Consideration of the neural crest and its skeletal derivatives in the context of novelty/innovation.

I examine the neural crest and skeletal tissues derived from neural crest cells in the context of novelty/innovation by asking whether the neural crest is a novel tissue and whether the evolutionary origin of the neural crest required innovative developmental processes. As a vertebrate autapomorphy, the neural crest is a novel structure. I equate novelty with innovation and take a hierarchical approach. Some other workers separate the two, using novelty for new structures not found in an ancestor and not homologous with a feature in an ancestor, and innovation for the new processes required to generate the novel structure. While development clearly evolves, I do not separate those processes that result in the production of novel features from those that lead to change in existing structures, whether that change is a transition or transformation from one homologous feature to another (fins-->tetrapod limbs or locomotory appendages-->crustacean maxilliped feeding appendages). The existence of novelties causes us to consider the concept of latent homology. Neural crest cells form cartilage, dentine and bone. Cartilage is found in invertebrates and so is not a vertebrate innovation. No invertebrate cartilage mineralizes in vivo, although some can be induced to mineralize in vitro. Mineralization of cartilage in vivo is a vertebrate innovation. Dentine is a novel tissue that only forms from neural crest cells. Bone is a vertebrate innovation but not one exclusive to the neural crest. The developmental processes responsible for the neural crest and for these skeletal tissues did not arise de novo with the vertebrates. Novelty/innovation results from tinkering with existing processes, from the flexibility that arises from modifications of existing gene networks, and from the selective advantage provided by gene duplications or modifications. Copyright 2005 Wiley-Liss, Inc.