Summary

This chapter opened with the observation that living organisms exist in an extraordinary variety — an estimated 1.7 to 1.8 million species described so far — a richness we call biodiversity. Because no science could function by studying each organism as an isolated, unconnected case, biologists classify organisms: grouping them systematically on the basis of shared characters, in order to identify them, communicate about them, and capture genuine evolutionary relationships between them.

At the broadest level, all cellular life is now organised into three domains — Bacteria, Archaea and Eukarya — a scheme proposed by Carl Woese on the basis of molecular evidence, replacing the earlier two-way prokaryote/eukaryote split. The science of naming, identifying and classifying organisms is taxonomy, while the broader study of their diversity and evolutionary relationships is systematics; the two are closely linked, since a sound classification should reflect real evolutionary history.

The basic unit of classification is the species — a group of individuals capable of interbreeding, distinguishable from every other such group by consistent characters. Species are organised into a nested taxonomic hierarchy of increasingly inclusive categories: species, genus, family, order, class, phylum (or division, for plants), and kingdom. Every valid species is given a unique two-part scientific name under the rules of binomial nomenclature, established by Linnaeus — a Latinised genus name (capitalised) followed by a specific epithet (lower case), both italicised, optionally followed by the author's name. …