Taxonomic categories

Classification is done in steps, from small groups to bigger and bigger ones. Each step is a rank or taxonomic category, and together they make the taxonomic hierarchy. From the lowest to the highest:

Species → Genus → Family → Order → Class → Phylum (called Division in plants) → Kingdom.

• Species: individuals with basic similarities, which differ from closely related species in form. Lion (Panthera leo) and tiger (Panthera tigris) are two species.
• Genus: a group of related species. Lion, tiger and leopard are all in the genus Panthera. Potato (Solanum tuberosum) and brinjal (Solanum melongena) are both in Solanum.
• Family: related genera. Solanaceae includes Solanum, Petunia and Datura. Felidae includes Panthera and Felis (cats).

• Order: families with a few similar characters. Carnivora includes Felidae (cats) and Canidae (dogs). For plants, the book's example is Polemoniales (printed "Polymoniales" in the book).
• Class: related orders. Mammalia includes Primata (monkeys, apes, humans) and Carnivora.
• Phylum: related classes. Chordata includes all animals with a notochord and a dorsal hollow nerve cord.
• Kingdom: the highest category, such as Animalia (animals) and Plantae (plants).

Species is the lowest category for all organisms. As you go up from species to kingdom, each group holds more organisms and they share fewer characters, so placing organisms in higher taxa becomes harder. Members of a lower taxon share more characters than members of a higher one.

The housefly, in full: species Musca domestica, genus Musca, family Muscidae, order Diptera, class Insecta, phylum Arthropoda, kingdom Animalia.