Exactly how new traits emerge in evolution is a question that has long puzzled evolutionary biologists. While some adaptations develop to address a specific need, others (called “exaptations”) develop as a by-product of another feature with minor or no function, and may acquire more or greater uses later. Feathers, for example, did not originate for flight but may have helped insulate or waterproof dinosaurs before helping birds fly.
How common such pre-adaptive traits are in relation to adaptive traits is unclear. SFI External Professor Andreas Wagner and colleague Aditya Barve, both evolutionary biologists at the University of Zurich, decided to get a systematic handle on how traits originate by studying all the chemical reactions taking place in an organism’s metabolism.
Starting with the metabolism of an E. coli that can survive on glucose as its sole carbon source, they subjected the complex metabolic chemical process to a "random walk" through the set of all possible metabolisms, adding one reaction and deleting another from it with each step. They kept constant the total number of reactions and the bacterium’s ability to survive on glucose alone, but allowed everything else to change. Every few thousand steps they analyzed the altered metabolism’s reactions.
They found that most metabolisms were viable on about five other carbon sources – sugars, building blocks of DNA or RNA, or proteins – that are naturally common but chemically distinct compounds. To be certain that viability on these other carbon sources wasn’t a natural consequence of viability on glucose, they tested metabolisms starting with viability on 49 other carbon sources, and each time found that exaptations emerged allowing the metabolism to survive on any one of several other carbon sources alone.
Image caption: In this network diagram of E. coli metabolism combinations, the color of each node corresponds to the combination of carbon sources the network is viable on. There are 247 different phenotypes in this graph, that is, different combinations of carbon sources the networks are viable on. Networks viable on glucose alone are black. Two nodes are connected if they are both viable on the same carbon source, while the size of a node is representative of the number of other nodes it is connected to. The figure shows that the majority of networks (96 percent) are viable on many different carbon sources. The figures were generated using the Gephi software.
“We observed an incredible abundance of viability on carbon sources that these metabolisms were never even required to use,” Wagner says.
By varying the number of reactions in a metabolic system, the team also found a relationship between the system’s complexity (determined by number of reactions) and the extent of the exaptations, with larger networks having more of them.
The findings underscore the idea that traits we see now – even complex ones, like color vision – may have had neutral origins that sat latent for generations before spreading through populations, Wagner says.
"Our work shows that exaptations exceed adaptations several-fold," he says.
If exaptations are pervasive in evolution, he adds, it becomes difficult to distinguish adaptation from exaptation, and it could change the way evolutionary biologists think about selective advantage as the primary driver of natural selection.
Their work was published July 14 in the online edition of Nature.
Read their paper in Nature (June 14, 2013)
Read a Q&A with Andreas Wagner in The Scientist (June 14, 2013)