The Living World: Class 11 Biology · Chapter 1

Biology studies living things and the processes that keep them alive. This chapter opens Unit 1, Diversity in the Living World, whose four chapters tackle a single problem: how can anyone make sense of millions of different kinds of organisms? Here you build the toolkit (identifying, naming and ranking organisms) that Chapters 2 to 4 then use to survey microbes, plants and animals.

One of biology's biggest discoveries came out of an unglamorous job: keeping records. As naturalists described more and more organisms, the descriptions had to be organised, and systems for identifying, naming and grouping living things followed. Those systems revealed resemblances running in two directions: across the living world today, and back through time between living and extinct forms. The best explanation is that every organism alive is related to every other, and to all that ever lived. It is a sobering thought that you share ancestors with a tiger and a neem tree, and it gives weight to today's efforts to conserve biodiversity.

For most of history people related to nature differently. Wind, fire and rivers were worshipped, the peepal tree and the cow are still sacred in many Indian homes, and the feeling behind it all was awe, sometimes fear. As long as living things were judged mainly by what they meant for humans, an anthropocentric (human-centred) outlook, biological knowledge advanced only slowly.

Life has settled nearly every corner of the planet, from snow leopards on Himalayan scree to crabs in Sundarbans mud and heat-loving microbes in the hot springs of Manikaran. It comes in every size, from bacteria to banyan trees that spread over a hectare. Faced with such variety, you might ask what all of it has in common: what is life?

That question hides two others. One is scientific: what separates a living thing from a non-living one? Evidence can settle it. The other is philosophical: what is life for? No experiment can test an answer to that, so biology works on the first and leaves the second to philosophers.

1.1 Diversity in the Living World

How many kinds of organisms share your neighbourhood? Biologists count kinds, not individuals: a hundred house crows are one kind, while a house crow and a myna are two. Each kind is a species. Start the count at your window and you may stop at three (a money plant, a gecko, a house crow). Survey the whole campus and the tally climbs; survey a Western Ghats forest and it climbs far higher (Fig. 1.1). Even then the list is incomplete, because bacteria, yeasts and most algae are invisible without a microscope.

Scale that count up to the whole planet and you are measuring biodiversity, the number and variety of kinds of organisms on Earth. So far about 1.7–1.8 million species have been described and named.22Global databases such as the Catalogue of Life now list a little over two million accepted species, living and extinct, so you may meet larger figures elsewhere. Nobody thinks the list is finished: every year brings new frogs from the Western Ghats, new microbes from ordinary garden soil, and new species even from regions naturalists have combed for centuries.

Naming organisms: identification and nomenclature

Ask people from six Indian states what they call the bitter-leaved tree whose twigs are chewed as a toothbrush, and you may hear neem, vembu, vepa, bevu, kadunimb and nim (Fig. 1.2). Each name works perfectly at home. Across a country, let alone the world, local names breed confusion: one name can cover two plants, and one plant can carry a dozen names. In Hindi, for instance, meetha neem means curry leaf, a completely different plant.

Solving this takes two steps. First comes identification: describing an organism accurately enough to decide which known kind it is, or to show that it is new. Only then can nomenclature do its job, which is to attach a standard name that everyone, everywhere, uses for that kind. A name stuck on the wrong organism is worse than no name at all.

Two rulebooks make this work. Plant names follow the International Code of Botanical Nomenclature (ICBN), and animal names the International Code of Zoological Nomenclature (ICZN).33In 2011 the botanical code was renamed the International Code of Nomenclature for algae, fungi, and plants (ICN). Exam questions usually still say ICBN. Between them they enforce three promises. A name, once used, is never given to a different organism. An organism has only one valid scientific name, however many local ones it collects. And its published description is detailed enough that a biologist in Chennai or in Chile, examining the same organism, reaches the same name.

Binomial nomenclature

Look at neem's scientific name, Azadirachta indica. The first word, Azadirachta, is the generic name: it names the genus. The second word, indica, is the specific epithet, which singles out one species within that genus. A two-word name like this is a binomial, and the system is binomial nomenclature. The eighteenth-century Swedish naturalist Carolus Linnaeus used two-word names consistently for the species he described, and they caught on because they are short and easy to sort. The tiger is Panthera tigris; you are Homo sapiens.

Four conventions go with the two-word format.

1. Order: the generic name always comes first and the specific epithet second.
2. Capitals: the generic name begins with a capital letter, the specific epithet with a small letter, even when the epithet comes from a place name: Ficus benghalensis, never Ficus Benghalensis.
3. Language: the words are Latin or Latinised, whatever language they started in. Azadirachta, for example, is a Latinised form of a Persian name for the tree.
4. Style: in print both words are set in italics; in handwriting, where italics are awkward, each word is underlined separately. Either style flags the name as Latin at a glance.

A full citation can end with the name of the scientist who first described the species, abbreviated and not italicised. Azadirachta indica A.Juss. records that the French botanist Adrien-Henri de Jussieu described neem (in 1830), and in Oryza sativa L. the ‘L.’ (also written Linn.) credits Linnaeus with naming rice. Once a genus has been written in full, later mentions may shorten it to its initial, as in P. tigris.

Classification and taxa

Say ‘birds’ to a friend and they picture feathers, a beak and wings; say ‘grasses’ and nobody pictures a mango tree. A single group name carries a whole bundle of features. That is the power of classification: sorting things into convenient groups on characters anyone can see. With 1.7 million described species, biology could not work without it, since nobody can study every species one by one.

Biologists call such groups taxa (singular taxon), and a taxon can be any size. Picture boxes nested one inside the next (Fig. 1.4): the box of all plants holds a smaller box of flowering plants, which holds the grasses, which holds rice. Each box is a taxon. Taxa therefore exist at many levels, and any organism belongs to several of them at once.

Taxonomy and systematics

The science behind all this sorting is taxonomy, which arranges organisms into taxa according to their characteristics. Outward appearance is only one clue. Today's taxonomists also compare internal anatomy, the structure of cells, the way an organism develops from egg or seed, and ecological information such as habitat and behaviour.

Four linked processes form the base of taxonomy, and you can follow them for a frog nobody has named yet (Fig. 1.5). Characterisation records its features. Identification compares them with those of known species. Classification places it in a genus, a family and the higher ranks. Nomenclature gives it a valid scientific name once it proves to be new.

The oldest classifications were practical ones. A farmer's first question about a plant was whether it fed the family, the cattle or the fire, so early groupings followed use: grain crops, fodder, timber trees, healing herbs, draught animals. Such groupings are handy, but they say little about how organisms are built or how they are related (the comparison below).

Systematics goes a step beyond sorting: it asks how organisms are related to one another, and today that means their evolutionary relationships. The name comes from the Latin systema, an orderly arrangement, and Linnaeus used it in the title of his catalogue of nature, Systema Naturae. The field began with relationships and later widened to take in identification, nomenclature and classification as well.

1.2 Taxonomic Categories

Classifying an organism is like writing a full postal address: country, state, district, town, street, house. Each line narrows the search, and each sits at a fixed level. In taxonomy the lines of the address are ranks, also called taxonomic categories, and the complete ordered ladder of them is the taxonomic hierarchy. Each category is a unit of classification, and in everyday use it is simply called a taxon.44Strictly, the category is the rank (say, genus) and the taxon is a real group placed at that rank (say, Panthera). NCERT and most exam questions use ‘taxon’ for both.

Ranks are not arbitrary boxes. Feathers occur in every bird and in no other living animal, and every bird also has a toothless beak. A set of features shared by all the members, and by no outsiders, marks a real, natural group, so birds earn a rank of their own: the class Aves. Taxonomic groups built this way are genuine biological units, not just collections of organisms that resemble each other on the surface.

The same seven ranks are used for every organism, plant or animal. From the top they are kingdom, phylum (in plants, division), class, order, family, genus and species, and species is always the bottom rung (Fig. 1.6). Deciding where an organism belongs needs one thing above all: a detailed knowledge of its characters. Only then can you judge which organisms it resembles most closely, within its own kind and beyond it.

1.2.1 Species

The species is the working unit of taxonomy. Its members share the same basic make-up, and a species must be separable from its nearest relatives by clear differences in form. The grey wolf and the golden jackal look broadly alike, yet size, coat and skull mark them as two species, Canis lupus and Canis aureus.

Notice how their names are built. The shared first word, Canis, names the genus, a rank above species; the second words, lupus and aureus, are the specific epithets that pick out each species. Genera vary in size. Ficus holds hundreds of species, among them the banyan (F. benghalensis), the peepal (F. religiosa) and the fig (F. carica). The sloth bear is the only member of Melursus, and humans are the only living members of Homo, so our name is Homo sapiens.

1.2.2 Genus

What holds a genus together is that its species share more characters with one another than with species of any other genus. The figs show this neatly: every Ficus hides its tiny flowers inside a hollow, fleshy receptacle that ripens into the ‘fig’, a structure no other genus has. In the dog family the same logic separates Canis (wolves and jackals) from Vulpes, the foxes, such as the slimmer, bushy-tailed Bengal fox, Vulpes bengalensis (Fig. 1.7). A genus, in short, is a cluster of closely related species.

1.2.3 Family

Lemons, oranges and the bael fruit look quite unlike one another, yet crush a leaf of any of them and you smell aromatic oils, held in tiny glands that dot the leaf; their flowers also share one basic plan. On such evidence the genera Citrus and Aegle go into one family, Rutaceae. A family is a group of related genera whose members have fewer features in common than the species of one genus do. For plants, botanists weigh both vegetative characters (root, stem, leaf) and reproductive ones (flower, fruit, seed) when drawing family lines. Among animals, Canis and Vulpes make up the dog family, Canidae, while the sloth bear's Melursus sits apart in the bear family, Ursidae.

1.2.4 Order

An order gathers related families. Each step up loosens the bond, so the families in one order agree in only a few characters, fewer than the genera inside a single family share. From this rank upwards taxonomists look less at single features and more at aggregates of characters, overall combinations that recur across the group. Floral evidence, such as a ring-shaped nectar disc inside the flower, helps unite the citrus, neem and mango families (Rutaceae, Meliaceae and Anacardiaceae) in the order Sapindales (Fig. 1.8). Among mammals, the dog, bear and cat families all belong to the order Carnivora.

1.2.5 Class

Bats, elephants, monkeys and tigers look very different, but all of them have hair and feed their young on milk from mammary glands. Their orders (Chiroptera, Proboscidea, Primata and Carnivora) are therefore grouped in one class, Mammalia: a class is a group of related orders.55NCERT writes the primate order as Primata; most modern books call it Primates. In the plant system used in this book, flowering plants fall into two classes: Dicotyledonae, whose seeds have two seed leaves, and Monocotyledonae, with one.

1.2.6 Phylum

Look inside the embryo of a fish, a frog, a lizard, a chick or a human and you find the same two structures running along the back: a notochord, a flexible supporting rod, and above it a dorsal hollow nerve cord. These shared features unite the five classes (fishes, amphibians, reptiles, birds and mammals) in one phylum, Chordata (Fig. 1.9). Plants use a different name for this rank, the division: plant classes that share a few characters form a division, such as Angiospermae, the flowering plants.

1.2.7 Kingdom

The top rung is the kingdom. Every animal phylum, Chordata and Arthropoda included, belongs to the kingdom Animalia, and every plant division belongs to the kingdom Plantae. In this unit we call them simply the animal kingdom and the plant kingdom; Chapter 2 explains why modern biologists split living things into more than two kingdoms.

The seven ranks are coarse. For finer placement taxonomists also use intermediate ranks such as subphylum, superorder and subfamily; the big cats, for instance, are often put in the subfamily Pantherinae of Felidae.

Compare the two ends of the ladder. All tigers share almost every feature, while all members of Animalia share only a handful, such as being many-celled and lacking cell walls. In general, a taxon low in the hierarchy has few members with much in common, and a high one has many members with little in common. That is also why the top of the ladder is the hardest to get right: with so few shared characters, judging how one class or phylum is related to another at the same level is difficult, and these placements are the ones most often revised.

Table 1.1 places five familiar organisms at every rank above species. The genus is simply the first word of each scientific name, so it needs no column of its own. Then use the Rank climber below to watch the groups grow as you go up.