Understanding life's diversity requires multiple perspectives and approaches. In this dissertation I analyze diversity in vertebrate appendages from an evolutionary developmental perspective and explore the question: how do new body parts evolve? My aim is to contribute to an emerging understanding of how complexity evolves in living systems. In Chapter 1, I introduce origin of new body parts as `the novelty problem' and present vertebrate appendages as a system in which conceptual progress might be made. In Chapter 2, I test hypotheses of the evolutionary origin of adipose fins, appendages found on teleost fishes posterior to the dorsal fin, and I conclude that adipose fins are homoplastic, having evolved repeatedly. In the light of these results, I explore adipose fin diversity to test for generalities in how form and function evolve in new vertebrate appendages. To do this, I characterize skeletal evolution in adipose fins and compare these data to what has been documented in other fin systems (Chapter 2), and I describe a musculoskeletal linkage system in an adipose fin and discuss how active control evolves in new vertebrate appendages (Chapter 3). These studies explore the causes of homoplasy and the predictability of evolutionary trajectory. Chapters 4 and 5 focus on the relations between serial homology, transformation, and the novelty concept. I describe the development of lepidotrichia in an adipose fin, an example of translocation, and explore how developmental context might affect the morphology of serial homologs (Chapter 4). I also describe adipose fin development to understand how these appendages might have originated (Chapter 5). I conclude in Chapter 6 by proposing avenues of future research in adipose fins and arguing that concepts of intra organismal homoplasy and morphological novelty confront a common challenge--the application of evolutionary concepts, which are rooted in individuality, at a level of biological organization where parts can only be quasi-individualized.
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