Domain (or its synonyms) is the most inclusive of these biological groupings. The arrangement of taxa reflects the fundamental evolutionary differences in the genomes. There are several modern alternative domain classifications of life. Among them are:
- The 'two-empire' system, with top-level groupings of Prokaryota (or Monera) and Eukaryota empires.
- The six-kingdom system with top-level groupings of Protista, Archaebacteria, Eubacteria, Fungi, Plantae, and Animalia.
- The three-domain system of Carl Woese, introduced in 1990, with top-level groupings of Archaea, Bacteria, and Eukaryota domains.
After defining Archaea as a new domain, Woese redrew the taxonomic tree. His three-domain system is based upon genetic relationships. It divides life into 23 main divisions in three domains: Bacteria, Archaea, and Eukarya. Archaea are neither bacteria nor eukaryotes. Looked at another way, they are prokaryotes that are not bacteria.