Biology
Chapter 15: 14-19
Lecture
Biological Classification Systems
WebLecture Topics
Classification
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.
- Phenetics or numerical taxonomists assign values to arbitrarily chosen traits, then rate an organism on whether the trait is present or absent. Organisms are closely related if they share many traits in common. Pheneticists don't care whether the traits are homologous or analogous structures. They avoid using any evolutionary evidence in making their classification decisions, since no evolutionary reconstruction can be certain. A pheneticist would say porpoises are more closely related to humans than to fish, since they share more characteristics (mostly mammalian) with humans.
- Cladists use assumptions of common ancestry to arrange groupings and determine common ancestors. A cladist would assume porpoises and humans descend from the same near ancestor because they have more common characteristics than porpoises and fish share. Ultimately, though, porpoises, humans, and fish share a common ancestor further back up the phylogenetic tree. Cladists try to construct the phylogenetic tree (or genealogical tree) based on commonly shared homologous characteristics, so it is important for them to distinguish between structures which which are physically similar and structures which are functionally similar.
- Classical evolutionists use both common features and the amount of divergence to stress unique attributes of certain groups. Common evolutionary ancestry, though necessary, is not sufficient for an organism to be included in a particular taxonomic group. Classical evolutionists classify birds and crocodiles, though they appear to share a reptilian ancestor, because birds now differ so much from reptiles in many structures.
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:
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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!
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