Traditional Biological Classification System
Before the Idea of Phylogeny
An organism's evolutionary history is called its phylogeny. Before the 20th century, biologists did not use evolutionary history in classification at all. The very concept of phylogeny did not arise until Charles Darwin published his theory of evolution, which included the idea of common descent. The roots of our current system of biological classification lie in the 18th century with the naturalist Carolus Linnaeus, long before Darwin.
Linnaeus' major publications, Species Plantarum (Plant Species) and Systema Naturae (Systems of Nature), divided living things into heirarchical categories and assigned each species a short but unique name. The categories were based on the physical characteristics of organisms. Plants were arranged according to the number of stamens in their flowers, which does not reflect their evolutionary relationships well. Some taxa developed under the Linnaean system, such as the mammal orders Insectivora and Pachydermata, are based only on superficial resemblances and have no bearing on phylogeny at all.
The current system includes eight major ranks: Domain, Kingdom, Division (plants and fungi) or Phylum, Class, Order, Family, Genus, and Species. Each rank is called a taxon (plural taxa). Every known organism is classified into taxa of these eight ranks, and often into a number of intermediate-ranking taxa as well (for example, Subphylum or Superorder).