Regulatory Evolution And The Origin Of Novelties

The recurring theme among the diverse examples of evolutionary novelties described in this chapter is the creative role played by evolutionary changes in gene regulation. The evolution of new regulatory linkages—between signaling pathways and target genes, transcriptional regulators and structural genes, and so on—has created new regulatory circuits that have shaped the development of myriad functionally important structures. These regulatory circuits also serve as the foundation of further diversification. The patterning of structures such as the tetrapod autopod and butterfly wing has diverged extensively throughout the radiation of the lineages in which these novelties first evolved.

The time scale over which new structures have evolved or body plans have diverged is considerable—of the order of many millions of years. One of the best-analyzed transitions, the evolution of tetrapod limbs from fish fins, is estimated to have transpired over the course of about 9 million years, for example. As we learn more about the architecture of the genetic regulatory differences between lineages, as well as the scope and time scale of anatomical evolution, we can better appreciate that changes in a potentially very large number of regulatory linkages accompany morphological evolution. Chapter 7 will examine the beginnings of that process—the origins of variations in morphological characters and the divergence of closely related species.

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