Classification

There are millions of different kinds of living things. To make sense of them, scientists sort them into groups by the features they share: this is classification. Animals with a backbone are vertebrates, those without are invertebrates, and every living thing belongs to a kingdom and has a two-part scientific name. Keys help us identify an organism step by step.

Key facts

  • Vertebrates have a backbone. There are five groups: fish (scales, gills, fins), amphibians (moist skin, lay eggs in water), reptiles (dry scaly skin, lay leathery eggs on land), birds (feathers, beaks, hard-shelled eggs) and mammals (hair or fur, feed their young on milk).
  • Invertebrates have no backbone: insects (6 legs, 3 body parts), arachnids such as spiders (8 legs, 2 body parts), crustaceans (hard shell, many legs), molluscs (soft body, often a shell), worms and jellyfish.
  • Carl Linnaeus sorted living things into groups inside groups: kingdom, phylum, class, order, family, genus, species.
  • Five kingdoms: animals, plants, fungi, protists (single-celled organisms with a nucleus) and prokaryotes (bacteria).
  • Every species has a two-part (binomial) name: the genus, then the species, written in italics. Humans are Homo sapiens; the lion is Panthera leo and the tiger Panthera tigris.
  • Two species in the same genus are closely related. Scientists now also group life into three domains, using chemical analysis of cells: Archaea, Bacteria and Eukaryota.
  • A key is a set of questions, each with two answers, that leads you to the name of an organism.

Worked examples

Example 1

An animal has dry, scaly skin and lays eggs with leathery shells on land. Which vertebrate group does it belong to?

  1. Fish have scales too, but they live in water and breathe with gills.
  2. Amphibians have moist skin and lay their eggs in water.
  3. Dry scaly skin and leathery eggs laid on land: it is a reptile, like a lizard, snake or tortoise.

Answer: reptiles

Example 2

Use the key to identify an animal with six legs and wings.

  1. Does it have legs? Yes, so go down.
  2. Does it have six legs or fewer? Yes (it has six), so go down.
  3. Does it have wings? Yes: it is the bee.

Answer: bee

Example 3

Put these in order from the biggest group to the smallest: species, kingdom, genus, family, class, phylum, order.

  1. The kingdom is the biggest group, such as all animals.
  2. Each group is split into smaller ones. kingdom → phylum → class → order → family → genus → species
  3. A memory aid: “King Philip Came Over For Good Soup”.

Answer: kingdom → phylum → class → order → family → genus → species

Example 4

The lion is Panthera leo, the tiger Panthera tigris and the domestic cat Felis catus. Which two are most closely related?

  1. The first word of a binomial name is the genus; the second is the species.
  2. The lion and the tiger share the genus Panthera; the cat is in a different genus, Felis.
  3. Species in the same genus are closely related, so the lion and the tiger are the closest.

Answer: the lion and the tiger