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5.4Development and Evolution of Animals

Now that molecular biology research on the mechanisms of development has progressed, and now that the shared molecular mechanisms that form the bodies of animals is understood, such knowledge is being used to explain the mechanisms of the evolution of animals. It is speculated that a mechanism similar to the molecular mechanism of animal development is also at work in the long-term evolution of the physical structures of animals.
Therefore, comparative studies have been conducted on the relationships between animal evolution and the expression patterns of homeotic genes in the developmental processes of various extant species of animals. The results have revealed a number of fascinating facts. For example, homeotic genes may have been involved in the evolution from fish fins to bird wings and human limbs (Fig. 5-5). Fin and wing buds are primordial precursors of the fins of extant fish and the wings of extant birds (and have the same structures as amphibian limbs). The expression patterns of homeotic genes that express these fin buds and wing buds were compared to the skeletal structures of the fins of ancient fish and the limbs of ancient amphibians. The results of this comparison revealed a correspondence relationship between the differences in homeotic gene expression patterns and the differences in the ancient skeletal structures. That is, homeotic gene expression patterns changed in animals during their evolution, and these changes may have caused major changes in the structures of their bodies. These structural changes were also suited to the organism's environment. The structures may have been passed on to their progeny, if the activities of these changed structures were advantageous to the animals. Such cases are known not only for homeotic genes but also for many genes involved in development. These facts suggest that many of the genes involved in development also play an important role in the evolution of animals.

Fig. 5-5. Evolution and the Expression Patterns of Homeotic Genes

The evolution of homeotic gene expression patterns is thought to have an important role in the evolution of fish fins into the four limbs of animals. The expression patterns of homeotic genes in primordial precursors of the fins of extant fish and the forelegs of extant chickens match the patterns of the main axial structures of the skeletons of fins and feet in fossils.

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