The taxonomic hierarchy, binomial names and evolutionary relationships

1One bird, many names

A tall grey bird with a golden crown of stiff feathers walks through the wetlands of East Africa. In English it is called the grey crowned crane, the crowned crane or, in Uganda, where it is the national bird, the crested crane. Each language in the region has its own name for it as well. A scientist in Kigali, Kampala or Tokyo, however, writes one name only: Balearica regulorum.

About two million species have been described, and many more await discovery. Biologists can only find, compare and talk about them if every species has one name and one place in an agreed system. That system is classification. In S1 you learned the ranks and the binomial name; at A-level you must also explain how biologists decide which organisms are related.

2The ranks, from domain to species

Each rank down the hierarchy groups fewer organisms that share more features and a more recent common ancestor. Figure 1 shows the eight main ranks for the grey crowned crane. The order of the ranks can be remembered as "Dear King Philip Came Over For Good Soup".

The human and the crane share every rank down to phylum, so they share a more recent common ancestor with each other than either does with the bean, which they share only at domain level. The names of flowering-plant divisions and classes differ between books; the ranks from order down are much more stable.

3Rules for binomial names

The Swedish naturalist Carl Linnaeus made two-part names the standard in his Species Plantarum (1753) for plants and the tenth edition of his Systema Naturae (1758) for animals.

• The genus name comes first and starts with a capital letter; the specific epithet comes second and is all lower case: Phaseolus vulgaris, never Phaseolus Vulgaris.
• The name is printed in italics, or underlined (each word separately) when handwritten.
• After the first full use, the genus may be shortened to its initial: P. vulgaris.
• The specific epithet is never used alone: "sapiens" does not name a species, but Homo sapiens does.
• Names are Latin or treated as Latin, so they are the same in every language.

4Natural and artificial classification

An artificial classification groups organisms by one or two convenient features chosen for a purpose: "trees and shrubs", "flying animals", "poisonous and edible fungi". It is useful but can put unrelated organisms together: bats and birds both fly, but bats are mammals.

A natural (phylogenetic) classification uses many features and aims to group organisms by common ancestry. It relies on two kinds of similarity:

• Homologous structures have the same basic structure and origin but may do different jobs. The five-fingered (pentadactyl) limb is built from the same bones in a human arm, a bat wing and a whale flipper: evidence that mammals share an ancestor.
• Analogous structures do the same job but have different structures and origins, such as the wing of a bird and the wing of a butterfly. They arise when unrelated organisms adapt to the same way of life, and are not evidence of close relationship.

Today the strongest evidence is molecular. Every species has DNA, and many proteins (such as cytochrome c, used in respiration) are found in almost all organisms. Mutations build up in DNA over time, so the more differences there are between two species' DNA base sequences or a protein's amino-acid sequences, the longer ago they shared a common ancestor.