Systematics, Taxonomy, and Classification
Alternative Methods of Classification
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.
The cladistic model is somewhat similar to previous models of organization, except for several notable differences. The one most widely reported is the cladistic location of birds on a cladogram in relation to the reptiles. A cladogram relates birds closer to crocodiles and dinosaurs than to snakes or lizards. Interestingly, it is now known that reptiles did not evolve from a common reptilian ancestor, but are more likely descended from several different ancestors, making the reptile classification a conglomeration of animals with similar characteristics but dissimilar backgrounds! The cladogram correctly shows the hereditary linkage between birds and certain reptiles. It clearly is different from classical taxonomic classifications, which place all reptiles in one category (Reptiles) and all birds in another (Aves).
Cladistic analysis is extremely objective: The organism either has the feature or not. This strength is also a weakness. Opponents charge that the technique does not account for the amount or degree by which a feature is present or is utilized. In their opinion, this omission ignores too much relevant data and fails to make an accurate assessment of the difference between groups. For example, the fact that penguins have wings but do not use them to fly would create a cladistic analysis problem.
A contrast to the no-bias approach of the cladistic analysis approach, the evolutionary systematic method deliberately builds in observer judgment. These taxonomists place heavier emphasis on the observed use or nonuse of a structure as well as the way it is used. Judgments are based on direct observation regarding the degree of evolutionary importance a particular feature is to that organism. For instance, in the previous bird and reptile analogy, the evolutionary systematic approach would lend more importance to the presence or absence of feathers than the derived homologous characteristics. So birds would be classified separate from reptiles. Most taxonomists agree that in the absence of data, the cladistic model is superior; with adequate data, the evolutionary systematic model has advantages.
Phenetics classifications are somewhat similar to evolutionary systematics in that both include all available data regarding the study of organisms and both are antagonistic to the cladistic analysis model. Phenetic classification does not attempt to establish evolutionary linkages but simply the clumping together of organism based on “overall” degrees of similarity. Phenetic classification requires access to the most data. Its overall effectiveness is diminished when the data are incomplete.
Excerpted from The Complete Idiot's Guide to Biology © 2004 by Glen E. Moulton, Ed.D.. All rights reserved including the right of reproduction in whole or in part in any form. Used by arrangement with Alpha Books, a member of Penguin Group (USA) Inc.