Patterns in Life: Diversity and Classification: Class 9 Science · Chapter 12

Scoop up a handful of damp soil from a garden bed. It may hold more living things than there are people in India: billions of bacteria, threads of fungi, tiny worms, mites and beetle grubs. Now widen the view to the whole country, from the cold deserts of Ladakh to the mangrove swamps of the Sundarbans. The number of different living things becomes almost impossible to picture.

This variety of life, from microbes to elephants and from mosses to mango trees, is called biodiversity (जैव विविधता). It includes the many kinds of organisms, the small differences between individuals of one kind, and the many habitats in which they live.

Each part of this variety has work to do. In a mustard field in February, honeybees carry pollen from flower to flower, so the crop sets seed. In a compost pit, bacteria and fungi turn peels and dry leaves back into minerals for the soil. Out at sea, drifting algae too small to see make a large share of the oxygen in the air. And green leaves everywhere trap sunlight to make the food on which almost every food chain depends.

Farmers have relied on this variety for centuries. In the hills of Uttarakhand, families traditionally grew about twelve crops together in one field, a system called baranaja ('twelve grains'): millets, pulses and beans side by side. If a dry spell ruins one crop, another survives and the family still eats. A field of many varieties also slows pests, which spread fast through rows of identical plants.

With so much variety, how do scientists study it at all? They sort organisms into groups by the features they share and by how closely they are related. This sorting, called classification (वर्गीकरण), is the backbone of this chapter.

12.1 India as a Biodiversity Hotspot

Few countries pack in as many kinds of land as India. Within its borders lie snowfields and cold deserts, the sand dunes of the Thar, rain-soaked forests, the dry Deccan plateau, river deltas, mangrove swamps and coral islands. Rainfall alone ranges from roughly 100 mm a year around Leh in Ladakh to more than 11,000 mm at Mawsynram in Meghalaya. Every mix of rain, warmth and soil suits a different set of plants and animals.

The hangul, a large deer, survives only in the forests of Kashmir. The Narcondam hornbill lives on one small volcanic island in the Andaman Sea. Species like these, which occur naturally in just one region, are called endemic species (स्थानिक प्रजाति). If an endemic species disappears from its one home, it is gone from the planet.

Conservation money and effort are limited, so scientists use a two-part test to find the places that need protection most urgently. A biodiversity hotspot is a region that has a very large number of endemic species and has already lost most of its natural habitat (Fig. 12.1). The usual bar is at least 1,500 endemic plant species, with at least 70 % of the original vegetation already gone.

Thirty-six regions of the world pass this test. Four of them reach into India. They are the Himalaya, Indo-Burma (most of the North East), the Western Ghats (shared with Sri Lanka) and Sundaland, which reaches India only through the Nicobar Islands. Protecting them protects far more than rare species. The forests of the Western Ghats, for instance, feed rivers that supply water to millions of people across southern India.

12.2 How has the Biodiversity Evolved?

Today's living world is a single frame from a story that is still running. Look closely at a group of one kind of organism, say the millet plants in one farmer's field, and you will find small differences between them. One plant grows deeper roots, another flowers a few days earlier, a third has hairier leaves. Many such differences are passed on from parent to offspring.

Most of the time these differences hardly matter. When conditions change, some of them do. In a run of dry years, plants with deeper roots can reach water that others cannot, so more of them survive and set seed (Fig. 12.2). Their seeds inherit the deep-root trait, so the next generation has more deep-rooted plants. Repeat this over hundreds or thousands of generations, and a population can change so much that it becomes a new form of life.

No individual plant changed its roots because it needed water; the population changed because survivors passed on what they already had. Give such processes millions of years, in every habitat on the Earth, and you get the living world we know. It is made of countless related but different forms, each branching from older ones. Working out who is related to whom is the job of classification.

12.3 How to Classify Organisms?

Grouping is something you already do. You sort your books by subject and your clothes by season. Sorting living things works the same way, except that the rule you choose decides which group each organism lands in. The activity below shows how much the rule matters.

Fig. 12.3 shows the same effect with six pond organisms. Sorted by 'can it fly?', the dragonfly sits with the birds and the bat. Sorted by 'does it have a backbone?', it moves to a group of its own, while the frog and the rohu fish join the birds and the bat.

12.3.1 Some criteria to classify living organisms

A field biologist meeting a strange creature begins with what the naked eye can see and works inwards, to features that need a microscope or a DNA laboratory. These are the main kinds of features compared:

• Outward form: shape, size and body plan. A starfish's arms radiate from a centre; an earthworm is a long tube.
• Inner structure: skeletons, organs and tissues. A fish has a backbone inside; a prawn has a hard covering outside instead.
• Cells: one cell or many; a true nucleus or not; a cell wall or not. A bacterium versus a cell from your cheek.
• How it gets food: an autotroph makes its own food, usually by photosynthesis; a heterotroph takes food from other organisms. A tulsi plant versus a goat.
• Reproduction: by seeds, spores, eggs or live birth; with one parent or two. A fern spreads by spores; a mango tree by seeds.
• Role in nature: producer, consumer or decomposer. Grass, a deer and a mushroom.
• Genes: how alike the DNA is. DNA shows that whales are closer relatives of hippos than of fish.

When a whole set of features is shared by many organisms, the simplest explanation is that they inherited those features from a common ancestor. So shared features are clues to family history, and DNA lets scientists test those clues.

12.4 The Need for Classification

Walk into a large wholesale medicine shop and ask for a cough syrup. The shopkeeper finds it in seconds among thousands of boxes, because the shelves are sorted: by type of product, then by use, then by brand. Without that order, every request would mean searching the whole shop.

Living things are far more numerous. Scientists have described about two million species, and estimates of the true total run to eight or nine million. Without a system, nobody could keep track of them, compare them or share what they know about them. The case study below shows the problem on a small scale.

Sorting organisms into groups by their shared features, which scientists call biological classification, pays off in several ways:

• A common language. A scientist in Chennai and one in Chile can be sure they mean the same organism.
• A place for newcomers. A newly found beetle can be placed among its relatives and given a name.
• Easier study. Millions of species become a manageable set of groups that can be studied one at a time.
• Relationships. Groups show which organisms are close relatives and which depend on one another.
• Likenesses and differences. A group's name tells you at once what its members have in common.
• Conservation. Knowing which species is which shows which ones are rare or shrinking, so they can be protected.

12.5 Biological Classification Systems Over Time

Classification has been rebuilt several times, each time because new tools revealed something the old system could not explain.

The Greek thinker Aristotle, in the 4th century BCE, grouped animals mainly by where they lived (land, water or air) and by how they looked. It was a start, but a habitat rule puts a whale with the fish and a bat with the birds, because it looks only at the outside.

In 1758 the Swedish naturalist Carolus Linnaeus used two kingdoms: Plantae, which stay in one place and make their own food, and Animalia, which move about and eat other organisms. Microscopes soon caused trouble. The single-celled Paramecium swims and eats like an animal, the single-celled Euglena is green like a plant yet swims, and bacteria fit neither kingdom.

So in 1866 Ernst Haeckel added a third kingdom, Protista, for microscopic single-celled life. Better microscopes then showed that bacteria lack a true nucleus while protists have one, so in 1938 Herbert Copeland moved bacteria into a kingdom of their own, Monera. Finally, fungi turned out to be quite unlike plants: they cannot make food and instead absorb it, usually from dead matter. In 1969 Robert Whittaker gave them their own kingdom too, producing the five-kingdom system: Monera, Protista, Fungi, Plantae and Animalia (Fig. 12.5).

12.6 Five Kingdom Classification

Whittaker's five kingdoms rest on four questions about an organism's cells and body:

• Cell type: is it a prokaryote (प्राक्केंद्रकी), whose DNA lies loose in the cell, or a eukaryote (सुकेंद्रकी), whose DNA sits inside a nucleus wrapped in a membrane?
• Body organisation: is the organism a single cell (unicellular) or made of many cells (multicellular)?
• Cell wall: does each cell have a wall, and if so, is it made of cellulose (as in plants) or chitin (as in fungi)?
• Nutrition: does it make its own food, or take food from others, and does it swallow that food or absorb it?

Ask these questions in a sensible order and you get a key: a chain of yes-or-no questions that leads to exactly one kingdom (Fig. 12.6). Try it in the activity below, then test yourself on ten mystery organisms in the Kingdom detective.

12.6.1 Kingdom Monera — Unicellular prokaryotes

Kingdom Monera holds the bacteria and cyanobacteria: single prokaryotic cells, often only about a thousandth of a millimetre long. They are champions of survival. They thrive in the scalding springs at Manikaran in Himachal Pradesh, in the brine of salt pans and deep inside rocks, and trillions of them live in your own gut.

A few bacteria are dangerous: they cause cholera, typhoid and tuberculosis. The great majority are harmless, and many are partners we cannot do without. Your breakfast curd is set by Lactobacillus. Gram and pea plants get nitrogen from Rhizobium, which lives in small lumps on their roots. Cattle depend on microbes in their stomachs to break down tough grass, and clean-up crews of bacteria are even used to digest oil spills.

Cyanobacteria, often called blue-green algae, make their own food by photosynthesis, which most bacteria cannot. So kingdom Monera contains both autotrophs and heterotrophs. What unites its members is the missing nucleus.

12.6.2 Kingdom Protista — Unicellular eukaryotes

A protist is a single cell, but a complete eukaryote, with a nucleus and other membrane-bound parts. Some protists have no cell wall; others have a wall made of cellulose. They live wherever there is water: in ponds, the sea and damp soil, and even inside other organisms.

They feed in very different ways. Euglena and diatoms carry chloroplasts and make food in light. Amoeba flows around a food particle and swallows it. Paramecium sweeps bacteria into its gullet with beating cilia. Plasmodium, which causes malaria, lives as a parasite in our blood.

Follow the food in a village pond and you will meet protists near the start. Diatoms and other green protists capture sunlight, water fleas graze on them, and small fish eat the water fleas. Other protists feed on bacteria and rotting scraps, helping to recycle what dies.

12.6.3 Kingdom Fungi — Multicellular, heterotrophic eukaryotes with a cell wall

Leave a slice of bread in a damp kitchen during the monsoon and within days it grows a fuzzy grey or green coat. That fuzz is a fungus, a relative of mushrooms and yeasts. Most fungi are multicellular eukaryotes whose cells have walls of chitin, the same tough material that forms an insect's outer covering. They have no chlorophyll, so they cannot make their own food.

Instead, a fungus grows as fine threads called hyphae, which spread through its food and together form a mycelium (कवकजाल) (Fig. 12.8). The threads release digestive enzymes into the food and then absorb the dissolved nutrients. Most fungi feed this way on dead matter, so they are called saprophytes. A mushroom is only the part that pushes up to scatter spores; the main body of the fungus stays hidden in the log or the soil.

Yeast is a puzzle at first, because it is a single cell. But its wall contains chitin and it absorbs its food, so it belongs with the fungi, not the protists. People put fungi to work in many ways. Yeast makes bread dough rise and helps ferment batters. The mould Penicillium gave us penicillin, the first widely used antibiotic, and moulds such as Aspergillus are grown in factories to make enzymes.

Imagine a forest where nothing rots: every leaf that ever fell would still be lying there, with its minerals locked inside. Fungi, together with bacteria, are the main recyclers that prevent this. Many fungi also team up with plant roots, helping them take up water and minerals in return for sugar. Some are parasites that cause diseases, such as rust in wheat and ringworm on human skin. Fungi spread by spores, which they can make with or without mating, and they thrive where it is warm and damp.

12.6.4 Kingdom Plantae — Multicellular, autotrophic eukaryotes with a cell wall

Every plant, from a pond alga to a banyan tree, is built from many eukaryotic cells boxed in by stiff walls of cellulose. Plants contain chlorophyll and make their own food in sunlight. On land, almost every food chain starts with them, and the oxygen they release keeps the air breathable.

Kingdom Plantae is divided into five groups. Read them in order and you can follow plants moving from water onto ever drier land, solving one problem at each step (Fig. 12.9).

Thallophyta: the simplest plants

Thallophytes, the algae, have a simple body called a thallus that is not divided into root, stem and leaves (thallos means an undivided body, phyton means plant). Because water surrounds them, every cell can take in water, gases and minerals directly, and the water holds the body up. Examples are Ulva (sea lettuce) on rocky shores and Chara in clear ponds; the slimy green threads in a village pond are often algae too.

Bryophyta: first steps onto land

On a damp wall after the monsoon you may see a velvety green carpet: moss. Mosses and liverworts are the bryophytes (bryon means moss), the first plants to make a life on land, but only just. Thin threads called rhizoids hold them down, and they have simple stem-like and leaf-like parts. They have no vascular tissue, so water must soak through them cell by cell, which keeps them short and tied to damp, shady spots. They also need water to breed: their male cells swim through a film of rainwater or dew to reach the egg. Living on land yet breeding in water earns them the nickname 'amphibians of the plant world'. Riccia (a liverwort) and Funaria (a moss) are common examples.

Pteridophyta: a transport system arrives

Ferns and horsetails, the pteridophytes (pteron means feather, after the feathery fronds), were the first plants with plumbing. Running through their true roots, stems and leaves are strands of vascular tissue (संवहन ऊतक). Xylem carries water and minerals up from the roots, and phloem carries food from the leaves to every other part. With water delivered where it is needed, a fern can hold up large fronds, and some tree ferns grow taller than a house. Their limits lie in reproduction. They make no seeds, spreading instead by spores, and their male cells still have to swim through water to reach the egg.

Gymnosperms: seeds without fruits

Gymnosperms, such as deodar, pine and Cycas, freed reproduction from water. Wind carries their pollen to the egg, so male cells no longer have to swim. And they make seeds: a young plant packed with stored food inside a protective coat, able to wait out cold or drought. The seeds lie uncovered on the scales of cones (gymnos means naked, sperma means seed). Needle-like or scaly leaves lose little water, so gymnosperms thrive on cold, dry mountain slopes.

Angiosperms: flowers and fruits

Most kinds of land plants alive today, roughly 3 lakh species, are angiosperms: the flowering plants, from grass and rice to neem and banyan. They also have the most complex bodies of all plants. Two inventions explain their success. Flowers attract insects, birds and bats that carry pollen from plant to plant far more reliably than wind. And the seeds develop inside a fruit (angeion means vessel), which shields them and helps them travel. Birds eat guavas and drop the seeds far away, the wind carries the fluffy seeds of the silk-cotton tree, and coconuts float across the sea. With these advantages, angiosperms grow in almost every habitat on land.

Taken together, the five groups show plants gaining transport tissue, then seeds, then flowers and fruits: each a new way to live farther from water. No design is perfect, though. Every gain brings its own costs, so each group survives best in its own kind of place.

12.6.5 Kingdom Animalia — Multicellular, heterotrophic eukaryotes

A goat walks to fresh grass, a spider spins a web and waits, and an owl hears a mouse rustle in the dark. Animals are many-celled eukaryotes with no cell walls, and none of them can make food. Getting food, and avoiding becoming food, explains why most animals move about, have sense organs and nerves that react in a split second, and behave in coordinated ways.

The first big split in the animal kingdom depends on a notochord (पृष्ठरज्जु), a flexible rod that runs along the back and stiffens the body (Fig. 12.10). Animals that never have one are non-chordates, the invertebrates. Animals that have one, at least at some stage of life, are chordates. In most chordates the notochord is the forerunner of the backbone: as the embryo grows, a vertebral column forms in its place. Chordates are further divided into protochordates and vertebrates.

Invertebrates: animals without a notochord

Think of the invertebrate groups as a series of design experiments, from sponges with no real tissues to insects with complex organ systems. Each group keeps most of the earlier features and adds something new, and each addition solves one problem while creating another (Fig. 12.11).

Porifera (sponges) means 'pore-bearers'. A sponge is a loose body of cells with no true tissues or organs. Water flows in through countless pores and out of a larger opening, bringing food and oxygen straight to the cells and carrying wastes away. Sponges stay fixed in one place, almost all in the sea; a few, like Spongilla, live in fresh water. Out of water, a sponge would collapse and dry out.

Cnidaria: sea anemones, jellyfish, corals and Hydra. A Hydra in a pond waits with its tentacles spread. When a water flea brushes past, stinging cells fire and the tentacles push the prey into its mouth. This needs true tissues, groups of stinging, muscle and nerve cells working together. The catch is that the mouth is the body's only opening, so undigested remains must leave the way the food came in.

Platyhelminthes (flatworms): the pork tapeworm holds on inside the human gut with a crown of hooks and suckers, and the liver fluke clings inside the liver of cattle and sheep. Free-living Planaria glide under stones in streams. A flatworm is as thin as a ribbon, so no cell is far from the surface and oxygen simply diffuses in: no lungs or gills are needed. It also has a front end. Its body shows 11Bilateral symmetry: only one cut, from head to tail, splits the body into two mirror-image halves. bilateral symmetry, and the head, where the sense organs sit, leads the way, so the animal moves with purpose. Its gut, however, still has only one opening.

Nematoda (roundworms) bring a big improvement: a gut that is a tube with two openings, a mouth at one end and an anus at the other. Food now moves one way and is processed in stages, like a factory line. Their bodies are long and round, and they live almost everywhere: in soil, in water and inside hosts. Examples are Ascaris in the human gut, hookworms, and the filarial worm that causes elephantiasis. Male and female worms are separate.

Annelida (segmented worms): earthworms, leeches and the sea worm Nereis. The body is divided into ring-like segments, each with its own muscles, so a worm can stretch one part while shortening another. A fluid-filled body cavity between the gut and the body wall gives the muscles something to push against. A nerve cord coordinates the movement, and the organs work together as organ systems.

Arthropoda: insects, spiders, scorpions, crabs, prawns and millipedes, the largest animal group on the Earth. Their segments are grouped into body regions with different jobs, such as the head, thorax and abdomen of an insect, and their legs are jointed (arthron means joint, pous means foot). Their body is cased in an exoskeleton that works like armour with built-in anchor points. Muscles attach to its inside to work the jointed legs, its waxy surface keeps water in, and its hardness keeps enemies out. This is how a cockroach survives in a dry kitchen cupboard, where an earthworm would shrivel within hours.

Mollusca: snails, slugs, oysters, octopuses and cuttlefish. Their organs are organised into organ systems inside a soft body, which is usually unsegmented, with a head, a muscular foot for creeping and a hump that holds the organs. In many molluscs a hard shell grows over the body for protection. A garden snail, a pearl oyster and an octopus look nothing alike, yet all three are variations on this one plan.

Echinodermata: starfish, sea urchins, brittle stars and sea cucumbers, all living in the sea. Their skin is spiny (echinos means spiny, derma means skin), and under it lies an internal skeleton of calcium carbonate plates, even though they have no notochord. Hundreds of tiny tube feet, worked by water pressure, let them creep along and grip rocks.

Line the eight groups up and a trend appears: true tissues, then a head end, then a through-gut, then segments, body cavities and skeletons. But more complex does not mean better. Sponges have thrived for more than 500 million years without a single organ, and an insect pays for its armour by having to moult every time it grows.

Chordates: animals with a notochord

Protochordates, such as lancelets and sea squirts, are small sea animals that have a notochord at least at some stage of life but never a backbone. A sea squirt larva swims with a notochord in its tail, then settles on a rock and loses it. They are living hints of what the ancestors of the first fish may have been like.

Run a finger down the middle of your back and you can feel a row of bumps: your vertebrae, the small bones that make up your backbone. Animals with a backbone are vertebrates. A backbone is a strong but jointed rod inside the body, so it can carry a heavy body and still bend. Muscles anchor firmly to it, and it shields the spinal cord while the skull guards the brain. Most of the largest and fastest animals on the Earth are vertebrates. They fall into five familiar groups, told apart by where they live, what covers their skin, how they breathe and how they produce young:

12.7 Adaptations as Outcomes of Structural Change

The variety of animal bodies we see today is the result of structural changes adding up over very long times, and each body suits a way of life. A kingfisher's dagger-shaped bill spears fish in one quick dive. A gharial's long, slim snout sweeps sideways through water to snap up fish. The snow leopard's thick fur and long, bushy tail keep it warm on icy Himalayan slopes. A fruit bat flies on wings of skin stretched between very long finger bones.

In mammals, glands that make milk let mothers feed their young for weeks or months after birth, which greatly improves the young ones' chances of surviving. Features like these work together rather than alone: a cheetah's long legs, bendy spine, wide nostrils and large heart all combine to power its sprint. The range of body forms reflects the range of environments in which animals live.

12.7.1 The hierarchical nature of classification

Classification is arranged in levels, like boxes inside boxes. The biggest box is the kingdom; inside it are smaller and smaller groups, down to the species: Kingdom → Phylum → Class → Order → Family → Genus → Species. (For plants, division is often used instead of phylum.)

At each step down, a group holds fewer kinds of organisms, and its members share more features. A family never straddles two orders; it lies wholly inside one. Follow the Indian peafowl down the levels in Fig. 12.12: the kingdom Animalia includes earthworms and crabs, but by the genus Pavo only two kinds of peafowl are left.

Think of finding one student in a big school. 'Class 9' narrows the search, 'Section B' narrows it further, and roll number 23 picks out one person. The students of Section B share far more (the same timetable, teachers and classroom) than all the students in the school do. In the same way, the levels of classification let scientists pin down any organism exactly and see at once which others are its closest relatives.

12.8 Scientific Naming — The Binomial System

Everyday names cause trouble in two ways. They change from place to place, so one fish may be sold under three different names in markets in Kolkata, Chennai and Mumbai. And they mislead: a starfish is not a fish, a silverfish is an insect, and a flying fox is a bat.

Scientists solve both problems by giving each species one official name, recognised in every country and every language. The system is called binomial nomenclature (द्विपद नामपद्धति). Carolus Linnaeus made it standard practice in the 18th century. 'Binomial' means two names: each scientific name has two parts, in Latin or a Latin-like form.

The first part is the genus (वंश), a group of closely related species. The second part is the species name; together with the genus it names one species (जाति). The Indian peafowl, Pavo cristatus, and the green peafowl of South-East Asia, Pavo muticus, share the genus Pavo because they are built alike, but they are separate species.

Scientific names follow a few simple rules:

1. A scientific name has two words: the genus first, then the species name.
2. The genus begins with a capital letter; the species name is written entirely in small letters (Pavo cristatus, never Pavo Cristatus).
3. In print the name is set in italics; when handwritten, each word is underlined separately (Fig. 12.13).
4. After the full name has been used once, the genus may be shortened to its first letter: P. cristatus.

12.9 Fossils as Evidence

Split a slab of shale from a coal mine in Jharkhand and you may find the print of a tongue-shaped leaf that no living plant has. It belongs to Glossopteris, a plant that vanished about 250 million years ago. Such a preserved remain or trace of past life is a fossil (जीवाश्म). It may be a bone, a shell, the print of a leaf, a footprint, or a whole insect trapped in amber.

Fossils form when an organism is buried quickly in mud or sand. Over time more layers settle on top and harden into rock, so in undisturbed rock the deeper layers are older (Fig. 12.14). All over the world, older layers mostly hold simpler life, such as bacterial mats and shells, while younger layers hold more complex forms, such as land plants, reptiles and mammals.

India has many other important fossil sites. Dinosaur eggs and nests have been found at Raiyoli in Balasinor, Gujarat. Rajasaurus narmadensis, a meat-eating dinosaur, is named after the Narmada valley where its bones were dug up. The Shivalik hills hold fossils of ancient apes, elephants and giraffe-like animals. And a fossil skull of an early human was found at Hathnora, also in the Narmada valley. Together they read like pages from the history of life on the Earth.

12.10 Biodiversity Under Threat

Think of a fig tree in a Western Ghats forest. Its tiny flowers, hidden inside the young fig, can be pollinated only by fig wasps. Its fruits feed hornbills, fruit bats, macaques and squirrels through the year, and those animals carry its seeds to new places.

If the fig wasps vanished, perhaps because of pesticides, the tree could set no new seed, and every animal that relies on figs would go hungry (Fig. 12.15). Ecologists call this a cascade: one disappearance triggers others, because no species lives alone.

Human activities are speeding up such losses. The main causes are:

• clearing forests, grasslands and wetlands for farms, roads and towns;
• polluting air, water and soil;
• over-hunting, over-fishing and over-collecting;
• invasive species brought from elsewhere, such as water hyacinth choking lakes;
• climate change, which shifts rainfall and temperature faster than many organisms can adjust.