Why classify?
There are millions of kinds of living things on Earth. To make sense of them, scientists classify them, or sort them into groups based on features they share. Classification is like organizing a library, so you can find a book quickly.
Big groups: the kingdoms
Scientists sort living things into very large groups. Many schools teach six kingdoms:
| Kingdom | Features | Examples |
|---|---|---|
| Animals | Many cells; eat other living things; most can move | Dogs, fish, insects |
| Plants | Many cells; make their own food from sunlight | Trees, grasses, ferns |
| Fungi | Absorb food from what they grow on | Mushrooms, mould, yeast |
| Protists | Mostly single-celled; varied | Amoebas, algae |
| Bacteria | Tiny single cells with no nucleus | E. coli, bacteria in yogurt |
| Archaea | Single cells that often live in extreme places | Microbes in hot springs |
Sorting animals
Animals are split into two big groups:
- Vertebrates have a backbone.
- Invertebrates do not have a backbone. More than 95% of animal species are invertebrates!
The five vertebrate groups
| Group | Body covering | Breathing | Young | Examples |
|---|---|---|---|---|
| Mammals | Hair or fur | Lungs | Most born alive; drink milk | Humans, whales, bats |
| Birds | Feathers | Lungs | Hard-shelled eggs | Robins, penguins |
| Reptiles | Dry scales | Lungs | Leathery eggs (mostly) | Snakes, turtles |
| Amphibians | Moist skin | Gills, then lungs | Jelly eggs in water | Frogs, salamanders |
| Fish | Wet scales | Gills | Eggs in water (mostly) | Salmon, sharks |
Some invertebrate groups
- Insects: six legs, three body parts (ants, beetles)
- Arachnids: eight legs, two body parts (spiders, scorpions)
- Molluscs: soft bodies, often with shells (snails, octopuses)
- Worms: long soft bodies (earthworms)
Sorting plants
- Flowering plants make seeds inside flowers and fruits (apple trees, roses).
- Conifers make seeds in cones (pines, spruces).
- Ferns and mosses don’t make seeds; they make tiny spores.
Species and scientific names
A species is one kind of living thing. Members of a species can breed together and have young that can also breed.
Each species gets a two-part scientific name, such as Canis lupus for the grey wolf. The first part is the genus; the second names the species. Scientists all over the world use these names, so there’s no confusion.
Using a dichotomous key
A dichotomous key helps you identify a living thing by answering two-choice questions.
- Does it have a backbone? Yes → go to 2. No → invertebrate.
- Does it have feathers? Yes → bird. No → go to 3.
- Does it have hair or fur? Yes → mammal. No → go to 4.
- Does it have moist skin? Yes → amphibian. No → go to 5.
- Does it have gills its whole life? Yes → fish. No → reptile.
Tricky animals
- A bat flies, but it has fur and feeds its young milk, so it’s a mammal.
- A whale lives in the ocean, but it breathes air and has babies, so it’s a mammal.
- A penguin can’t fly, but it has feathers, so it’s a bird.
Common mistakes
- Classifying by where an animal lives. Classify by body features instead.
- Thinking spiders are insects. Spiders have eight legs; insects have six.
Try it yourself
Use the dichotomous key above to classify: a salamander, a shark, an eagle, a turtle and a dolphin.
Quick recap
- Classifying sorts living things into groups by shared features.
- Kingdoms are the largest groups.
- Vertebrates have backbones; invertebrates do not.
- A species is one kind of living thing with a two-part scientific name.
- A dichotomous key uses yes-or-no questions to identify organisms.