界 (分類学)
歴史 > 六界（1998）

1998年、キャバリエ＝スミスは六界モデルを発表したが、その後の論文で改訂されている。次に、2009年に発表された改訂版を説明する。キャバリエ＝スミスは、ウーズらによって提唱され、最近の研究によって裏付けられている基本的な真正細菌（Eubacteria）と古細菌（Archaebacteria）を区別することの重要性を否定している。細菌界（原核生物帝の唯一の界）は膜トポロジー（英語版）によって、ウニバクテリア（Unibacteria）とネギバクテリア（Negibacteria、グラム陰性細菌）の2つの亜界に分割された。ウニバクテリアは古細菌（Archaebacteria）とポジバクテリア（Posibacteria、グラム陽性細菌）に分けられた。古細菌の遺伝的距離の長い枝よりも、二膜性－単膜性を区別する方がはるかに本質的であり、これには特に生物学的意義はないと見なすものであった。
キャバリエ＝スミスは、分類群は単系統でなければならないという要件を有効とは認めていない。彼は、原核生物、細菌、ネギバクテリア、ウニバクテリア、ポジバクテリアを有効な側系統分類群として定義し、（生物学的ニッチの概念に関して）生物学的意義の重要な革新を示している。
同じように、彼の側系統的な原生動物界には、動物界、菌界、植物界、クロミスタのそれぞれの祖先が含まれている。系統発生学的な研究の進歩により、キャバリエ＝スミスは、アーケゾア（すなわちミトコンドリアを持たない原始的な真核生物）であると考えられていたすべての系統が、実際には、ミトコンドリアを二次的に失い、典型的にはミトコンドリアを新しい細胞小器官であるハイドロジェノソームに変化させていたことに気づいた。つまり、キャバリエ＝スミスによって与えられた用語の意義によれば、すべての生きている真核生物は実際にはメタカリオタである。微胞子虫門（phylum Microsporidia）のように、消滅したアーケゾア界の一部は、菌界に再分類された。他には、現在エクスカバータ下界に属しているメタモナーダのように、原生動物界に再分類されたものもある。
キャバリエ＝スミスは側系統を認めているので、下の図は「祖先図」ではなく「組織図」であり、進化系統樹を表しているわけではない。
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|  Kingdom Bacteria     Subkingdom Negibacteria         Infrakingdom Lipobacteria             Superphylum Eobacteria         Phylum Heliobacteria         Phylum Hadobacteria           Subphylum Chlorobacteria           Subphylum Deinobacteria       Superphylum Endoflagellata         Phylum Spirochaetae           Subphylum Euspirochaetae           Subphylum Leptospirae     Infrakingdom Glycobacteria       Superphylum Pimelobacteria         Phylum Sphingobacteria           Subphylum Chlorobibacteria           Subphylum Flavobacteria         Phylum Eurybacteria           Subphylum Sclenobacteria           Subphylum Fusobacteria           Subphylum Fibrobacteria         Phylum Cyanobacteria           Subphylum Gloeobacteria           Subphylum Phycobacteria         Phylum Proteobacteria           Subphylum Rhodobacteria             Infraphylum Alphabacteria             Infraphylum Chromatibacteria           Subphylum Thiobacteria       Superphylum Planctobacteria         Phylum Planctobacteria   Subkingdom Unibacteria     Infrakingdom Posibacteria       Phylum Posibacteria         Subphylum Teichobacteria           Infraphylum Endobacteria           Infraphylum Actinobacteria         Subphylum Togobacteria     Infrakingdom Archaebacteria       Phylum Mendosicutes         Subphylum Euryarcheota           Infraphylum Halomebacteria           Infraphylum Eurytherma         Subphylum Sulfobacteria |  Kingdom Protozoa   Subkingdom Archezoa     Phylum Metamonada       Subphylum Eopharyngia       Subphylum Axostylaria     Phylum Trichozoa       Subphylum Anaeromonada       Subphylum Parabasala   Subkingdom Neozoa     Infrakingdom Sarcomastigota       Phylum Neomonada         Subphylum Apusozoa         Subphylum Isomita         Subphylum Choanozoa       Phylum Cercozoa         Subphylum Phytomyxa         Subphylum Reticulofilosa         Subphylum Monadofilosa       Phylum Foraminifera       Phylum Amoebozoa         Subphylum Lobosa         Subphylum Conosa           Infraphylum Archamoebae           Infraphylum Mycetozoa             Superclass Eumyxa             Superclass Dictyostelia     Infrakingdom Discicristata       Phylum Percolozoa         Subphylum Tetramitia         Subphylum Pseudociliata       Phylum Euglenozoa         Subphylum Plicostoma         Subphylum Saccostoma     Infrakingdom Alveolata       Superphylum Miozoa         Phylum Dinozoa           Subphylum Protalveolata           Subphylum Dinoflagellata         Phylum Sporozoa           Subphylum Gregarinae           Subphylum Coccidiomorpha           Subphylum Manubrispora       Superphylum Heterokaryota         Phylum Ciliophora           Subphylum Tubulicorticata           Subphylum Epiplasmata           Subphylum Filocorticata     Infrakingdom Actinopoda       Phylum Heliozoa       Phylum Radiozoa         Subphylum Spasmaria         Subphylum Radiolaria |  Kingdom Fungi   Subkingdom Eomycota     Phylum Archemycota       Subphylum Dictyomycotina         Class Chytridiomycetes           Subclass Rumpomycetidae           Subclass Spizomycetidae         Class Enteromycetes       Subphylum Melanomycotina         Infraphylum Allomycotina           Class Allomycetes         Infraphylum Zygomycotina           Superclass Eozygomycetia             Class Bolomycetes             Class Glomomycetes           Superclass Neozygomycetia             Class Zygomycetes               Subclass Mucoromycetidae               Subclass Meromycetidae             Class Zoomycetes               Subclass Entomycetidae               Subclass Pedomycetidae                 Superorder Trichomycetalia                 Superorder Pyxomycetalia     Phylum Microsporidia       Class Minisporea       Class Microsporea         Subclass Pleistophorea         Subclas Disporea   Subkingdom Neomycota     Phylum Ascomycota       Subphylum Hemiascomycotina         Class Taphrinomycetes         Class Geomycetes         Class Endomycetes           Subclass Dipomycetidae           Subclass Saccharomycetidae       Subphylum Euascomycotina         Class Discomycetes           Subclass Calycomycetidae           Subclass Lecomycetidae           Subclass Pezomycetidae         Class Pyrenomycetes           Subclass Verrucomycetidae           Subclass Ostiomycetidae         Class Loculomycetes           Subclass Dendromycetidae           Subclass Loculoascomycetidae         Class Plectomycetes     Phylum Basidiomycota       Subphylum Septomycotina         Class Septomycetes           Subclass Sporidiomycetidae           Subclass Uredomycetidae       Subphylum Orthomycotina         Superclass Hemibasidiomycetia           Class Ustomycetes         Superclass Hymenomycetia           Class Gelimycetes             Subclass Tremellomycetidae             Subclass Dacrymycetidae             Subclass Auromycetidae           Class Homobasidiomycetes             Subclass Clavomycetidae             Subclass Pileomycetidae |  Kingdom Animalia   Subkingdom Radiata     Infrakingdom Spongiaria       Phylum Porifera         Subphylum Hyalospongiae         Subphylum Calcispongiae         Subphylum Archaeocyatha †     Infrakingdom Coelenterata       Phylum Cnidaria         Subphylum Anthozoa         Subphylum Medusozoa     Infrakingdom Placozoa       Phylum Placozoa   Subkingdom Myxozoa     Phylum Myxosporidia   Subkingdom Bilateria     Branch Protostomia       Infrakingdom Lophozoa         Superphylum Polyzoa           Phylum Bryozoa             Subphylum Gymnolaemata             Subphylum Lophopoda           Phylum Kamptozoa             Subphylum Entoprocta             Subphylum Cycliophora         Superphylum Conchozoa           Phylum Mollusca             Subphylum Bivalvia             Subphylum Glossophora               Infraphylum Univalvia               Infraphylum Spiculata               Infraphylum Cephalopoda           Phylum Brachiozoa             Subphylum Brachiopoda             Subphylum Phoronida         Superphylum Sipuncula           Phylum Sipuncula         Superphylum Vermizoa           Phylum Annelida             Subphylum Polychaeta               Infraphylum Operculata               Infraphylum Pharyngata             Subphylum Clitellata             Subphylum Echiura             Subphylum Pogonophora           Phylum Nemertina       Infrakingdom Chaetognathi         Phylum Chaetognatha       Infrakingdom Ecdysozoa         Superphylum Haemopoda           Phylum Arthropoda             Subphylum Cheliceromorpha               Infraphylum Pycnogonida               Infraphylum Chelicerata             Subphylum Trilobitomorpha †             Subphylum Mandibulata               Infraphylum Crustacea               Infraphylum Myriapoda               Infraphylum Insecta           Phylum Lobopoda             Subphylum Onychophora             Subphylum Tardigrada         Superphylum Nemathelminthes           Phylum Nemathelminthes             Subphylum Scalidorhyncha               Infraphylum Priapozoa               Infraphylum Kinorhyncha             Subphylum Nematoida               Infraphylum Nematoda               Infraphylum Nematomorpha       Infrakingdom Platyzoa         Phylum Acanthognatha           Subphylum Trochata             Infraphylum Rotifera             Infraphylum Acanthocephala           Subphylum Monokonta         Phylum Platyhelminthes           Subphylum Turbellaria             Infraphylum Mucorhabda             Infraphylum Rhabditophora           Subphylum Neodermata             Infraphylum Trematoda             Infraphylum Cercomeromorpha     Branch Deuterostomia       Infrakingdom Coelomopora         Phylum Hemichordata           Subphylum Pterobranchia           Subphylum Enteropneusta         Phylum Echinodermata           Subphylum Homalozoa           Subphylum Pelmatozoa             Infraphylum Blastozoa             Infraphylum Crinozoa           Subphylum Eleutherozoa             Infraphylum Asterozoa             Infraphylum Echinozoa       Infrakingdom Chordonia         Phylum Urochorda           Subphylum Tunicata             Infraphylum Ascidiae             Infraphylum Thaliae           Subphylum Appendicularia         Phylum Chordata           Subphylum Acraniata             Infraphylum Cephalochordata             Infraphylum Conodonta †           Subphylum Vertebrata             Infraphylum Agnatha             Infraphylum Gnathostomata   Subkingdom Mesozoa     Phylum Mesozoa |  Kingdom Plantae   Subkingdom Biliphyta     Infrakingdom Glaucophyta       Phylum Glaucophyta     Infrakingdom Rhodophyta       Phylum Rhodophyta         Subphylum Rhodellophytina         Subphylum Macrorhodophytina   Subkingdom Viridiplantae     Infrakingdom Chlorophyta       Phylum Chlorophyta         Subphylum Chlorophytina           Infraphylum Prasinophytae           Infraphylum Tetraphytae         Subphylum Phragmophytina           Infraphylum Charophytae           Infraphylum Rudophytae     Infrakingdom Cormophyta       Phylum Bryophyta         Subphylum Hepaticae         Subphylum Anthocerotae         Subphylum Musci           Infraphylum Sphagneae           Infraphylum Bryatae       Phylum Tracheophyta         Subphylum Pteridophytina           Infraphylum Psilophytae           Infraphylum Lycophytae           Infraphylum Sphenophytae           Infraphylum Filices         Subphylum Spermatophytina           Infraphylum Gymnospermae           Infraphylum Angiospermae |  Kingdom Chromista   Subkingdom Cryptista     Phylum Cryptophyta   Subkingdom Chromobiota     Infrakingdom Heterokonta       Superphylum Sagenista         Phylum Sagenista           Subphylum Bicoecia           Subphylum Labyrinthista         Phylum Ochrophyta           Subphylum Phaeista             Infraphylum Hypogyrista             Infraphylum Chrysista           Subphylum Diatomeae         Phylum Bigyra           Subphylum Bigyromonada           Subphylum Pseudofungi           Subphylum Opalinata     Infrakingdom Haptophyta       Phylum Haptophyta |