Integrating micro- and macroevolution of development through the study of horned beetles

A major challenge in evolutionary developmental biology is to understand how developmental evolution on the level of populations and closely related species relates to macroevolutionary transitions and the origin of evolutionary novelty. Here, I review the genetic, developmental, endocrine, and ecological basis of beetle horns, a morphological novelty that exhibits remarkable diversity both below and above the species level. Integrating from a variety of approaches three major insights emerge: the origin of beetle horns relied at least in part on the redeployment of already existing genetic, developmental and endocrine mechanisms. At the same time little to no phylogenetic distance appeared to have been necessary for the evolution of diverse modifier mechanisms that permit substantial modulation of trait expression at different time points during development in different species, sexes, alternative male morphs or even different tissue regions of the same individual. Lastly, at least a subset of these modifier mechanisms can evolve rapidly in geographically isolated populations, apparently driven by relatively simple, and probably ubiquitous, changes in ecological conditions. I discuss the implications of these results for our understanding of the genesis of morphological novelty and diversity.