The classification of organisms into kingdoms and the hierarchy of taxonomic groups, the binomial naming system, the meaning of a species, the use of keys, and the meaning and measurement of biodiversity.

What this dot point is asking

WJEC wants you to describe how organisms are classified into kingdoms and a hierarchy, explain the binomial naming system, define a species, use keys to identify organisms, and explain what biodiversity is and how it is measured.

Why we classify

Classification is the sorting of living things into groups based on their similarities and differences. It helps scientists organise the huge variety of life, identify organisms, and show how closely related different species are.

The kingdoms and the hierarchy

The largest groups are the kingdoms. The five kingdoms are:

• Animals (multicellular, no cell wall, feed on other organisms),
• Plants (multicellular, cell wall, photosynthesise),
• Fungi (such as moulds and mushrooms; feed by breaking down material around them),
• Protoctists (mostly single-celled, such as amoeba),
• Prokaryotes (bacteria; single-celled with no nucleus).

Within each kingdom, organisms are sorted into smaller and smaller groups in a hierarchy:

kingdom, phylum, class, order, family, genus, species

As you move down the hierarchy, the groups get smaller and the organisms in them are more closely related and more alike.

The binomial system

Every species is given a two-part scientific name using the binomial system.

This is useful because each species has a unique name used by scientists all over the world, avoiding the confusion of common names, which differ between languages and regions.

What a species is

Members of the same species are similar and can breed together; members of different species usually cannot produce fertile offspring.

Using keys

A key is a tool for identifying an organism by answering a series of questions about its features. A common type is a dichotomous key, where each step gives two choices that lead either to the next step or to the name of the organism. Following the choices that match the organism leads to its identity.

Biodiversity and how it is measured

To estimate the biodiversity or the number of a species in an area, scientists use sampling:

• A quadrat is a square frame placed on the ground; you count the organisms inside it, repeat at several random positions, find a mean, and scale up to the whole area.
• A transect is a line across an area; you record the organisms along it to see how they change, for example from a path into a meadow.

Sampling positions must be chosen randomly so the results fairly represent the whole area and are not biased.