Sorting organisms into five kingdoms.

• R.H. Whittaker (1969) proposed the Five Kingdom Classification: Monera, Protista, Fungi, Plantae and Animalia.
• He sorted organisms on five criteria: how the cell is built, how the body is organised, how the organism gets food, how it reproduces, and its phylogenetic (evolutionary) relationships.
• Older schemes put bacteria, blue-green algae, fungi, mosses, ferns, gymnosperms and angiosperms all under 'Plants', because all of them have a cell wall. That one shared feature lumped together groups that differ widely in everything else.
• The old 'Plants' mixed prokaryotic bacteria and cyanobacteria with eukaryotes, mixed unicellular Chlamydomonas with multicellular Spirogyra under algae, and mixed heterotrophic fungi (walls of chitin) with autotrophic green plants (walls of cellulose).
• Whittaker gave fungi their own kingdom, collected every prokaryote into Monera, and placed unicellular eukaryotes in Protista.
• Protista now holds Chlamydomonas and Chlorella, which have walls and were once algae among plants, together with Paramoecium and Amoeba, which lack walls and were once animals. The members moved because the criteria changed.
• Table 2.1 in brief. Monera: prokaryotic, non-cellulosic wall (polysaccharide + amino acid), no nuclear membrane, cellular, autotrophic (chemosynthetic or photosynthetic) or heterotrophic (saprophytic or parasitic). Protista: eukaryotic, wall in some, cellular, autotrophic (photosynthetic) or heterotrophic.
• Table 2.1 continued. Fungi: eukaryotic, wall with chitin, multicellular or loose tissue, heterotrophic (saprophytic or parasitic). Plantae: eukaryotic, cellulose wall, tissue or organ level, autotrophic (photosynthetic). Animalia: eukaryotic, no wall, tissue, organ or organ system level, heterotrophic (holozoic, saprophytic and so on).
• A three-domain system has also been proposed. It splits Monera into two domains and puts all eukaryotic kingdoms in the third, which gives a six kingdom classification.
• Classification keeps being revised as our knowledge of characters and evolutionary relationships improves; the aim is a phylogenetic system, based on evolutionary relationships, not only on form, physiology and reproduction.

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