Carl Woeses Competing Three Kingdom Taxonomy

Although the accumulation of knowledge tends to create redundancies and contradictions in classifications, the five kingdoms taxonomy enjoys a position as the most up-to-date classification scheme based on morphology, cell biology, and genetics. Its main rival, the three kingdom scheme innovated and championed by biochemist Carl Woese at the University of Illinois, Urbana, is based almost entirely on genetic sequences of ribosomal RNA. Since genes may jump and change, and spontaneously mutate without necessarily causing changes in the proteins of the bodies of organisms, however, the three kingdom system is less well rounded in its informational base.

Moreover, while genetic studies will certainly alter future classificatory schemes, one of the main insights of Woese's three kingdom taxonomy (in which two of the kingdoms are bacteria) supports the five kingdoms scheme. That is the distinction between two kinds of bacteria, the archaebacteria and the eubacte-ria. (All nonbacterial organisms in the Woese system—that is, plants, animals, fungi, and protoctists—are lumped into the Eukarya.) This distinction supports the five kingdom notion of a common, symbiotic origin to bacteria and visible life forms. That is because gene sequences in RNA show similarities between archaebacteria—"old bacteria" in Greek, a group that includes prokaryotes inhabiting extreme environments such as salt flats and tolerating extreme conditions (similar to those thought to prevail on the early earth) of high heat and acidity—and the cells of modern plants and animals. This makes sense if we assume that the ancient Thermoplasma-like forms joined in an earthly partnership with the predecessors to mitochondria (and, later, the predecessors to chloroplasts and plastids) to make cells with nuclei. Indeed, in the three kingdom taxonomy, animals and archaebac-teria are about as related as archaebacteria and eubacteria. Such solely genetic-based classification, however, chafes against the naturalist's sense, based on morphology and paleo-biology as well as molecular biology, that plants and animals are more closely related to each other than either is to bacteria. Nonetheless, both the modern three kingdom and five kingdom systems of taxonomy are more reflective of evolution than the antiquated (but highly persistent) plant-animal dichotomy.

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