Classification, taxonomy and systematics

No one can study millions of living things one by one. So we sort them into convenient groups, based on characters we can easily see. This is classification. When you hear "dog", "mammal" or "wheat", a picture comes to mind at once: that is because each word names a group.

Such a group is called a taxon (plural taxa). Plants, animals, mammals, dogs and wheat are all taxa, but at very different levels: a dog is a mammal, and mammals are animals. Arthropoda and algae are taxa too.

Taxonomy is the process of classifying organisms into taxa. It has four basic steps. Imagine finding a cockroach for the first time.

• Characterisation: note all its features: three pairs of legs, antennae, wings.
• Identification: compare it with organisms already known, like a butterfly, which also has jointed legs, antennae and wings.
• Classification: place it in a group: kingdom Animalia, phylum Arthropoda, an insect.
• Nomenclature: give it its name: Periplaneta americana.

In early days, organisms were grouped by their uses: for food, clothing and shelter. Later, classical taxonomy used mainly external features (shape of leaf, root, stem). Modern taxonomy also uses internal structure, the structure of cells, the process of development, and ecological information about the organism.

Systematics goes one step further. The word comes from the Latin systema, meaning a systematic arrangement; Linnaeus called his book Systema Naturae. Systematics studies the kinds and diversity of organisms and the relationships among them, including how they are related through evolution.