The history of evolutionary biology is replete with operational definitions of novelty. For our purposes, which are primarily to understand the developmental and genetic basis of the evolution of novel animal forms and patterns, a "novelty" is defined as a structure or pattern element, or even an entire body plan, that has a new adaptive function. This chapter focuses on the best examples of morphological novelty for which developmental genetic knowledge has been elucidated. We do not address other forms of innovation, though they are fascinating in their own right, such as the evolution of physiological adaptations through protein evolution (for example, antifreeze proteins, lens crystallins, keratins, lactose synthesis, immune systems), because they do not concern morphological evolution per se.
Quantitative morphological variation, even when extreme, is not considered novel unless it encompasses a fundamental functional shift. Thus most homologous body parts are not considered novelties, even when the range of a vertebrate forelimb, for example, extends from the gigantic size of a whale flipper to the tiny arm of a tree shrew. Certainly, a gray area exists in terms of the degree of functional change considered sufficient to warrant classification as a novelty. Here, we limit our discussion to cases where a functional shift is accompanied by fundamental changes in development. These examples include the evolution of feathers, the origin of the insect wing and the spider spinneret, and the evolution of lepidopteran pigmented wing scales.
Other examples of novelty represent the evolution of a new structure, cell layer, or pattern element with no clear morphological antecedent. The cryptic origins of these innovations make them particularly interesting to evolutionary biologists. Several key innovations evolved in the chordate lineage, including the notochord, cranial placodes, the neural crest, and paired limbs. The wealth of developmental genetic knowledge about higher vertebrates and a few key outgroups, such as the cephalochordate amphioxus and urochordate ascid-ians, allows us to trace the evolution of some chordate novelties through comparisons of these taxa.
This chapter closes by analyzing radical transformations of body organization. Some ascid-ians have evolved direct development that eliminates the notochord, the defining structure of the chordate lineage. Snakes are "limbless tetrapods," having an extended axial skeleton associated with a novel mode of locomotion. Although the loss of ancestral characters typically is not considered to be a novelty, in these examples key morphological characters disappear in the context of a new ontogeny. Other examples of body plan changes include turtles, which have an armored shell resulting from rearrangements of tetrapod characters combined with redeployment of specific cell types, and cephalopods and echinoderms, which exhibit novel body plans that are characterized by new structural components as well as reorganization of ancestral features.
The origins of most novelties have long puzzled biologists. Observing an anatomically and developmentally complex structure such as a bird feather, a butterfly wing, or the vertebrate brain does not provide insight into the means by which that structure evolved. Nevertheless, taking a comparative approach—integrating the study of paleobiology, comparative embry ology, and developmental genetics—provides a window into the history of some important morphological novelties. The evolution of these novelties involves changes in the regulation of transcription factors, signaling molecules, and structural proteins.
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