Millions of kinds of organism share the planet. To study them we classify them — grouping organisms by shared features into a tidy, universal system that also reveals how they are related.
Classification (taxonomy) sorts living things into groups based on shared characteristics. It makes an overwhelming diversity manageable, gives every organism a single universal name, and shows evolutionary relationships — organisms in the same group share a common ancestor.
Organisms are placed in a nested series of ranks, from the broadest to the most specific:
The species is the basic unit — a group of similar organisms that can interbreed to produce fertile offspring. As you move down the ranks, the groups get smaller but the members become more alike.
Carolus Linnaeus gave us the binomial system: each species has a two-part Latin name — the Genus then the species. Rules: the Genus is capitalised, the species is lower case, and the whole name is italic (or underlined when handwritten).
For example, humans are Homo sapiens and the domestic mango is Mangifera indica. The same name is used by scientists everywhere, avoiding the confusion of local names.
Robert Whittaker grouped all life into five kingdoms, based on cell type (prokaryotic/eukaryotic), number of cells, and mode of nutrition:
| Kingdom | Cells | Key features |
|---|---|---|
| Monera | Prokaryotic | Bacteria — single-celled, no nucleus |
| Protoctista | Eukaryotic | Mostly single-celled (algae, protozoa) |
| Fungi | Eukaryotic | Mostly multicellular; absorb food (saprotrophs) |
| Plantae | Eukaryotic | Multicellular; make food by photosynthesis |
| Animalia | Eukaryotic | Multicellular; ingest food (holozoic) |
Viruses are acellular, so they don't fit any of the five kingdoms — they're studied separately (see the Viruses chapter). Modern biology also adds a higher level, the three domains (Archaea, Bacteria, Eukarya).