How many Kingdoms?

The concept of Kingdom, as well as most other taxonomic levels predate the realisation that living organisms have undergone evolution and that phylogenies can be traced through a combination of morphology, anatomy, ultrastructure and (more recently) molecular sequences. Rather than modifying our taxonomic systems, we have rather attempted to mould phylogenies so that they fit into the Kingdom-Division-Class-Order-Family-Genus-Species series. Therefore, the squeezing of organisms into these categories is somewhat arbitrary. For example, the evolutionary distance between divisions varies enormously from seed plants to red algae, showing that the concept of "division" is not used consistently in different groups.

The classification of organisms into Kingdoms carries the implicit assumption that once lineages diverge, genetic information does not cross back again from other lineages. We now know that there are a number of ways that genetic material has crossed lineages, the most obvious one being the endosymbiosis of chloroplasts and mitochondria. Therefore, the concept of Kingdom is an imperfect concept, and one that biologists are still trying to figure out how to deal with in the light of our current and steadily advancing knowledge.

It has long been recognized that the 2-Kingdom system was inadequate. In 1969, Robert H Whittaker proposed a five-kingdom system as an alternative to the 2-Kingdom System. The Archaebacteria and the Eubacteria are treated as subkingdoms under this system. Various authors have used either Protista or Protoctista for the kingdom that includes most eukaryotic algae. Under this system, photosynthetic organisms occur in three kingdoms: Monera, Protista, and Plantae. In most treatments that use this system, the Chlorophyta are included in the Protista and not the plant kingdom, something which is clearly not supported by the current evidence.

The Monera were eventually split into the Kingdoms: Eubacteria and Archaebacteria. A further Kingdom, the Chromista was proposed at the same time for the heterokontic organisms, although this was and remains controversial. The kingdom Archaezoa was proposed for three phyla (Archaemoebae, Metamonada, Microsporidia) which differ from all other eukaryotes in lacking mitochondria, peroxosomes, Golgi dictyosomes and cisternae, and probably also in having 70S rather than 80S ribosomes.

Modern studies show that even the eight kingdom system is not adequate. Some groups are paraphyletic, others are polyphyletic. Current evidence is pointing towards the idea that there is a single eukaryotic kingdom, if indeed the concept of "Kingdom" has any fundamental meaning at all in evolutionary phylogeny. However, there is no consensus in this regard, and you are still likely to see photosynthetic organisms treated according to any of the above systems, or without regard to their position within Kingdoms at all.