The Living World

NCERT starts with a simple observation. Look around you and you find an enormous variety of living organisms, and the variety multiplies the moment you widen your search to a forest, a lake or the sea. Every new area shows organisms you have not seen before. The number of species that are known and described lies between 1.7 and 1.8 million. This number and variety of organisms present on earth is what the chapter calls biodiversity. A large number of organisms still remain unknown, so the number keeps rising as new ones are described.

Two problems follow at once from such variety. First, how do you refer to an organism so that a scientist anywhere in the world knows exactly which one you mean? Local names change from place to place, and one plant can carry different names in different regions, or one name can be used for different plants. So there has to be a standardised way of naming, which NCERT calls nomenclature. Second, with millions of kinds, you cannot study each one on its own; you need to put them into groups. That is classification.

One more step comes before naming, and students often skip it: identification. Nomenclature is possible only when the organism has been correctly described and you know to which organism the name is attached. The order the examiner wants is identification, then nomenclature, then classification. NCERT summarises it as the processes basic to taxonomy: characterisation, identification, classification and nomenclature.

Why classify at all? Because classification lets you study a small sample and apply what you learn to the whole group, it makes the study of the enormous diversity manageable, and it shows the relationships between different groups. NCERT exercise Q1 (why are living organisms classified) and Q2 (why do classification systems keep changing) are straight from this paragraph and are asked almost every year.

Biologists have agreed on principles so that each organism gets a scientific name that is accepted everywhere and used by everyone. For plants the rules are laid down in the International Code for Botanical Nomenclature (ICBN), and for animals in the International Code of Zoological Nomenclature (ICZN). The aim is stated exactly in NCERT: a scientific name ensures that each organism has only one name, and that no other organism has the same name.

The system in use is binomial nomenclature, given by Carolus Linnaeus. Every name has two components: the generic name and the specific epithet. In Mangifera indica, the mango, Mangifera is the genus and indica is the specific epithet. Write the rules as a numbered list; the marking scheme rewards each one.

1. Biological names are generally in Latin and written in italics. They are Latinised or derived from Latin irrespective of their origin.
2. The first word represents the genus; the second is the specific epithet.
3. When handwritten, both words are separately underlined; when printed they are in italics, to indicate their Latin origin.
4. The genus starts with a capital letter; the specific epithet starts with a small letter.

A fifth point that examiners like: the name of the author appears after the specific epithet, at the end of the name, in an abbreviated form. Mangifera indica Linn. tells you that the species was first described by Linnaeus. The author's name is not italicised or underlined.

Common examples you should be able to write correctly: man Homo sapiens; mango Mangifera indica; potato Solanum tuberosum; brinjal Solanum melongena; lion Panthera leo; tiger Panthera tigris; leopard Panthera pardus; housefly Musca domestica; wheat Triticum aestivum. A specific epithet can appear under more than one genus (indica is common), but the two-word combination is unique.

Classification, in NCERT's words, is the process by which anything is grouped into convenient categories based on some easily observable characters. When we say plants or animals, dogs or cats, insects, we are already using categories. The scientific term for these categories is taxa (singular taxon). Notice that taxa can be at very different levels: plants, wheat, animals, mammals and dogs are all taxa, but wheat is a much smaller group than plants, and dogs a much smaller group than mammals. So classification is not just naming a group; it involves ranking groups relative to each other.

Taxonomy is the branch that does this work. Its processes are characterisation, identification, classification and nomenclature. Modern taxonomy does not depend on external appearance alone; NCERT says organisms are classified on the basis of external and internal structure, along with the structure of the cell, the development process and ecological information.

People classified organisms long before there was a science of taxonomy, because they needed to know which plants and animals were useful for food, shelter and clothing. The systematic study that grew out of that interest is systematics. The word comes from the Latin systema, meaning a systematic arrangement of organisms; Linnaeus used Systema Naturae as the title of his publication. Systematics includes identification, nomenclature and classification, and, in addition, it takes into account the evolutionary relationships between organisms. That last phrase is the difference the examiner wants when asked to distinguish taxonomy from systematics.

A frequently tested pair is taxon versus category. A category is the rank (species, genus, family and so on); a taxon is the actual group of organisms placed at that rank. Mammalia is a taxon at the rank of class; Solanaceae is a taxon at the rank of family. Keep this distinction ready for a 2-mark question.

Classification is not a single-step process. It involves a hierarchy of steps, and each step represents a rank or category. Since a category is part of the overall taxonomic arrangement, it is called a taxonomic category, and all the categories together constitute the taxonomic hierarchy. Each category is a unit of classification, and each represents a rank; NCERT says it is commonly termed a taxon.

NCERT illustrates with insects. Insects share a common feature, three pairs of jointed legs, so the group 'insects' represents a class. NCERT stresses that taxonomic categories are distinct biological entities, not merely morphological aggregates: a category is a real group of organisms that share features because they are related, not just a pile of things that happen to look alike.

The categories every student must know, in ascending order, are: species → genus → family → order → class → phylum (division for plants) → kingdom. Species is at the lowest end of the hierarchy and kingdom at the top. To place an organism in the categories, you must know the characters of the individual or group and identify the similarities and dissimilarities with other organisms.

Two trends run through the whole hierarchy and are asked in almost every paper. First, as you go higher from species to kingdom, the number of common characteristics goes on decreasing: the lower the taxon, the more characters its members share. Second, the number of organisms included in a taxon increases as you go up; a species contains fewer organisms than its genus, a genus fewer than its family, and so on. NCERT adds a consequence: the higher the category, the greater the difficulty in determining the relationship of that taxon to other taxa at the same level, so the problem of classification becomes more complex at higher ranks.

Every organism is placed in all seven categories at once. This is the reason a scientific name, which uses only the genus and species, is enough to locate the organism in the entire hierarchy.

NCERT defines a species as a group of individual organisms with fundamental similarities. It is the lowest category of the taxonomic hierarchy and the basic unit of classification. Closely related species can be distinguished from one another on the basis of distinct morphological differences.

The standard examples are Mangifera indica (mango), Solanum tuberosum (potato) and Panthera leo (lion). In each name the second word, indica, tuberosum and leo, is the specific epithet, while Mangifera, Solanum and Panthera are the genera. A genus may contain one or more specific epithets, each representing a different organism, but the organisms share morphological similarities. So Panthera has another species, Panthera tigris (tiger), and Solanum includes Solanum nigrum and Solanum melongena (brinjal) in addition to potato. Human beings belong to the species sapiens in the genus Homo, so the scientific name is Homo sapiens.

Exercise Q8 asks you to collect the currently accepted meanings of the word species. The taxonomic meaning, which is what NCERT uses here, is a group of organisms that share fundamental similarities and can be distinguished from related groups by morphology. The biological meaning, which you will meet in Class 12 Evolution, is a group of natural populations whose members can interbreed and produce fertile offspring and which are reproductively isolated from other such groups. Outside biology the word is used more loosely: in chemistry a species is any kind of atom, ion or molecule under discussion, and in everyday speech it means simply a kind or sort. The point of the exercise is that in biology the word has a precise meaning, the lowest taxonomic category.

Exemplar asks what makes potato and brinjal separate species though both are Solanum. The answer: they share the generic characters that place them in Solanum, but they differ in distinct, stable morphological features (the tuber-bearing underground stem of potato, the fruit and floral details of brinjal), and they do not interbreed, so they are separate species within the same genus.

A genus comprises a group of related species which have more characters in common when compared with species of other genera. NCERT's phrase is that genera are aggregates of closely related species. The genus is the category just above species and gives the first word of the binomial name.

Plant example: potato and brinjal are two different species, but both belong to the genus Solanum, because they share the floral and vegetative features that define that genus. Animal example: lion (Panthera leo), leopard (Panthera pardus) and tiger (Panthera tigris) have several common features and are all species of the genus Panthera. This genus differs from another genus, Felis, which includes the cats. The contrast between Panthera and Felis is the example the examiner expects when you are asked to explain genus; it also sets up the next category, because both genera will be placed together in one family.

Points to remember for marks: a genus can contain a single species or many; all species of a genus share the generic name; species of the same genus resemble each other more than they resemble species of other genera; and the genus name, when written alone, is still a valid taxon (you can say the genus Panthera), whereas the specific epithet alone is not.

A short comparison that answers many 2-mark questions: species is a group of individuals with fundamental similarities and is the lowest category; genus is a group of related species with more characters in common than with species of other genera and is the next higher category. The number of shared characters is greater within a species than within a genus.

A family is a group of related genera with still fewer similarities as compared to genus and species. In plants, families are characterised on the basis of both vegetative and reproductive features of the plant species. The reproductive features (flower, fruit, seed) are more stable and are the main basis of plant families, which is why the same family can include plants that look very different vegetatively.

Plant example: three different genera, Solanum, Petunia and Datura, are placed in the family Solanaceae. Animal example: the genus Panthera, comprising lion, tiger and leopard, is put along with the genus Felis (cats) in the family Felidae. NCERT then makes the comparison the other way: if you look at the features of a cat and a dog, you find some similarities and some differences, and they are separated into two different families, Felidae and Canidae.

The naming pattern is worth memorising because it appears as an MCQ. Plant family names end in -aceae (Solanaceae, Poaceae, Anacardiaceae, Convolvulaceae). Animal family names end in -idae (Felidae, Canidae, Hominidae, Muscidae). The suffix tells you the rank at a glance.

Families of the four NCERT organisms: man belongs to Hominidae, housefly to Muscidae, mango to Anacardiaceae and wheat to Poaceae. Learn these four as a set with their orders, because the 5-mark hierarchy question can be asked about any of them.

Order and the categories above it are identified on the basis of an aggregate of characters. An order is an assemblage of families which exhibit a few similar characters. The similar characters are fewer in number than those of the different genera included in a family; that is the sentence NCERT uses and it is the sentence the marking scheme carries.

Plant example: the families Convolvulaceae and Solanaceae are included in the order Polymoniales, mainly on the basis of floral characters. Animal example: the order Carnivora includes the families Felidae and Canidae. So cats, lions, tigers, leopards and dogs are all Carnivora, even though they sit in two families and several genera.

Plant order names end in -ales (Polymoniales, Sapindales, Poales). Animal order names have no fixed suffix (Carnivora, Primata, Diptera), so learn them as words. The orders of the four NCERT organisms: man in Primata, housefly in Diptera, mango in Sapindales, wheat in Poales.

A typical 3-mark question asks you to explain why an order shows fewer common characters than a family. The answer runs through the logic of the hierarchy: a family is a group of related genera, so its members already share fewer characters than the species within one genus; an order groups several such families, and the only characters common to all of them are the few that all the families happen to share. Each step up unites more diverse groups on fewer shared characters. Use the Solanaceae and Convolvulaceae example to make it concrete: within Solanaceae the genera share many floral and vegetative characters; across the order Polymoniales only a few floral characters are common.

A class includes related orders. NCERT's example is from the animal side. The order Primata, comprising monkey, gorilla and gibbon, is placed in the class Mammalia along with the order Carnivora, which includes animals like tiger, cat and dog. The class Mammalia has other orders as well; the two named are only examples. What unites all the orders in Mammalia is the set of mammalian characters (mammary glands, hair, and the other features you will meet in Animal Kingdom), which are shared by monkeys and dogs alike even though those animals belong to different orders and families.

The earlier insect example is also a class: insects, with three pairs of jointed legs, form the class Insecta, to which the housefly belongs. For plants, the classes named in Table 1.1 are Dicotyledonae (mango) and Monocotyledonae (wheat), based on the number of cotyledons in the seed along with other characters.

Classes of the four NCERT organisms: man in Mammalia, housefly in Insecta, mango in Dicotyledonae, wheat in Monocotyledonae.

At this level of the hierarchy the number of organisms in a taxon is very large and the shared characters are few but fundamental. This is a good place to remember the second trend: a class like Mammalia contains many orders, each order many families, each family many genera, each genus one or more species, so the count of organisms rises steeply as you climb. If a question asks which taxon has the largest number of organisms, the highest category listed in the options is the answer; if it asks which has the most characters in common, the lowest category is the answer.

Classes comprising animals like fishes, amphibians, reptiles and birds, along with mammals, constitute the next higher category, the phylum. All of these are included in the phylum Chordata on the basis of common features such as the presence of a notochord and a dorsal hollow neural system. NCERT names these two characters and an MCQ on the shared features of Chordata is common; give both.

In plants, classes with a few similar characters are assigned to the higher category called a division. Division is the plant equivalent of phylum, and this equivalence is one of the most repeated 1-mark questions from the chapter (NCERT Exemplar asks it directly). In Table 1.1 both mango and wheat are placed in the division Angiospermae, the flowering plants, even though one is a dicot and the other a monocot.

Phylum or division of the four NCERT organisms: man in Chordata, housefly in Arthropoda, mango in Angiospermae, wheat in Angiospermae. Arthropoda, which the housefly shares with crabs, spiders and all other jointed-legged animals, is the largest phylum of the animal kingdom, a fact used in Animal Kingdom later.

The phylum is the level at which the characters shared are the broad body-plan features rather than details. A fish and a human share a notochord at some stage of development and a dorsal hollow nerve cord, and little else that is obvious; that is enough to put them in one phylum. This is exactly what NCERT means when it says the higher the category, the fewer the shared characters and the harder it is to determine relationships among taxa at that level.

All animals belonging to the various phyla are assigned to the highest category, the kingdom Animalia, and all plants to the kingdom Plantae. These are the two kingdoms named in this chapter; the five-kingdom classification of Whittaker with Monera, Protista and Fungi comes in Chapter 2, Biological Classification. Kingdom is the top of the hierarchy and contains the greatest number of organisms with the fewest shared characters.

NCERT closes the chapter with Table 1.1, which every student should be able to reproduce from memory. Read it as four columns.

• Man: species Homo sapiens, genus Homo, family Hominidae, order Primata, class Mammalia, phylum Chordata, kingdom Animalia.
• Housefly: species Musca domestica, genus Musca, family Muscidae, order Diptera, class Insecta, phylum Arthropoda, kingdom Animalia.
• Mango: species Mangifera indica, genus Mangifera, family Anacardiaceae, order Sapindales, class Dicotyledonae, division Angiospermae, kingdom Plantae.
• Wheat: species Triticum aestivum, genus Triticum, family Poaceae, order Poales, class Monocotyledonae, division Angiospermae, kingdom Plantae.

Notice what the table teaches. Man and housefly share only the kingdom. Mango and wheat share the division and the kingdom but part at the class. Exercise Q11 (illustrate the taxonomic hierarchy with a plant and an animal) is answered by writing any one plant column and any one animal column of this table, preferably as a flow chart from species to kingdom.

NCERT ends with the observation that the problem of classification becomes more complex as you go higher, because each higher category contains more organisms sharing fewer characters, and so determining how one taxon relates to the others at the same rank becomes progressively harder. That sentence is the conclusion of the chapter and makes a good last line for any long answer on the hierarchy.