Identifying Coopted Networks and Causative Mutations in the Origin of Novel Complex Traits

One of the central goals of the field of evo-devo is to understand how novel complex traits originate. Novel complex traits are often old, and this makes understanding the genetic basis of their origin difficult. The traditional genetics approach for identifying the causative mutations for trait origin, of crossing species with and without the trait, is often impossible when the species are too distantly related. Alternatively, if the species are closely related, the genetic basis of their differences is often the recent loss, rather than the gain, of the trait in one of them, and mutations resulting in trait loss are not always equivalent to those that led to trait gain. Here, we reexamine an evo-devo study of the origin of melanic spots in the wings of flies, which is presented in more than one mainstream undergraduate textbook on Evolution, as an example of molecular evolution leading to the origin of a novel trait. We put forth an alternative to the previously proposed scenario and, in our view, a more likely evolutionary framework that explains the data, the CRE–DDC model, and then review other case studies and avenues of research that should help identify where new complex traits come from, as well as the actual causative mutations underlying their origin.