Evolution is littered with examples of opportunism. Hosts infected by viruses found new uses for the genetic material the agents of disease left behind; metabolic enzymes somehow came to refract light rays through the eye’s lens; mammals took advantage of the sutures between the skull bones to help their young pass through the birth canal; and, in the signature example, feathers appeared in fossils before the ancestors of modern birds took to the skies.
In cases like these, evolution has made do by co-opting an existing trait for a new use when the right circumstances arose. These instances offer the lesson that a trait’s current use does not always explain its origin.
In 1982, Stephen Jay Gould and Elisabeth Vrba gave a name to this phenomenon: exaptation. As they described it, exaptation is a counterpart to the more familiar concept of adaptation. While exaptations are traits that have been enlisted for new uses, adaptations have been shaped by natural selection for their current function, they wrote.
The order and arrangement of the bones in the four limbs of land-dwelling animals are an exaptation for walking on land, since these limbs originally evolved for navigating water; by contrast, changes to the shape of the bones and to the musculature are adaptations, Gould and Vrba wrote.
The concept has been controversial since it first arose, largely because it has been so difficult to distinguish between the forces of exaptation and adaptation in the historical context of evolution. Until recently, evidence for the co-opting of traits has been limited to case studies, such as the evolution of the feather. But examples from the morphological, behavioral and, increasingly, molecular realms have led some biologists to suspect that this phenomenon may play a much more sizable role in evolution than is generally appreciated.
A new study in Nature offers what may be the first attempt to comprehensively identify potential exaptations. The results of the study, which focused on metabolism, complement anecdotal examples and take an initial step toward quantifying exaptation’s contribution, at least within this system, said researchers not involved in the work.
Scientists used computational modeling to create randomized metabolic systems tuned to use one kind of fuel, which, they showed, often have the latent potential to use other fuels they have never before consumed. Thus, a hypothetical organism deprived of its usual food source could manage just fine on a second, completely new fuel. In this scenario, that capacity to switch fuels gives rise to an exaptation.
“I think it’s becoming increasingly clear that exaptation is very important in the evolution of biologically important processes,” said Joe Thornton, a molecular evolutionary biologist at the University of Chicago and the University of Oregon, who was not involved in the study. “There is now a growing body of evidence indicating the actual importance of these processes that Gould and Vrba were pointing to.”
Seeking Hidden Potential
Identifying an exaptation requires a look back at history, which is not easy to do with most biological traits. Andreas Wagner and Aditya Barve, of the University of Zurich, sidestepped this problem by simulating evolution and testing the results. They focused on metabolism, using a computational representation of the networks of reactions organisms use to break down food and produce the molecules necessary for survival and growth.
They wanted to know: If a network was adapted to use a particular carbon source, such as glucose, could it also use other carbon sources, such as adenosine or acetate?