Morphological novelties abound in the history of animal evolution and have been essential to the diversification of animal forms. The scope of novelties encompasses many familiar structures, from antlers to ear bones, from whale tails to panda thumbs. New structures and pattern elements are often key features that distinguish animal groups. For example, the vertebrates are characterized by an elaborate forebrain, the neural crest, and cartilage—all novelties that were critical to the success of the vertebrate lineage and that allowed the evolution of further innovations such as teeth and jaws.

Structural novelties (or "key innovations") are associated with adaptive radiations into new ecological territories. The movement of vertebrates onto land is tied to the evolution of the tetrapod limb; the escape of insects into the air required the evolution of wings. The distinctive molar tooth shape has evolved independently several times in association with herbivory. Other novelties such as feathers and butterfly scales permit the display of colors and color patterns used for communication or predator avoidance.

New structures require the evolution of new developmental programs. To understand the origin of morphological novelties, we must look again to the genetic control of development. Have new developmental regulatory genes evolved to sculpt novel body parts? Or are "old" genes recruited for new patterning roles? And, if regulatory genes are reused, are they co-opted individually or as part of larger, preexisting circuits?

This chapter uses case studies to examine various ways in which new structures and pattern elements evolved through changes in developmental gene regulation. In several instances, shared aspects of development and regulatory gene expression reflect the evolution of novelties from preexisting ancestral structures. Such novelties evolved through downstream regulatory changes in developmental programs. In other cases, "old" developmental regulatory genes evolved a new role in the formation

. . . on these expanded membranes nature writes, as on a tablet, the story of the modifications of species. —Henry Walter Bates The Naturalist on the River Amazons (1863)

Novelties come from previously unseen association of old material. To create is to recombine.

of novel pattern elements. In the best-documented examples, groups of genes or entire genetic circuits were recruited to carry out a new developmental function.

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