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Classification of Life

The classification of life on earth is changing the whole time as scientists discover new relationships between organisms using DNA sequencing. Margulis and Schwartz (1988) divided life into five kingdoms (Bacteria, Fungi, Protoctista, Animals, Plants). The most problematic kingdom in this classification is the Protoctista (also called protists) because the Fungi, Animals and Plants are all derived from groups within the Protoctista, thus making Protoctista paraphyletic. Similarly all eukaryotes (i.e. everything except Bacteria) probably evolved from a bacteria ancestor, hence rendering Bacteria paraphyletic.

Life here is divided into three main domains, two of them bacterial (prokaryotes) and the other the eukaryotes.Recent genetic studies are revealing that there is extensive horizontal gene transfer between prokaryotes and that hence a strictly hierarchical classifcation is probably not valid. Eukaryotes most probably evolved from Archaea but with the contribution of mitochondria from Eubacteria. It is also possible that the cytoplasm of eukaryotes originated from Eubacteria. Hence putting these major groups into a strict phylogenetic hierarchy is impossible. Viruses are added on as a fourth group, merely for convenience as they don't have a common early origin but are rather intracellular parasites derived from bits of genetic material from their hosts. 

Read more about The diversity and classification of living organisms.

Prokaryotes

Microscopic, typically unicellular organisms, surrounded by a plasma membrane but lacking a nuclear membrane. There are no membrane-bound organelles, such as mitochondria or plastids, but ribosomes are present. The essential genetic material occurs as a single, circular ribbon of DNA. Smaller circular DNA ribbons, known as plasmids, may also occur within the cell but these are not essential to cellular functioning.

 
 

Eubacteria (true bacteria)

Besides the normal bacteria we are familiar with, this group also includes the symbiotic mitochondria found within our cells and symbiotic chloroplasts found in the cells of green plants.

 
 

Archaea (Archaebacteria)

Most Archaea are found in very extreme environments such as round rift vents in the deep sea or in saline ponds at salt works.

 

Eukaryotes (protists, plants, fungi and animals)

Have a nuclear membrane.

Tomopterna delalandii (Cape sand frog)

 

Viruses

Not cellular unlike all the other groups above. Although they reproduce, they can only do so only by entering a host cell and making use of its machinery for replication. Viruses are bits of genetic material, apparently derived from their hosts. Thus, as Margulis & Schwartz (1988) put it, the polio and flu viruses are probably more closely related to people, and the tobacco mosaic virus (TMV) to tobacco, than polio and TMV are to each other.

 

Ranks

The aim of Biodiversity Explorer is to present a natural classification of life such that the classification of each organism reflects its evolutionary history. When one presents a natural classification, it is difficult to designate groups to specific ranks. For instance, in conventional classifications, amphibians, reptiles, birds and mammals are each designated as classes. However, in reflecting the natural classifcation of birds, we find that they are classified as reptiles as well as birds because they evolved from reptiles (they are in fact derived from dinosaurs). So birds can't belong to both the class Aves and the class Reptilia. It all becomes a lot simpler when one dispenses with ranks. In Biodiversity Explorer, ranks have been dispensed with at the higher levels of classification. For the plants, for instance, ranks are only employed from Order level downwards and this is because plant classification uses these ranks in a consistent way. For animals, ranks are usually only used from superfamily or family downwards.

References

Text by Hamish Robertson with contributions also from UCT Zoology staff

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