In the course of evolution organisms sometimes acquire completely new and sometimes dramatic features, like horns or new appendages. The evolutionary origins of new structures are much more difficult to study than modifications to existing ones. One approach, however, is to study the development of newly arisen structures in as many different species as possible. The genome does not code for a body plan directly; rather, it encodes genes that coordinate the process of development. Development is a series of events that pattern a fertilized egg into a multicellular organism. The timing and spatial organization of gene function in an embryo is therefore central to creating body structure. Again and again it has been seen that new structures don’t necessarily mean new genes; the development of many new structures is controlled by previously existing genes that have been deployed in new contexts. The use of existing genes for a new purpose is called co-option.
A recent study by Moczek et al. provides fresh detail on the development of new structures in a particularly interesting group of animals. Many species of beetle possess rather gaudy horns on their heads and thoraxes. Horns are not modified mouthparts or limbs; they exist in addition to a full set of these other structures. Certain limb genes, though, are turned on in the horns. These genes, distal-less, dachshund, and homothorax, play central roles in the limb development of other insect species. When the authors disrupted the function of these genes in beetle larvae, the animals grew abnormally short horns and limbs. Their experiment indicates their dual functions in beetles: an ancestral function for making legs, and the more recently evolved functions in making horns. You therefore might think of these genes simply as tools for making something that sticks out, be it a limb or horn.
Genetic co-option is not limited to beetles, but by studying creatures like these we can develop a more general picture of how body structure evolves. These studies have made it clear that, just as your brain doesn’t have a neuron that is specific to your grandma, there aren’t new genes that are specific to each new structure.
The photo above, by Alex Wild, is of two Onthophagus taurus males.