Five Kingdom Classification

R.H. Whittaker, an American taxonomist, proposed the Five Kingdom classification in 1969, sorting all life into Monera, Protista, Fungi, Plantae and Animalia, using a combined set of criteria: cell structure (prokaryotic vs eukaryotic), thallus organisation, mode of nutrition, reproduction, and phylogenetic relationships. Its merits are that it is grounded in the real complexity of cell structure and thallus organisation, it factors in mode of nutrition, it correctly separates Fungi away from Plantae, and it reflects the phylogeny (evolutionary relatedness) of the groups. Its demerits are that Kingdom Monera and Kingdom Protista both end up as very heterogeneous grab-bags - each one still mixing together autotrophic and heterotrophic organisms, and organisms with and without a cell wall - and, crucially, viruses are left out of the system entirely. The scheme did not stand still after 1969: Carl Woese and co-workers (1990) proposed three Domains of life - Bacteria, Archaea and Eukarya - based on differences in rRNA sequence and cell-membrane lipid structure; Thomas Cavalier-Smith (1998) proposed a revised Six Kingdom scheme that split Monera into Archaebacteria and Eubacteria; and Ruggiero et al. (2015) extended this further into a practical Seven Kingdom scheme with two SuperKingdoms - Prokaryota (Archaebacteria and Eubacteria) and Eukaryota (Protozoa, Chromista, Fungi, Plantae and Animalia) …