Modern Basis for Classification

Homologous Structure

Carolus Linnaeus used structural features as the basis for his classification system. He grouped organisms according to their structural similarities. Today, taxonomists still use structural similarities as a basis for classification. They look for homologous structures just as Linneaus did.
Homologous structures are structures that show the same basic pattern, the same general relationship to other parts, and the same pattern of development.
However, they need not have the same function. For example, the human arm, the whale flipper, and the bat’s wing, all these appendages are homologous structures that show the same basic pattern. Also, all three appendages are found in the same part of the body. Finally, the bones in all three appendages develop in similar ways. Although their functions are different, they are homologous structures.

Similar Biochemistry

Biochemistry is the study of the chemical compounds formed by living things. Many biologists believe that closely related organisms form similar chemical compounds in their body. They use this belief to help classify organisms. For example, the horseshoe crab was, at one time, classified as a close relative of the true crab. However, chemical analysis showed that its blood was more like spider’s blood than crab’s blood. Thus, the horseshoe crab is now classified as a close relative of spiders.

Genetic Similarity

Most biologists agree that genetic similarity is the best evidence that organisms are closely related. Every organism makes a special compound called DNA that bears hereditary characters. Thus it seems reasonable to assume that the greater the similarity of DNA among organisms, the more closely they may be related.

Selecting a Classification System

Some biologists feel that two kingdom, Plantae and Animalia, are enough to classify all living things. Others prefer three kingdoms; still others use four, and some use five kingdoms.