A note on classification

These pages do not follow the fundamentally flawed and now outdated "5-kingdom" classification still seen in many textbooks. A few (well, several) words of explanation may be useful.

Firstly, it should be said that, within reasonable scientific limits, no one classification can be said to be right, and others wrong. There is a need to balance the need for a workable (and inherently conservative) classification with the aim of reflecting phylogenetic relationships. The steady flow of new ultrastructural and molecular information necessitates fairly continuous review of the way we classify living organisms, and it should be understood that there is scope for differing, but equally valid, scientific opinions. The 7-kingdom classification used here does, however, accord with widely held modern views.

A historical perspective

Historically it was once believed that all organisms are either "plants" or "animals", and they were grouped into two kingdoms, the highest formal unit* of organism classification. The algal groups, the fungi and even the bacteria were all treated as plants and grouped together as the "Thallophyta" ("lower plants").

This historical view carries forward to the present day in the way that the rules of classification are managed. "Animals" are governed by the International Code of Zoological Nomenclature (ICZN) and "plants", in this early broad sense, are all governed by the International Code of Nomenclature for algae, fungi and plants (ICN (formerly the International Code of Botanical Nomenclature (ICBN)), except that bacteria (with the exception of the Cyanobacteria) have their separate code, the International Code of Nomenclature of Bacteriology (ICNB).

These are separate and largely independent codes, each formally ratified and updated by international congresses. Their independence is demonstrated by the fact that Pieris is a genus of butterflies (including the Cabbage White), but with equal validity is also a genus of flowering shrubs. Biologists who report their findings on Drosophila do not always pause to make clear that they are working on the genus of fruit-flies and not on the identically named genus of toadstools!

The present situation is, theoretically at least, unsatisfactory. The slime moulds (if not treated totally independently) are best regarded as belonging to the kingdom Protozoa. Yet traditionally they have been regarded as fungi, and fungi have been regarded as plants. They are specifically governed by the ICN whereas other protozoans come under the ICZN. There are many such anomalies, and while there are already provisions to allow for organisms claimed by both codes, harmonisation and eventual unification of the codes is seen as a long term aim.

One recent harmonisation is the adoption in the ICBN/ICN of the rank of 'phylum' as an alternative to 'division' (Tokyo Congress, 1994). Previously, 'phylum' was strictly a zoological term (though often misused), but it is now generally available and is used throughout these pages.

A further consequence of the tying of the codes to the old two-kingdom view is that they do not provide rules governing the names of kingdoms themselves. There is general acceptance that such names should be latinised and plural, but nothing to establish that any one such name is correct, or to prevent biologists making up and using their own names. Names of kingdoms used here are those most widely accepted, but you may see alternatives elsewhere. 'Fungi' is an authentically Latin, plural word, but some seem not to realise that, and so the kingdom Fungi, in particular, has acquired a number of synonyms.

* "domain" is used by some biologists as a new rank higher than 'kingdom', but it has no formal status, at least for eukaryotic organisms.

Yes, but what has happened to the Protista?

Of the many classifications that have been proposed, one of the most influential in recent times has been that by Whittaker in 1969. The major eukaryotic kingdoms Animalia, Plantae and Fungi were maintained, but, regardless of other relationships, unicellular eukaryotes were placed in the Protista (or better, with some allied multicellular organisms, in the Protoctista). All prokaryotic organisms were placed in the kingdom Monera. This was found to be convenient by many biologists, but was quickly rejected by others. Further molecular and biochemical evidence has since supported the view that the Protista are an unnatural and artificial entity that should not be recognised at any formal level. Even informal recognition could be held to be misleading.

A more acceptable modern treatment is to reincorporate these organisms into the existing eukaryotic kingdoms when this appears to reflect phylogenetic relationships, and to divide the residue between the Protozoa and the Chromista, the latter containing photosynthetic organisms (many of the non-green 'algal' groups) plus some apparently related 'fungi'. This is essentially the treatment of the protistan organisms by Cavalier-Smith (1998) and in recent editions of the Dictionary of Fungi (e.g. Kirk et al., pg. 748, in a confusingly separate section from the main text).

The major split of prokaryotic organisms into the Eubacteria ("true" bacteria) and the Archaebacteria is also adopted here, resulting in the seven kingdoms rather than the five of the Whittaker system.

The present approach is not the last word. An important group of fungi, the Oomycota (including potato blight and the water moulds), differ fundamentally from other fungi in a number of ways. They have been placed in the Protista, and here they are accepted in the Chromista. A growing view is that they (along with many other chromists) should be moved to yet another kingdom, the Straminipila (or "Stramenopila", an alternative spelling that is becoming common).

Kingdom concepts in disarray

Other kingdoms are being proposed as the flow of new molecular data continues, and a very radical treatment, based on new knowledge of phylogeny, is presented by Adl et al. (2005). It features a substantially changed major classification of living organisms at the highest level. The classification used in this website is certainly unnatural, and in a few years will be as outdated as the '5-kingdom' classification is now. Fortunately, most biological work itself remains unaffected!

Though using a different classification to that adopted here, an excellent book giving a very comprehensive overview of living organisms is that by Barnes (1998).

•   Adl, S.M., et al.* (2005). The new higher level classification of Eukaryotes with emphasis on the taxonomy of protists. Journal of Eukaryotic Microbiology 52: 399-451.
•   Barnes, R.S.K. (ed.) (1998). The diversity of living organisms, Blackwell Science, Oxford.
•   Cavalier-Smith, T. (1998). A revised six-kingdom system of life. Biological Reviews 73: 206-266.
•   Kirk, P.M., Canon, P.F., Minter, D.W., & Stalpers, J.A. (eds.) (2008). Ainsworth & Bisby's dictionary of the fungi, 10th ed., CAB International, Wallingford.
* 28 co-authors!


A.J. Silverside, January 2009, last updated May 2014
(first hosted at, September 1998) home page