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Chapter 15: 14-19

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Biological Classification Systems

WebLecture Topics


One of the primary tasks of biologists is the classification of individual organisms into groups which share similar characteristics: classification is the first response of the human mind in trying to understand any mass of observational data. The task and problems of classification predate evolution—they go back to the original records of the first systematic naturalists, Aristotle and Theophrastus.

Classification Systems

Modern classification systems are driven by two requirements: the need to account for similarity between organisms, and the need to account for diversity. The latter requirement means that classification schemes will affect evolution theories, since similarity between organisms is interpreted as a measure of how closely related the organisms are by common ancestry. Modern biological taxonomy involves dividing the largest groups or taxons, kingdoms, into subsequently finer taxa. The traditional divisions are (using humans as the examplar):

Kingdom Animalia: multicellular organisms which cannot make their own food
Phylum Chordata: animals with spinal nerve cords
Class Mammalia: chordates with hair, mammry glands, giving live birth to their young
Order Primates: mammals with forward-looking eyes and opposible thumbs
Family Hominidae: walking upright, aligned toes, short pelvis
Genus Homo: large brain, uses tools, lives in groups
Species Sapiens: larger brain, more complex social structures

Approaches to Classifying Organisms

Three approaches are used to identify a given organism as a member of a group at an taxonomic level.

The different methods have resulted in many controversies over whether a particular species belongs in one genus or family or another. Hence there is no universally agreed-on classification system. The most commonly used system, the five-kingdom Whittaker system, although it is not very old is already under attack by biologists who want to make divisions based on differences at the cellular level. Note the three systems proposed below, which do not agree even at the kingdom level, breaking the top taxon into three, two, or five groups:

1 Eubacteria Archaebacteria Eukaryotes
2 Prokaryotes Eukaryotes
3 Monera Protista • Fungi • Plantae • Animalia

Ultimately, the three-part division won, as a new level, the domain, allowing biologists to retain prokaryotes under two groups (Archaea and Bacteria), and the four remaining kingdoms under Eukaryotes.

All classification systems are ultimately constructions of the human mind, reflecting criteria that humans think is important at some point in time, for the convenience of humans in organizing and communicating their thoughts. If one system doesn't work, another will be substituted for it.

Trying to organize every living thing into a particular slot is a daunting task, but one well-suited to the peculiar abilities of the Web. Take a look at the Tree of Life, the current Web-based attempt to create the ultimate phylogenetic tree!