The real ‘kingdoms’ of eukaryotes

The diversity of life has generally been divided into a few — four to six — fundamental ‘kingdoms’. The most influential system, the ‘Whittaker’ five kingdom structure, recognises Monera (prokaryotes) and four eukaryotic kingdoms: Animalia (Metazoa), Plantae, Fungi and Protista. Whittaker’s system, somewhat modified, was presented as more realistic than the traditional division of life into animals and plants. Fungi, as well as prokaryotes, were separated from the plants, to which they are not related, while various protists were also released from artificial ‘plant-like’ or ‘animal-like’ designations. Yet, the improvement is limited. In five kingdom systems, Protista is a grab-bag for all eukaryotes that are not animals, plants or fungi. This grab-bag is sometimes considered as an interim convenience or, very dubiously, to recognise a distinct ‘grade’ of organisation. In any case, most systematists and biology textbooks now hold that only monophyletic groups — an ancestor and all of its descendents — should be formally classified as higher taxa, overriding notions of convenience or grades. Nevertheless, the five kingdom system is still presented in some form in most biology textbooks. The long popularity of the five kingdom system may be due more to the pedagogical and rhetorical utility of simply dividing life into a few groups than any biological realism. Summarising all organisms in five points on a slide or five chapters in a textbook is of great appeal in traditional, eukaryotecentric, biology (remember, four of these kingdoms are eukaryotic). With our historically poor knowledge of the deep-level relationships amongst eukaryotes, biologically realistic alternatives would have involved literally dozens of eukaryotic ‘kingdoms’ — unpalatable for many communication purposes. The last few years have seen dramatic improvements in our understanding of deep-level evolutionary relationships, thanks to advances in molecular phylogenetics, particularly in analytical methods, and in the diversity of organisms for which data are available, as well as a maturing of knowledge about some key features of eukaryotic cells. Eukaryotes at least can now be divided into just a few major groups that are probably all monophyletic. For the first time systems that reflect the actual relationships amongst eukaryotes can also be useful for communication. The following sections introduce each of these major groups.