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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 and the other the eukaryotes. 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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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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References
Text by Hamish Robertson
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