Taxonomy

Taxonomy is the branch of biology that characterises, identifies, names and classifies living organisms. About 1.7-1.8 million species are known, so every organism needs one universal scientific name and a fixed place in a ranked system. This page covers The Living World, Class 11 NCERT Biology Chapter 1: nomenclature, the rules of binomial nomenclature, taxa, taxonomy and systematics, the seven taxonomic categories from species to kingdom, and the ranks of man, housefly, mango and wheat. NEET tests this topic through exact names, naming rules and the order of categories.

1. Biology and the Diversity of Living Organisms

1.1 Early views of the living world

• Biology: the science of life forms and living processes.
• The living world contains an amazing diversity of living organisms.
• Early humans could easily tell inanimate (non-living) matter from living organisms.
• They deified (worshipped as gods) some inanimate matter, such as wind, sea and fire, and some animals and plants. All of these evoked a sense of awe or fear.
• The description of living organisms, including human beings, began much later in human history.
• Societies with an anthropocentric (human-centred) view of biology made only limited progress in biological knowledge.
• Systematic and large-scale description of life forms made systems of identification, nomenclature and classification necessary.
• The biggest outcome: living organisms share similarities both horizontally and vertically.
• All present-day organisms are related to each other, and also to all organisms that ever lived on earth.
• This revelation humbled humans and led to cultural movements for the conservation of biodiversity.

1.2 Ernst Mayr (1904-2004)

1.3 What is life?

• Organisms live in extraordinary habitats: cold mountains, deciduous forests, oceans, freshwater lakes, deserts and hot springs.
• A galloping horse, migrating birds, the valley of flowers or an attacking shark evoke awe and wonder.
• Ecological conflict and cooperation occur among members of a population and among populations of a community.
• Even the molecular traffic inside a cell makes us ask: what indeed is life?
• This question hides two questions. The technical one asks what living is, as opposed to non-living.
• The philosophical one asks what the purpose of life is. Scientists do not attempt to answer it; biology reflects only on what is living.
• The range of organisms in size, colour, habitat, physiological and morphological features makes us seek the defining characteristics of living organisms.

1.4 Diversity and species

• Around us are potted plants, insects, birds, pets and other animals and plants. Many organisms are too small to see with the naked eye.
• A larger area of observation shows a greater range and variety of organisms; a dense forest shows far more kinds.
• ★ Exam imp Each different kind of plant, animal or organism represents a species.
• ★ Exam imp The number of species known and described ranges between 1.7 and 1.8 million.
• ★ Exam imp Biodiversity: the number and types of organisms present on earth.
• New organisms are continuously being identified as we explore new areas, and even old ones.
• Millions of plants and animals have been identified and described, but a large number still remains unknown.

2. Nomenclature and Identification

• We know local plants and animals by their local names.
• Local names vary from place to place, even within a country. Without a common system, people could not be sure which organism is being discussed.
• So the naming of living organisms is standardised: a particular organism gets the same name all over the world.

• Scientists have set procedures to give each known organism a scientific name, acceptable to biologists all over the world.
• The rules are agreed principles and criteria, written as international codes.

Scientific names do three jobs:

• ★ Exam imp They ensure that each organism has only one name.
• The description of an organism lets people in any part of the world arrive at the same name.
• They ensure that the name has not been used for any other known organism.

• The basics of taxonomy, that is identification, naming and classification, have been developed universally under these international codes.

3. Binomial Nomenclature

• The two-word format was found convenient.
• Example: the scientific name of mango is Mangifera indica.
• ★ Exam imp Mangifera is the genus; indica is the particular species, called the specific epithet.

3.1 Universal rules of nomenclature

1. Biological names are generally in Latin and written in italics. They are Latinised or derived from Latin, whatever their origin.
2. The first word of a biological name is the genus; the second is the specific epithet.
3. When handwritten, the two words are underlined separately; when printed, they are in italics. Both show their Latin origin.
4. ★ Exam imp The genus starts with a capital letter; the specific epithet starts with a small letter, as in Mangifera indica.

3.2 Author citation

• The name of the author appears after the specific epithet, at the end of the biological name.
• It is written in an abbreviated form, e.g. Mangifera indica Linn.
• ★ Exam imp "Linn." shows that this species was first described by Linnaeus.
• Each organism is identified on the basis of its resemblances and distinct differences, and is then given a correct two-word scientific (biological) name.

4. Classification, Taxa and Taxonomy

4.1 Classification

• It is nearly impossible to study every living organism, so a method is needed to make study possible.
• ★ Exam imp Classification: the process of grouping anything into convenient categories based on some easily observable characters.
• We easily recognise groups such as plants, animals, dogs, cats or insects.
• Each group name brings certain characters to mind. "Dog" brings an image of dogs, not cats.
• "Alsatians" narrows the image further, to one particular kind of dog.
• "Mammals" brings to mind animals with external ears and body hair.
• "Wheat" brings an image of wheat plants, not rice or any other plant.

4.2 Taxa

• Taxa: the scientific term for such convenient categories (singular: taxon).
• Dogs, cats, mammals, wheat, rice, plants and animals are all taxa.
• Taxa can indicate categories at very different levels. "Plants" is a taxon; "wheat" is also a taxon.
• A dog is a mammal and mammals are animals. So animals, mammals and dogs are taxa at different levels.

4.3 Taxonomy

Modern taxonomic studies are based on:

• External and internal structure
• Structure of the cell
• Development process
• Ecological information about organisms

• Taxonomy is not new. Humans have always wanted to know more about organisms, especially their uses.
• Early humans needed sources of food, clothing and shelter.
• ★ Exam imp So the earliest classifications were based on the uses of various organisms.
• Taxonomic studies are useful in agriculture, forestry and industry, and in knowing our bio-resources and their diversity.

5. Systematics

• Humans wanted to know not only the kinds of organisms and their diversity, but also the relationships among them. This branch of study is systematics.
• The word comes from the Latin systema, meaning the systematic arrangement of organisms.
• ★ Exam imp Linnaeus used Systema Naturae as the title of his publication.
• The scope of systematics was later enlarged to include identification, nomenclature and classification.
• ★ Exam imp Systematics takes into account the evolutionary relationships between organisms.

6. Taxonomic Categories and Hierarchy

• Classification is not a single-step process. It involves a hierarchy of steps, and each step represents a rank or category.

• Example: insects share common features such as three pairs of jointed legs.
• So insects are recognisable, concrete objects that can be classified; they were given a rank or category.
• Groups represent categories; a category denotes a rank; each rank or taxon is a unit of classification.
• ★ Exam imp Taxonomic groups are distinct biological entities, not merely morphological aggregates (groups based only on form).
• Taxonomic studies of all known organisms led to common categories: kingdom, phylum or division (for plants), class, order, family, genus and species.
• ★ Exam imp All organisms, in the plant and the animal kingdoms alike, have species as the lowest category.
• To place an organism in the categories, we need knowledge of the characters of an individual or a group.
• These characters show similarities and dissimilarities among individuals of the same kind and of other kinds.
• Every organism occupies a definite place in the system of classification.

6.1 Species

• Species: a group of individual organisms with fundamental similarities.
• One species can be distinguished from closely related species by distinct morphological differences.
• In Mangifera indica, Solanum tuberosum (potato) and Panthera leo (lion), the words indica, tuberosum and leo are specific epithets.
• Mangifera, Solanum and Panthera are genera: a higher level of taxon or category.
• Each genus may have one or more specific epithets. They represent different organisms with morphological similarities.
• Panthera has another specific epithet, tigris. Solanum includes species such as nigrum and melongena.
• ★ Exam imp Human beings belong to the species sapiens of the genus Homo: Homo sapiens.

6.2 Genus

• Genus: a group of related species that have more characters in common with each other than with species of other genera.
• Genera are aggregates of closely related species.
• ★ Exam imp Potato and brinjal are different species, but both belong to the genus Solanum.
• Lion (Panthera leo), leopard (P. pardus) and tiger (P. tigris) share several features. All are species of the genus Panthera.
• Panthera differs from another genus, Felis, which includes cats.

6.3 Family

• Family: a group of related genera, with still fewer similarities than genus and species.
• Plant families are characterised on the basis of both vegetative and reproductive features.
• ★ Exam imp Plants: the genera Solanum, Petunia and Datura are placed in the family Solanaceae.
• Animals: the genus Panthera (lion, tiger, leopard) and the genus Felis (cats) are placed in the family Felidae.
• ★ Exam imp A cat and a dog show some similarities and some differences. They are placed in different families: Felidae and Canidae respectively.

6.4 Order

• Species, genus and family are based on a number of similar characters.
• Order and the higher categories are identified on the basis of aggregates of characters.
• Order: an assemblage of families that show a few similar characters.
• These similar characters are fewer than those shared by the genera within a family.
• ★ Exam imp Plant families Convolvulaceae and Solanaceae are placed in the order Polymoniales, mainly on the basis of floral characters.
• The animal order Carnivora includes families such as Felidae and Canidae.

6.5 Class

• Class: a category that includes related orders.
• ★ Exam imp The order Primata (monkey, gorilla, gibbon) and the order Carnivora (tiger, cat, dog) are placed in the class Mammalia.
• Class Mammalia has other orders too.

6.6 Phylum (Division in plants)

• Phylum: the next higher category, made of classes.
• Classes of fishes, amphibians, reptiles, birds and mammals share common features.
• ★ Exam imp These features are the presence of a notochord and a dorsal hollow neural system, so they are included in the phylum Chordata.
• ★ Exam imp In plants, classes with a few similar characters are assigned to a higher category called Division.

6.7 Kingdom

• All animals of the various phyla are assigned to the highest category, Kingdom Animalia.
• ★ Exam imp Kingdom Plantae is distinct; it comprises all plants from the various divisions.
• These two groups are called the animal kingdom and the plant kingdom.

6.8 The hierarchy at a glance

• Figure 1 shows the categories from species to kingdom in ascending order.
• These are broad categories. Taxonomists have also developed sub-categories in this hierarchy, for more sound and scientific placement of taxa.

• ★ Exam imp Going higher from species to kingdom, the number of common characteristics decreases.
• ★ Exam imp The lower the taxon, the more characteristics its members share.
• The higher the category, the greater the difficulty of determining its relationship to other taxa at the same level.
• Hence, the problem of classification becomes more complex at higher levels.

7. Taxonomic Categories of Some Common Organisms

• ★ Exam imp Housefly belongs to the order Diptera, class Insecta and phylum Arthropoda.
• Mango and wheat are both in Angiospermae, but mango is in Dicotyledonae and wheat in Monocotyledonae.
• Man and housefly are animals; mango and wheat are plants (so their top rank here is a division).

8. Pairs, Exceptions, Numbers and Examples

8.1 Pairs to Match

8.2 Exceptions and "only" facts

8.3 Numbers to Remember

• 1.7-1.8 million: species known and described.
• 2: components of a binomial name (generic name and specific epithet).
• 4: universal rules of nomenclature; 4: basic processes of taxonomy.
• 7: main taxonomic categories, species to kingdom.
• 3 pairs: jointed legs in insects.
• 1904-2004: Ernst Mayr's life (born 5 July 1904; died at the age of 100).
• 1953-1975: Mayr at Harvard; nearly 80 years of research career.
• 1983, 1994, 1999: Balzan Prize, International Prize for Biology, Crafoord Prize.

8.4 Examples to Remember

9. Quick Revision

• Biodiversity is the number and types of organisms; 1.7-1.8 million species are known.
• Nomenclature standardises names; identification must come first.
• ICBN names plants; ICZN names animals.
• Binomial nomenclature (Linnaeus): generic name + specific epithet.
• Names are Latin, in italics; handwritten words are underlined separately.
• Genus starts with a capital, epithet with a small letter; author abbreviated at the end (Linn.).
• Classification groups organisms into convenient categories (taxa) on easily observable characters.
• Taxonomy: characterisation, identification, classification and nomenclature.
• Systematics (systema) also considers evolutionary relationships; Linnaeus wrote Systema Naturae.
• Categories: species, genus, family, order, class, phylum (division), kingdom.
• Species is the lowest category for all organisms; kingdom is the highest.
• Going up: fewer common characters, harder to find relationships at the same level.
• Order Polymoniales: Convolvulaceae and Solanaceae, mainly on floral characters.
• Chordata: notochord and dorsal hollow neural system.
• Housefly: Musca domestica, Muscidae, Diptera, Insecta, Arthropoda.

10. Solved Examples

Common Mistakes to Avoid