Class 9 BiologyChapter 12NCERT 12.3 and 12.4
How to classify organisms, and why we must
The same owl can sit in four different groups. It depends on the question you ask.
Think it over
- An owl and an eagle both have feathers. An owl and a fruit bat both hunt at night. Which pair is more alike, and who decides?
- A dolphin and a shark share a sleek shape and fins, yet only the dolphin breathes air and feeds its young on milk. Which features would you trust to group them?
- You could sort a jar of buttons by colour, by size or by the number of holes. Which choice tells you most about how each button was made?
In one minute
- One organism can fit many groups: the group it lands in depends on the criterion you choose (Activity 12.1).
- Scientists start with broad visible features and move to finer ones: external features, nutrition, internal structure, cell structure, ecological role, reproduction, and finally DNA.
- Shared features hint at shared ancestry.
- Biological classification is the scientific system of grouping organisms by their similarities and differences.
- It organises study, shows relationships, helps name new species, guides conservation and gives the world one common language.
Same animals, many groupings
Fig. 12.2 in your book shows a forest by day and by night. In the version below, ten animals of such a forest wait to be grouped. Start by sorting them by where you would see them. Then switch the criterion: when they are active, what covers the body, what they eat. Keep an eye on the owl.
A forest by day and night
0 of 10 placed
Drag a card into a group (on a phone, press and hold it first), or tap a card and then tap a group.
Where you see it
- In the air: Eagle, Fruit bat, Butterfly, Honeybee
- On a tree: Owl, Squirrel
- In or near water: Frog, Rohu fish
- On the forest floor: Tiger, Spotted deer
When it is active
- Day: Eagle, Butterfly, Honeybee, Squirrel, Spotted deer
- Night: Owl, Fruit bat, Frog
- Both, or unsure: Tiger, Rohu fish
Body covering
- Feathers: Owl, Eagle
- Fur or hair: Fruit bat, Squirrel, Tiger, Spotted deer
- Hard outer covering: Butterfly, Honeybee
- Scales: Rohu fish
- Moist bare skin: Frog
What it eats
- Other animals: Owl, Eagle, Tiger, Frog
- Plant parts: Fruit bat, Butterfly, Honeybee, Squirrel, Spotted deer, Rohu fish
Notice what happened. By body covering the owl sits with the eagle; by where it lives it sits with the squirrel; by when it is active it sits with the fruit bat and the frog. None of these groupings is wrong. Each answers a different question. So which features should scientists choose? Answering that is what classification is about: a planned way of arranging all of life.
Quick check
In Activity 12.1, an owl is grouped with an eagle. Which criterion was most likely used?
Show the answer
B. Body covering Both have feathers. By time of activity the owl (night) and eagle (day) would split up.
Some criteria to classify living organisms
Scientists sort like a careful detective: the obvious clues first, then the fine ones. Here are the kinds of evidence they use, from a quick look to the lab.
From a glance to the DNA
Broad, easy features first; finer ones after.
- External featuresShape, size, body organisationA starfish has arms in a star; a fish has a streamlined body.
- Mode of nutritionAutotrophic or heterotrophicGrass makes its own food; a goat eats the grass.
- Internal structuresSkeleton pattern, organs, tissuesA frog has a backbone inside; a crab wears its skeleton outside.
- Cell structureOne cell or many, prokaryote or eukaryote, cell wall or notA bacterium has no true nucleus; an amoeba has one.
- Ecological roleProducer, consumer or decomposerA mango tree produces; a mushroom decomposes its fallen leaves.
- ReproductionAsexual, sexual or bothYeast buds off new cells; a hen lays eggs.
- Genetic similarityShared inherited features, read from DNAA tiger's DNA is closer to a lion's than to a wolf's.
Shared features are clues to a shared ancestor: the more two groups have in common, the more closely related they are likely to be.
Each criterion shows a likeness or a difference. When organisms share many features, especially deep ones like cell structure and DNA, it suggests they came from common ancestors.
- Autotrophic
- Makes its own food, as green plants do by photosynthesis.
- Heterotrophic
- Depends on other organisms for food.
- Producer, consumer, decomposer
- Makes food; eats other organisms; breaks down dead matter.
Quick check
Which criterion needs DNA to be studied in detail?
Show the answer
C. Genetic similarity Comparing what organisms inherit is done most precisely by reading their DNA.
The need for classification
Think of a grocery store with no shelves and no labels: rice, soap, pens and biscuits in one heap. You could find a packet of salt eventually, but not quickly. Now swap the groceries for living things, and the heap for a planet holding millions of kinds. With no order to them, studying any one would be just as slow.
Now a real case. Birdwatchers in Arunachal Pradesh's Pakke Tiger Reserve have logged close to 300 kinds of bird: almost a quarter of India's roughly 1,300, in a single forest. Four of them are hornbills: the Rufous-necked, Oriental Pied, Great and Wreathed. These large birds nest only in holes in big, old trees and feed on particular fruits, so each species keeps to the part of the forest where its kind of tree and fruit grow.
Pakke Tiger Reserve, Arunachal Pradesh
~300 bird species recorded in Pakke
~1,300 in all of India
Tap a hornbill. They look alike at first; the bill, casque and neck colour tell them apart. You may also meet
Grouping organisms by what they share, from their outer features to their tissues, organs and cells, turns a jumble into something we can study. That scientific system is biological classification. It earns its place in six ways, grouped here by who gains:
For study: it turns a heap of millions into an ordered system, and lays likenesses and differences side by side. For understanding: it maps who is related to whom and who interacts with whom. For discovery: a new find can be slotted in and named. For the planet: it flags species close to extinction. For science everywhere: researchers in any country use the same system, so they always know they mean the same organism. That is six uses in all.
Quick check
Hornbills in Pakke nest only in large old trees. Why does that make the old trees so important to protect?
Show the answer
B. Without them the birds have nowhere to nest They need cavities in big, old trees to nest. Remove the trees and the hornbills cannot breed there.