Ways of Classifying organisms
The Linnaean binomial system of classifying animals brought organization from chaos; but recently, with the application of modern technology, new methods have surfaced that yield additional information. Methods of establishing ancestral kinship are helpful in establishing new taxonomic procedures that often relate species in new ways. Although no one method is without drawback, each offers unique insights and information in reference to the organisms in question.
Cladistic analysis is probably the most widely used alternative method. Cladistic analysis is a means to classify organisms to match their evolutionary history. Common phylogenetic features are used to establish relatedness between organisms with the help of sophisticated computer programs that quickly sort organisms according to shared evolutionary structures.
Cladistic analysis sorts homologous structures into either a primitive character or a derived character. Primitive characters establish the broad classification that generates the basic grouping of organisms. For instance, a cladistic primitive character for plants is the presence of chloroplasts. Those organisms that contain chloroplasts are clumped into the same large grouping.
Derived characters are also homologous structures, but they represent features that have been modified for specific functions. Derived characters are more unique than primitive characters and tend to sort organisms by their presence or absence in the organism. The presence of a derived character or set of derived characters establishes a greater degree of relatedness. The more derived characteristics organisms share, the greater their degree of kinship. For instance, a derived characteristic in plants is the presence of vascular tissue. Advanced plants contain vascular bundles, but simple aquatic plants do not. This relatively simple anatomical feature demonstrates the vast difference between vascular and nonvascular plants. Review the example that follows to distinguish primitive and derived characters in mammals.
Mammalian primitive characters:
- Mammary glands and hair (for example, humans, dogs, cows, whales)
- Appendages modified for aquatic movement (for example, whales)
- Appendages with an opposable thumb (for example, humans)
- Appendages designed for running (for example, dogs)
- Appendages designed for grazing on uneven ground and carrying heavy body weight (for example, cows)
After the primitive and derived characters are known, a cladogram can be constructed to show evolutionary linkages between groups of animals. Examine the illustration Simple cladogram.
This cladogram shows the evolutionary relatedness of major plant types by using simple derived characters on the right side in ascending order. Organisms located next to each other horizontally across the top are more related than those not in close proximity. For more specific classifications, such as the kinship between an oak tree and an elm tree, the cladogram would need more specific derived characters.