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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.
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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.
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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.
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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. |
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Eukaryotes (protists, plants, fungi
and animals)
Have a nuclear membrane.
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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.
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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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