From
biological taxonomy, a
kingdom or
regnum is a
taxonomic rank in either (historically) the highest rank, or (in the new
three-domain system) the
rank below
domain. Each kingdom is divided into smaller groups called
phyla (or in some contexts these are called "divisions"). Currently, textbooks from the United States use a system of six kingdoms (
Animalia,
Plantae,
Fungi,
Protista,
Archaea, and
Eubacteria), while British and Australian textbooks describe five kingdoms (
Animalia,
Plantae,
Fungi,
Protista, and
Prokaryota or
Monera).
It gradually became apparent how important the prokaryote/eukaryote distinction is, and Stanier and van Niel popularized Edouard Chatton's proposal in the 1960s.[1]
In the years around 1980 there was an emphasis on phylogeny and redefining the kingdoms to be monophyletic groups, groups made up of relatively closely related organisms. The Animalia, Plantae, and Fungi were generally reduced to core groups of closely related forms, and the others placed into the Protista. Based on RNA studies Carl Woese divided the prokaryotes (Kingdom Monera) into two kingdoms, called Eubacteria and Archaebacteria. Carl Woese attempted to establish a Three Primary Kingdom (or Urkingdom) system in which Plants, Animals, Protista, and Fungi were lumped into one primary kingdom of all eukaryotes. The Eubacteria and Archaebacteria made up the other two urkingdoms. The initial use of "six Kingdom systems" represents a blending of the classic Five Kingdom system and Woese's Three Kingdom system. Such six Kingdom systems have become standard in many works.[3]
A variety of new eukaryotic kingdoms were also proposed, but most were quickly invalidated, ranked down to phyla or classes, or abandoned. The only one which is still in common use is the kingdom Chromista proposed by Cavalier-Smith, including organisms such as kelp, diatoms, and water moulds. Thus the eukaryotes are divided into three primarily heterotrophic groups, the Animalia, Fungi, and Protozoa, and two primarily photosynthetic groups, the Plantae (including red and green algae) and Chromista. However, it has not become widely used because of uncertainty over the monophyly of the latter two kingdoms.
