Patterns in Life: Diversity and Classification Class 9 Notes | CBSE Science Chapter 12

Key Concepts

1. Biodiversity, Hotspots and the Need for Classification

The Earth holds an enormous variety of life. Some organisms are microscopic and invisible to the naked eye; others are giant trees. Life exists in the snow-clad Himalayas and in the coral reefs of the Andaman Sea. This immense variety of living organisms is called biodiversity.

Every organism helps keep nature stable:

• Microscopic algae in the oceans release most of the oxygen we breathe.
• Fungi and bacteria decompose fallen leaves and convert waste into manure, which makes the soil fertile.
• Birds, bees and bats pollinate flowers.
• Plants capture sunlight to prepare food that supports nearly all life on the planet.

Humans depend on biodiversity for food, shelter, medicines and livelihoods. For centuries, farmers conserved many crop varieties with useful traits such as drought tolerance, pest resistance and the ability to grow in nutrient-poor soils. They realised that diversity reduces the risk of crop failure and strengthens food security.

India as a Biodiversity Hotspot

India has mountains in the north, desert in the west, rainforests in North East India, plateaus in the south and long coastlines along the Arabian Sea and the Bay of Bengal. Each region has its own soil types and its own climate (temperature and rainfall). These varied habitats support a wide variety of species.

Some species are restricted to particular regions of the world and are not found naturally anywhere else. These are endemic species. Four examples found only in India:

• Nilgiri tahr
• Lion-tailed macaque
• The Indian variety of the pitcher plant, Nepenthes khasiana
• Neelakurinji

A biodiversity hotspot is a region that supports a large number of endemic species and has undergone significant habitat loss. Both conditions are part of the definition. Examples of global hotspots named in the chapter: the Western Ghats, Indo-Burma (including North East India), the Himalayas and Sundaland (including the Nicobar Islands). Protecting hotspots matters because they support food webs and help ecosystems stay healthy.

How Biodiversity Evolved

The biodiversity we see today was not always the same. Small differences among individuals affected their chances of survival and reproduction, because some differences helped them adapt to changed conditions. These differences built up over many generations and gave rise to new forms of life. Today’s diversity is the outcome of continuous change over a vast span of time, shaped by interactions between organisms and their surroundings.

Why Classify at All

The chapter uses a library analogy. Picture thousands of books scattered on the floor of a huge library, and you want On the Origin of Species by Charles Darwin. Without arranging books by subject, author or section, finding it would be very hard. Millions of organisms need the same kind of order.

Case study: Pakke Tiger Reserve, Arunachal Pradesh. Scientists have recorded nearly 300 bird species here, while India as a whole has about 1,300. Pakke supports four hornbill species: the Rufous-necked Hornbill, the Oriental Pied Hornbill, the Great Hornbill and the Wreathed Hornbill. They nest only in large, old trees with suitable cavities and feed on specific fruits, so each species occupies the parts of the forest that match tree size and fruit availability. Classifying them lets scientists ask precise questions about how species are distributed. If the large, old trees disappeared, the hornbills would lose their nesting sites.

The scientific system of grouping living organisms on the basis of their similarities and differences is called biological classification. It helps us in six ways:

1. It makes the study of living organisms organised and systematic.
2. It helps us understand similarities and differences among living beings.
3. It shows how different organisms are related and how they interact.
4. It helps in identifying and naming newly discovered organisms.
5. It supports biodiversity conservation by identifying organisms under threat of extinction.
6. It lets scientists all over the world discuss organisms using a common system.

2. Criteria Used to Classify Organisms

In Activity 12.1 you group the animals of an ecosystem in more than one way: by where they are seen, by when they are active (day, night or both), and by what they eat. An owl sits in the “tree”, “night” and “feathers” groups at once; an eagle, a tiger and a leopard fall together as carnivores. The lesson is that the same organism can fit into different groups depending on the criterion you choose.

Scientists begin with broad, easily visible features and then look at more detailed ones. The chapter lists seven characteristics:

Similar features in organisms suggest that they evolved from common ancestors.

3. How Classification Systems Changed Over Time

Scientists built different systems over time, each based on the knowledge available then. The chapter’s timeline:

Why each change happened:

• Aristotle’s limit: his system relied mainly on easily observable external features.
• Two kingdoms: Plantae were organisms that do not move and make their own food; Animalia move and depend on others for food. This caused confusion over Amoeba, Paramecium and bacteria. Amoeba and Paramecium move like animals but are unicellular and heterotrophic, while plants and animals are multicellular.
• Three kingdoms: Protista was added for unicellular microscopic organisms.
• Four kingdoms: better microscopes showed that Amoeba has a true, membrane-bound nucleus while bacteria do not. Both are unicellular yet very different, so bacteria moved to a separate kingdom, Monera.
• Five kingdoms: fungi such as mushrooms do not move like plants but are heterotrophic. They obtain nutrients by absorption, many from dead and decaying matter (some are symbiotic, some parasitic). So fungi got their own kingdom.
• Three domains: with better microscopes and genetic studies, scientists compared organisms at the DNA level. Organisms with similar DNA are considered to share a common ancestry. This showed that microscopic life is far more diverse than previously believed.

The Rigveda and the Brihat Samhita also classify animals, on the basis of habitat (terrestrial, aquatic and aerial), behaviour patterns and ecological roles.

Classification keeps changing as new tools, such as microscopes and staining techniques, reveal new organisms. It is an ongoing process of reasoning and change.

4. The Five Kingdoms

Whittaker’s five kingdom system uses four criteria, applied in a fixed order in the chapter’s concept map:

1. Cell type: prokaryote (no membrane-bound nucleus) or eukaryote (membrane-bound nucleus)
2. Level of organisation: unicellular or multicellular
3. Cell structure: presence or absence of a cell wall (and what it is made of)
4. Mode of nutrition: autotrophic or heterotrophic

The concept map in words. All living organisms first split by cell type. Prokaryotes form Monera (bacteria, archaea, cyanobacteria). Eukaryotes split by level of organisation. Unicellular eukaryotes form Protista (Amoeba, Paramecium, Chlamydomonas, Euglena), which may or may not have a cell wall. Multicellular eukaryotes split by cell structure. Those with a cell wall of chitin are heterotrophic decomposers: Fungi (mushroom, yeast, Aspergillus). Those with a cell wall of cellulose are autotrophic producers that photosynthesise: Plantae (mosses, grasses, ferns, pines, roses). Those without a cell wall are heterotrophic consumers: Animalia (ant, snake, frog, bird, cat, human).

Kingdom Monera: Unicellular Prokaryotes

Bacteria and cyanobacteria are single-celled prokaryotes. Bacteria live everywhere: soil, water, air, hot springs and other extreme environments where most organisms cannot survive, and even inside the human body. Bacteria in the gut of ruminants are responsible for producing biogas from their dung. Some bacteria are harmful pathogens that cause disease, but many are useful, such as Lactobacillus and Rhizobium. Cyanobacteria are autotrophs. Besides nutrient cycling, some bacteria break down pollutants such as oil, pesticides and sewage.

Kingdom Protista: Unicellular Eukaryotes

All single-celled eukaryotes that either have no cell wall or have a cell wall made of cellulose are grouped in Protista. They are microscopic, highly diverse and live in water or moist places. Some are autotrophic, others heterotrophic. Protists are an important link in aquatic food chains: some produce oxygen, others serve as food for small animals, and some act as decomposers that help in nutrient cycling.

Kingdom Fungi: Multicellular, Heterotrophic Eukaryotes with a Cell Wall

Fungi are mostly multicellular eukaryotes with cell walls made of chitin. They do not make their own food. They absorb nutrients from dead or decaying matter through fine filaments, which together form a network called the mycelium. Most fungi are saprophytes, so they work as decomposers: they break down complex organic matter (fallen leaves, twigs, dead organisms) into simpler substances and make them available in soil as minerals. Some fungi form mutualistic (symbiotic) relationships with other organisms; others are parasites that cause disease in plants and animals. They reproduce both sexually and asexually, often by spores, and grow best in warm, moist conditions.

• Yeast is unicellular, yet it is placed in Fungi because its cell wall is made of chitin.
• Mushrooms are macroscopic fungi that reproduce by spores.
• Aspergillus and Penicillium are used to make enzymes and antibiotics.

Without fungi, the decay of dead plants and animals would slow down greatly, which would harm soil fertility and ecological balance.

5. Classes of Kingdom Plantae

Kingdom Plantae is divided into five classes: Thallophyta, Bryophyta, Pteridophyta, Gymnosperm and Angiosperm.

Thallophyta (algae): Primitive Plants

The name comes from thallos (undifferentiated body) and phyton (plant). Thallophytes are among the simplest plants and are mostly found in water or moist places. Their body is a thallus, a simple body that allows direct exchange of gases, nutrients and water with the surroundings. Example: Spirogyra. They are well adapted to water but cannot live on land.

Lichens are the white-green patches seen on tree trunks, damp walls and stones. They are symbiotic associations of an autotrophic alga and a heterotrophic fungus: the fungus gives protection, the alga photosynthesises and provides food. Lichens change colour with air pollutants, so they are natural bioindicators of air quality. Some are used as spice, medicine or dye, and some are poisonous, so correct identification matters.

Bryophyta: First Steps on Land, Still Need Water

From bryon (moss) and phyton (plant). Examples: Marchantia and mosses, seen as green mats on damp rocks, old walls and soil, especially in the monsoon. Bryophytes have a more differentiated body than thallophytes. They have root-like structures called rhizoids and may have simple stem-like and leaf-like structures, but no true roots, stems or leaves. They lack vascular tissue. They need water for reproduction because male reproductive cells must swim to reach female cells. So bryophytes are called the ‘amphibians’ of the plant kingdom.

Pteridophyta: Land Plants with a Transport System

From pteron (feather) and phyton (plant). Example: ferns. Pteridophytes have true roots, stems and leaves and vascular tissues: xylem carries water and phloem carries food throughout the plant. A cross-section of a fern stem under the microscope shows these tissues. Pteridophytes still need water for reproduction and do not produce seeds.

Gymnosperms: Reproduction Without Water

From gymnos (naked) and spermos (seed). Examples: pines and cycads. They are adapted to cold and dry regions; their needle-like or scale-like leaves reduce water loss. They produce seeds, which protect the developing embryo and store food. They do not need water for fertilisation. Their seeds are not enclosed in fruits and are often exposed on cones, hence “naked seed”.

Angiosperms: Flowers, Fruits and Efficient Seed Dispersal

From angeion (vessel) and spermos (seed). Angiosperms are the flowering plants, with the most complex body organisation. Flowers attract pollinators, which makes reproduction more efficient. Fruits enclose the seeds and help spread them to new places through insects, birds, animals, wind or water. So angiosperms occupy the widest range of environments and are the most diverse plant group on the Earth. Example: Gulmohar. Their leaves can be grouped as monocots or dicots by shape and venation (Activity 12.8).

Plant Classes Compared

Early plants relied on water for support and reproduction. Plant groups gradually evolved transport tissues, then seeds, then flowers.

6. Groups of Kingdom Animalia

Most animals show locomotion, rapid response to stimuli and coordinated behaviour, which lets them search for food, avoid predators and interact with their surroundings. A major criterion for classifying animals is the presence or absence of a notochord, a flexible rod-shaped structure.

• No notochord: non-chordata (invertebrata)
• Notochord present: chordata, further divided into protochordata and vertebrata. In some chordates the notochord is a precursor for the development of the vertebral column.

The Eight Invertebrate Phyla

From sponges to echinoderms, body organisation grows more complex; each new feature improves feeding, movement or protection and often brings a new challenge.

Porifera (pore-bearers): multicellularity without tissues. Sponges are one of the simplest animal body plans. They are multicellular but lack tissues and organs (cellular level). Many pores let water flow through the body, bringing food particles and oxygen straight to each cell and carrying waste away. They stay fixed in one place in water (marine). One kilogram of sponge can filter up to 24,000 litres of sea water per day.

Cnidaria: true tissues and active feeding. Hydra, jellyfish and corals show tissue-level organisation, so specialised cells do specific jobs. Tentacles capture prey, so they no longer depend on water currents to bring food in. But they have a single opening for both food intake and waste removal; the chapter asks you to think about the limits this places on the animal. Habitat: fresh and marine water.

Platyhelminthes (flatworms): bilateral symmetry and directional movement. Their body can be divided into two halves along one plane (bilateral symmetry), with distinct head-tail and front-back regions. This allows better coordination and is the beginning of directional activity. The flat body lets gases diffuse without special respiratory organs. There is still a single opening. Many flatworms are parasites and have hooks and suckers to attach to host tissue. Level: organ. Habitat: water or inside a host.

Nematoda (roundworms): two openings. Elongated, cylindrical bodies move efficiently through soil, water or host tissue. The body has two openings, mouth and anus, and shows organ system level organisation (digestive system). Male and female worms are distinct. Habitat: soil, water or inside a host.

Annelida (segmented worms): segmentation and body cavity. Earthworms have cylindrical bodies divided into segments. Segmentation gives greater flexibility and more precise control of movement. They have organ system level organisation, muscles for locomotion, a nerve cord for control and coordination, and a body cavity. Habitat: moist soil or water. No skeleton.

Arthropoda: jointed appendages and an external skeleton. Insects, crabs and spiders have segmented bodies with segments specialised for different jobs, and jointed appendages. Their defining feature is a hard external skeleton (exoskeleton). It gives protection, reduces water loss and supports powerful muscles, which lets arthropods survive in dry and exposed places. Habitat: land or water.

Mollusca: organ system level organisation with soft bodies. Snails, squids and octopuses have soft bodies with a distinct head, a muscular foot and a hump. In many molluscs a shell protects the soft body. The group shows how one basic body plan can be modified in different directions. Habitat: water or moist land.

Echinodermata: internal support without a notochord. From echinos (spiny) and derma (skin). Starfish and sea urchins have a hard internal skeleton made of calcium carbonate, which gives protection and controlled movement even though they lack a notochord. Habitat: marine water.

Protochordates and Vertebrates

Protochordates (like Amphioxus) have a notochord at least once during their life. It gives internal support without restricting movement. They are primitive chordates and show a transition from invertebrates to vertebrates.

Vertebrates have a vertebral column (backbone), an internal skeleton that supports the body and protects vital organs such as the brain and spinal cord. This framework allows larger body size, efficient movement and complex organ systems, along with advanced senses and coordinated behaviour. Vertebrates are classified into five groups, fish, amphibians, reptiles, birds and mammals, based on broad patterns of habitat use, body covering and reproduction.

The fish, bird and mammal features come from Section 12.7, which calls adaptations outcomes of structural change. The chapter does not describe amphibians and reptiles, so those two rows add the standard school-level features used to tell them apart.

7. Hierarchy of Classification and Binomial Nomenclature

Classification follows a step-by-step order from very broad groups down to small, specific ones:

Kingdom → Phylum → Class → Order → Family → Genus → Species

At each lower level, organisms share more common features, and every lower group is part of the group above it. It works like a house address, letting scientists locate, compare and study an organism exactly.

Binomial Nomenclature

A tiger is bagh in Hindi, puli in Tamil, tiger in English and tigre in French. People from different regions may get confused. So scientists use a universal system of naming called binomial nomenclature, introduced by Carolus Linnaeus in the 18th century. Every organism gets a scientific name of two parts, written in Latin or a Latinised form.

A genus groups closely related species with common features. The tiger and the lion share the genus Panthera, the roaring cats, which can roar and have a similar skull structure. A species is a group of similar individuals capable of interbreeding and producing offspring. Genus plus species gives a unique name used worldwide.

Rules for writing scientific names:

1. The name has two parts: genus and species.
2. The genus name comes first and begins with a capital letter; the species name follows in small (lower-case) letters.
3. The name is printed in italics, or underlined when handwritten.

The Three Domain System, Fossils and Threats to Biodiversity

The five kingdom system was more complete than earlier systems but still could not fully explain the diversity of life. Every living cell contains DNA, which carries instructions for growth and function. Using genetic data, Carl Woese (1977) proposed three domains: Bacteria, Archaea and Eukarya.

Fossils are preserved remains of plants and animals found in layers of rock, sand and mud. Generally, older layers contain simpler organisms and newer layers show more complex forms, so fossils are natural records of how life changed over millions of years. Birbal Sahni studied fossil plants and founded the BSIP in Lucknow.

Biodiversity under threat: pollution, deforestation, overuse of resources and climate change are reducing biodiversity. When one species disappears, others that depend on it may also decline and eventually disappear. Diverse forests also act as barriers: villages with more mangroves suffered less damage in the 1999 Orissa super cyclone. The Sangai, the dancing deer endemic to Manipur, lives on the floating grasslands called phumdis in Loktak lake (Keibul Lamjao National Park). It was declared extinct in 1951, rediscovered in 1953, and is now listed in the IUCN Red Data list as its phumdi habitat degenerates.

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