The Problem

Currently 1.7 million living organisms have been identified on planet Earth.
Estimates of the actual number of species range from as low as 10 million to as high as 100 million (see The Encylopedia of Life ).
How is it possible to classify all these species using a system that makes sense and is biologically useful and accurate?

History of Classification

Two Kingdom System
- Developed by Carolus Linnaeus in the late 1700s.
- All living organisms were placed in two categories:
  - Animals
  - Plants
Five Kingdom System
- Developed by Robert Whittaker in 1969.
- All living organisms were placed in five categories.
  - Monera
  - Protista
  - Plantae
  - Fungi
  - Animalia
Three Domain System
- Developed by Carl Woese in the late 1970s.
- All living organisms were placed in three categories, above the level of Kingdom:
  - Bacteria
  - Archaea
  - Eukarya
- Some scientists consider a domain to be a “super kingdom.”

Figure 26.10, page 499, Campbell's Biology 5th Edition

We use the three domain system.
- Bacteria (prokaryotes)
- Archaea (prokaryotes)
- Eukaryota (all eukaryotes)
Why domains?
- They are based on degree of similarity or difference in the structure of important molecules that all organisms share, like rRNA.
- They more accurately indicates evolutionary relationships.