history of the scarabs. One study of 31 species of the genus Onthophagus revealed at least 20 reversals in the presence/absence of horn dimorphism (Emlen et al., 2005a).
Combined, these four trajectories account for most of the extant diversity in horn forms. But identifying these trajectories does a great deal more than describe taxonomic patterns; it also provides an essential first step toward elucidating the underlying mechanisms responsible for generating this diversity of animal forms. Only by identifying biologically meaningful trajectories of morphological change is it possible to begin to consider how these changes are generated and which developmental and physiological mechanisms are involved. In the following sections, we briefly describe three mechanisms now thought to be involved in the development of a