The binomial system

There are millions of species on our planet. Although species can be very different from each other, many have similar features that allow us to put them into groups. This is called classification.

Kingdoms

The first big division of living things in the classification system is to put them into one of five kingdoms. These are based on what an organism's cells are like.

The five kingdoms are:

1. animals (all multicellular animals)
2. plants (all green plants)
3. fungi (moulds, mushrooms, yeast)
4. prokaryotes (bacteria, blue-green algae)
5. protoctists (Amoeba, Chlorella and Plasmodium)

Further divisions

Living things can then be ranked according to:

• phylum
• class
• order
• family
• genus
• species

The binomial system of naming species uses Latin words. Each name has two parts, the genus and the species. For example, human beings belong to the genus Homo, and our species is sapiens - so the scientific name is Homo sapiens.

The binomial system is important because it allows scientists to accurately identify individual species. For example, the European robin is Erithacus rubecula. It is much smaller than the American robin, Turdus migratorius, which belongs to a different genus.

Other classification systems

Cladistics is another way to classify organisms. It can use data from DNA or RNA sequences, rather than just physical characteristics. It emphasises the evolutionary relationships between different species.

Vertebrates

Vertebrates are animals with backbones. They can be classified according to their features, and include bony fish, amphibians, reptiles, birds and mammals.

Bony fish

Bony fish absorb oxygen through their gills and they have wet scales. Their body temperature varies according to their surroundings (organisms that do this are called poikilotherms). Bony fish reproduce by external fertilisation and lay eggs (they are oviparous).

Amphibians

Amphibians usually have lungs but they can also absorb oxygen…