The origin and subsequent evolutionary diversification of complex morphological structures have long puzzled biologists (Darwin, 1859b; Goldschmidt, 1940; Mayr, 1960). These traits may arise suddenly, and their size and complexity, as well as their lack of visible homology with existing structures (novelty), were thought for many years to be incompatible with traditional gradualistic views of genetic variation, selection, and evolution (Müller and Wagner, 1991). Modern analytical techniques, including phylogenetic analyses and molecular genetics, have greatly improved our understanding of how complex structures arise, revealing in several instances how subtle perturbations to existing developmental mechanisms can generate substantial and unprecedented changes in animal form (Nitecki, 1990; Müller and Wagner, 1991). These studies demonstrate unequivocally that both novelty and complexity can arise from simple changes to development, and they illuminate how an understanding of development can inform studies of character evolution. We illustrate this approach using the example of beetle horns, skeletal outgrowths that function as weapons in intraspecific combat.
Beetles with horns include some of the most magnificent and bizarre organisms alive today. The sizes of these horns relative to the sizes of the beetles that bear them can dwarf even the most extreme antlers of ungulates, and the diversity of horn forms is breathtaking. Darwin used beetle horns when he first described sexual selection (Darwin, 1871) and Teissier and Huxley used beetle horns when they first described the concept of relative growth and allometry (Huxley, 1932; Teissier, 1935).
How did the first beetle horns arise? And once present, how were these structures modified so dramatically in form? In this chapter, we approach these questions from two vantages, comparative phylogenetic studies of horn evolution and developmental studies of the regulation of horn growth, and show that these disparate biological perspectives converge on the same basic conclusions regarding horn evolution: beetle horns do not appear to be difficult structures to gain or lose, and they appear capable of rapid and radical changes in form. We end this chapter with a conceptual model for how beetle horns evolve. Specifically, we identify three developmental mechanisms that are now thought to underlie the principle trajectories of beetle horn evolution. This integration of perspectives comprises an important step in our attempts to elucidate the myriad ways in which these exaggerated structures have radiated in form. It also illustrates the more general theme of this colloquium: that “Darwinian” processes of selection, combined with subtle genetic variations in basic developmental processes, can account for the origin, and the subsequent diversification of even the most extreme animal structures.