Classifying Living Things
Learn how scientists organise the millions of species on Earth into groups based on their characteristics.
Why Do We Classify?
There are millions of different species of living things on Earth. To make sense of this incredible diversity, scientists sort them into groups based on shared characteristics. This is called classification.
Classification helps us:
- ●Identify organisms and give them scientific names
- ●Understand relationships between different species
- ●Communicate about organisms around the world
- ●Study and protect biodiversity
The Major Kingdoms
Living things are sorted into broad groups called kingdoms. The four main kingdoms you need to know are:
Animals
Multicellular, can move, eat other organisms for food. No cell walls.
Plants
Multicellular, make their own food (photosynthesis), have cell walls, cannot move freely.
Fungi
Mushrooms, moulds, yeasts. Absorb nutrients from dead matter. Have cell walls but no photosynthesis.
Bacteria
Microscopic, single-celled organisms. Found everywhere. Some helpful, some cause disease.
Vertebrates vs Invertebrates
Animals are divided into two major groups based on whether they have a backbone (spine).
Vertebrates (have a backbone)
Only about 5% of all animal species! Five main classes:
- Mammals -- Warm-blooded, have fur/hair, feed young milk (humans, whales, kangaroos)
- Birds -- Warm-blooded, have feathers, lay eggs (eagle, penguin, emu)
- Reptiles -- Cold-blooded, have scales, lay eggs on land (lizards, snakes, crocodiles)
- Amphibians -- Cold-blooded, moist skin, live in water and on land (frogs, salamanders)
- Fish -- Cold-blooded, have scales and fins, breathe through gills, live in water
Invertebrates (no backbone)
About 95% of all animal species! Examples include:
- Insects -- 6 legs, 3 body parts (ants, bees, butterflies)
- Arachnids -- 8 legs (spiders, scorpions)
- Molluscs -- Soft bodies, often have shells (snails, octopus, mussels)
- Crustaceans -- Hard exoskeleton (crabs, lobsters, prawns)
- Worms -- Long, soft bodies (earthworms, leeches)
Dichotomous Keys
A dichotomous key is a tool used to identify organisms by asking a series of yes/no questions about their features. Each answer leads to the next question until you identify the organism.
Example: Identify the Animal
Does it have a backbone?
Does it have feathers?
INVERTEBRATE
Keep asking more questions
Key Vocabulary
Classification
Sorting organisms into groups based on shared characteristics.
Species
A group of organisms that can reproduce together to produce fertile offspring.
Vertebrate
An animal with a backbone (e.g. fish, birds, mammals).
Invertebrate
An animal without a backbone (e.g. insects, worms, jellyfish).
Kingdom
The largest group in classification (animals, plants, fungi, bacteria).
Dichotomous Key
A tool for identifying organisms using a series of yes/no questions.
Worked Examples
Classify a kangaroo: What kingdom, vertebrate or invertebrate, and what class?
Step 1: Is it living? Yes. Does it eat food? Yes. → Animal kingdom.
Step 2: Does it have a backbone? Yes. → Vertebrate.
Step 3: Is it warm-blooded? Yes. Does it have fur? Yes. Does it feed milk to young? Yes.
Answer: A kangaroo is an animal, vertebrate, mammal (specifically a marsupial).
A mushroom grows in the garden. Is it a plant?
Step 1: Does it make its own food through photosynthesis? No.
Step 2: Does it absorb nutrients from dead/decaying matter? Yes.
Answer: A mushroom is not a plant. It belongs to the fungi kingdom. Unlike plants, fungi cannot make their own food.
Is a frog a reptile or an amphibian?
Step 1: Does it have dry, scaly skin? No -- frogs have moist, smooth skin.
Step 2: Does it spend part of its life in water? Yes -- frogs start as tadpoles in water.
Answer: A frog is an amphibian. Amphibians have moist skin and typically live partly in water, partly on land.
Knowledge Check
Select the correct answer for each question. Click "Check Answer" to see if you are right.
Question 1
Which of these organisms belongs to the fungi kingdom?
Question 2
A shark has a backbone, lives in water, and breathes through gills. It is a:
Question 3
Approximately what percentage of animal species are invertebrates?
Question 4
Which class of vertebrates has feathers?
Question 5
What tool uses yes/no questions to identify living things?
Key Concepts Summary
- ●Classification means sorting organisms into groups based on shared characteristics.
- ●The main kingdoms are animals, plants, fungi, and bacteria.
- ●Vertebrates have backbones (mammals, birds, reptiles, amphibians, fish); invertebrates do not.
- ●Each vertebrate class has unique features: mammals (fur, milk), birds (feathers), reptiles (scales), amphibians (moist skin), fish (gills, fins).
- ●A dichotomous key uses yes/no questions to identify organisms.