Classification

There are millions of species on our planet. Although species ca

n be very different from each other, many have similar features that allow us to put them into groups. This is called classification.

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)

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.

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.

More information on arthropods:

Arthropods are an important phylum of invertebrates. They have:

• a hard exoskeleton (their skeleton is on the outside rather than on the inside)
• a segmented body (their body has different sections)
• jointed legs

Arthropods can be divided into different groups depending on how many legs they have.

Flowering plants

Flowering plants can be classified according to the nature of their seeds, seedlings and leaves. The two largest groups of flowering plants are:

• monocotyledons
• eudicotyledons (also called dicotyledons)

Monocotyledons

Wheat plants are monocotyledons

The seeds of monocotyledons each contain one embryonic leaf (the ‘cotyledon’). Many monocotyledons have leaves with parallel veins and the parts of their flowers come in threes. Examples of monocotyledons include:

• palms
• orchids
• grasses

Eudicotyledons

Sunflower plants are eudicotyledons

The seeds of eudicotyledons each contain two embryonic leaves. Eudicotyledons have leaves with branching veins, and the parts of their flowers come in fours or fives. Examples of eudicotyledons include:

• buttercups
• dandelions
• oak trees

Keys and identification

Keys are used to identify different species. A key will usually ask questions based on easily identifiable features of an organism. Dichotomous keys use questions to which there are only two answers. They can be presented as a table of questions, or as a branching tree of questions.

Branching tree example

This tree could help you identify a new vertebrate. For example, if it had no fur or feathers and dry skin, you would follow the right-hand pathway at the first and second junctions, but the left-hand pathway at the third junction. This would lead you to identify the animal as a reptile.

Topic Quiz: