NSB31 Recap

Classification: sorting living things

Why we classify

There are millions of species on Earth. Classification means sorting them into groups based on shared features. It lets scientists identify unknown organisms, communicate clearly across languages, and see how living things are related. Good classification uses structural features (body parts, number of legs, type of skeleton) rather than things like colour, size or where an animal happens to live, because those vary too much.

The five kingdoms

The largest groups are kingdoms. The five most commonly used are:

  • Animals — multicellular, eat other organisms, most can move.
  • Plants — multicellular, make their own food by photosynthesis, have cell walls.
  • Fungi — mushrooms, moulds and yeasts. They absorb food from dead or living material and have cell walls, but no chloroplasts, so they are not plants.
  • Protists — mostly single-celled organisms with a nucleus, such as amoeba and algae.
  • Bacteria (Monera) — single-celled organisms with no nucleus.

Levels of classification

Each kingdom is divided into smaller and smaller groups. From largest to smallest:

Kingdom, Phylum, Class, Order, Family, Genus, Species.

A common mnemonic is “King Philip Came Over For Good Spaghetti”. As you move down the list, the groups get smaller and the organisms in them become more similar. Two animals in the same family share more features than two animals that are only in the same class.

A species is the smallest group: organisms that can breed together to produce fertile offspring. A horse and a donkey can mate, but their mule offspring are infertile, so horses and donkeys are separate species.

Scientific names

Every species has a two-part scientific name — its binomial name — invented by Carl Linnaeus. The first word is the genus (capital letter) and the second is the species (lower case), and the whole name is written in italics or underlined when handwritten:

  • Humans: Homo sapiens
  • Dog: Canis familiaris
  • Dingo: Canis dingo — same genus as the dog, so closely related

Scientific names are the same in every country, so a biologist in Japan and one in Brazil know exactly which organism is meant. Common names cause confusion: “robin” is a different bird in Australia, Britain and America.