number of similar characters; order and the categories above it are identified on aggregates of characters.
• An order is an assemblage of families that show only a few similar characters, fewer than the genera within a family share.
• The plant families Convolvulaceae and Solanaceae are placed in the order Polymoniales, mainly on floral characters. The animal order Carnivora includes the families Felidae and Canidae.
• A class is a group of related orders. Order Primata (monkey, gorilla, gibbon) and order Carnivora (tiger, cat, dog) are both in class Mammalia, which has other orders as well.

7. Phylum, kingdom and the hierarchy table

NCERT §1.2.6–1.2.7

• A phylum is a group of classes. Fishes, amphibians, reptiles, birds and mammals share a notochord and a dorsal hollow neural system, so they are placed in the phylum Chordata.
• In plants, classes with a few similar characters are grouped into a higher category called a division, the plant counterpart of a phylum.
• Kingdom is the highest category. All animals from the various phyla form Kingdom Animalia; all plants from the various divisions form Kingdom Plantae.
• Moving up from species to kingdom, the number of characters shared by members goes down. The lower the taxon, the more characteristics its members share.
• The higher the category, the harder it is to work out its relationship to other taxa at the same level, so classification grows more complex towards the top.
• Man: Homo sapiens, genus Homo, family Hominidae, order Primata, class Mammalia, phylum Chordata.
• Housefly: Musca domestica, genus Musca, family Muscidae, order Diptera, class Insecta, phylum Arthropoda.
• Mango: Mangifera indica, family Anacardiaceae, order Sapindales, class Dicotyledonae, division Angiospermae. Wheat: Triticum aestivum, family Poaceae, order Poales, class Monocotyledonae, division Angiospermae.
• Taxonomic studies of plants and animals are useful in agriculture, forestry and industry, and in knowing our bio-resources and their diversity.

Must-know facts

1. Known and described species number 1.7 to 1.8 million, and new ones are still being found.
2. Plant names follow ICBN; animal names follow ICZN.
3. Binomial nomenclature was given by Carolus Linnaeus: generic name + specific epithet.
4. Genus starts with a capital letter, specific epithet with a small letter: Mangifera indica.
5. Printed names are italic; handwritten names have each word underlined separately.
6. Mangifera indica Linn.: 'Linn.' is the abbreviated author, showing Linnaeus first described it.
7. Four processes basic to taxonomy: characterisation, identification, classification, nomenclature.
8. Systematics comes from Latin 'systema'; Linnaeus wrote Systema Naturae; systematics considers evolutionary relationships.
9. Order of categories upward: species, genus, family, order, class, phylum (division in plants), kingdom.
10. Species is the lowest category; kingdom is the highest.
11. Going up the hierarchy, the number of shared characters decreases.
12. Solanum, Petunia and Datura are in Solanaceae; Solanaceae and Convolvulaceae are in order Polymoniales.
13. Panthera and Felis are in family Felidae; Felidae and Canidae are in order Carnivora.
14. Order Primata and order Carnivora are both in class Mammalia.
15. Chordata is defined by a notochord and a dorsal hollow neural system.
16. Housefly: Muscidae, Diptera, Insecta, Arthropoda.
17. Man: Hominidae, Primata, Mammalia, Chordata.
18. Mango: Anacardiaceae, Sapindales, Dicotyledonae, Angiospermae; wheat: Poaceae, Poales, Monocotyledonae, Angiospermae.

Common traps

Key terms

Biodiversity

The number and kinds of organisms present on earth.

Species

A group of individuals with fundamental similarities, set apart from close relatives by distinct morphological differences; the lowest taxonomic category.

Identification

Describing an organism correctly so that we know which organism a name refers to.

Nomenclature

Standardised naming, so that an organism has the same name all over the world.

ICBN

International Code for Botanical Nomenclature, the rules for naming plants.

ICZN

International Code of Zoological Nomenclature, the rules for naming animals.

Binomial nomenclature

Naming each organism with two words, the generic name and the specific epithet, as introduced by Linnaeus.

Specific epithet

The second word of a scientific name, marking the particular species within the genus.

Classification

Grouping into convenient categories on the basis of easily observable characters.

Taxon

A group of organisms at any rank of classification, such as dogs, mammals or animals; plural taxa.

Taxonomy

Classifying organisms into taxa on the basis of their characteristics.

Systematics

The study of the diversity of organisms and the relationships among them, including evolutionary ones.

Taxonomic hierarchy

All the taxonomic categories arranged in order of rank, from species to kingdom.

Genus

A group of closely related species that share more characters than they do with species of other genera.

Family

A group of related genera, characterised in plants on vegetative and reproductive features.

Order

An assemblage of families that show a few similar characters.

Division

The plant category equivalent to a phylum, grouping related classes.