The Three Domains of Life

Taxonomy: The science of naming and classifying organisms.
Classical evolutionary taxonomy: A way to describe relationships among organisms based upon similarities of structure and

the relatedness due to a common evolutionary history.
Cladistics: A way to describe relationships among organisms based upon a group of

organisms sharing a common evolutionary history.

Your Great-Grandparent's Kingdoms of Life

Based upon visual (including SEM, TEM microscopy), some DNA work but

still centered on larger organisms.
Changes proposed by Whittaker (U.S.) in 1969 and later modified by Lynn Margulis.
Two domains based upon structure and physiology of the cells:
1. Prokaryote - single cell organisms lacking a membrane-enclosed nucleus and organelles.
2. Eukaryote - organisms with cells having a membrane-enclosed nucleus and organelles.
And those two domains broken into five kingdoms:
1. Monera - single cell prokaryotes.
2. Protista - single cell eukaryotes.
3. Fungi - absorptive heterotrophic multicellular eukaryotes.
4. Plants - autotrophic multicellular eukaryotes.
5. Animals - ingestive heterotrophic multicellular eukaryotes.

Based upon ribosomal RNA sequences, Woese et al, in 1970's proposed three-domain system:
1. Archaea - single celled organisms lacking a membrane-enclosed nucleus and organelles with unique RNA and DNA sequences adapted for life in extreme conditions. They are more closely related to eukaryotes than bacteria!
2. Bacteria - single celled organisms lacking a membrane-enclosed nucleus and organelles.
3. Eukarya - organisms with cells having a membrane-enclosed nucleus and organelles.
Six-kingdom system:
1. Archaea - organisms of extreme environments divided into groups based upon their producing methane, living in extreme salt concentrations or extreme temperatures.
2. Bacteria
3. Protists
4. Plants
5. Fungi
6. Animals