Kingdom (biology)

In biology,kingdom istop-level, or nearlytop-level, groupingorganismsscientific classification. Originally, two kingdoms were distinguished,Animalia (animals), which typically could move about, andPlantae (plants), which typically could not. Early authors also hadthird kingdomminerals. Each kingdom was divided into classes, later into phylaanimalsdivisionsplants. This simplistic classification has been largely abandoned thanksnew developments, however.

When single-celled organisms were first discovered,were split betweentwo kingdoms: motile forms were placed inphylum Protozoa, while colored forms (algae)bacteria were categorizedseveral divisionsplants. A numberforms ended up being placedboth -instance Euglenaslime molds. Asresult,third kingdom,Protista, was createdhold these groups. This was first suggested by Ernst Haeckel, thoughwas some time beforekingdom gained much currency.

Copeland introducedfourth kingdombacteria, which haveprokaryotic cell organization rather thaneukaryotic organization foundhis other three kingdoms. He called themMychota, but this was later replacedMonera from their primitive form. The fungi, which he included amongProtoctista (an alternate name forProtista), were given their own kingdom by Whittaker. Thus, he had three kingdomsmulticellular organisms, depending on whetherwere autotrophic (Plantae), saprotrophic (Fungi), or heterotrophic (Animalia),twounicellular or colonial organisms (ProtistaMonera). With some variation inexact circumscriptionthese groups, this five-kingdom system has been standard forlong time,is still usedmany works.

However, newer findings have ledalternative systems. Most notable wasfinding by Carl Woese that prokaryotes comprised two distinct groups, which he called EubacteriaArchaebacteria butnow called BacteriaArchaea, whichnot ornot much closer relatedeach other than theytoeukaryotes. This promptedthree-domain system,eachthese groupstreated asdomain. The domains were originallyreplacementkingdoms, butmore commonly used ashigher level rank, withEukaryota divided into several different kingdoms. Alternatively, some have simply treatedBacteriaArchaea as two kingdomsplace ofMonera. This six-kingdom system has replacedfive-kingdom systemmany works.

A comparison ofmore notable systems:

Haeckel (1894) Three kingdoms	Whittaker (1959)¹ Five kingdoms	Woese (1977) Six kingdoms	Woese (1990) Three domains
Protista	Monera	Eubacteria	Bacteria
		Archaebacteria	Archaea
	Protista	Protista	Eukarya
Plantae	Fungi	Fungi
	Plantae	Plantae
Animalia	Animalia	Animalia

The Protista havebeen recognized asjunk-basket categoryorganisms that don't fit intoother eukaryotic kingdoms,asresult some workers have promoted various protist groupskingdoms. The most notablethese iskingdom Chromista, proposed by Thomas Cavalier-Smith, which corresponds essentially toheterokontsincludes groups like brown algae, diatoms,water molds. He also proposedkingdom Archezoaprimitive eukaryotes that lack mitochondria, but this gained little support due togroup's implausible heterogeneity,in later revisions he abandoned bothandArchaebacteria. Other groups which have been considered kingdoms, though considerably less often, includealveolateseuglenozoa.

See also: Binomial nomenclature, Scientific classification, Taxonomy

Footnote

¹ R. H. Whittaker's Five Kingdoms system, first propounded1959, has remained substantially unchanged since then, but subsequent refinements were made (see References). The terms protoctistaprokaryotaeoften substitutedprotistamonera respectively.

References

Whittaker, R.H. (1959). Onbroad classificationorganisms. Quart. Rev. Biol. 34, 210-226.

Whittaker, R.H. (1969). New conceptskingdomsorganisms. Science 163, 150-160.