Pathways to Function via the Emergence of a Mechanical Switch in Evolvable Matter

The underlying principles of how sharp switches occur in rugged fitness landscapes, while integral for understanding evolution of function and adaptation in biological systems, remain elusive. Here I use elastic mechanical networks as a platform for probing the physical principles governing single-mutation transitions between two highly-fit, incompatible functions. The function used is an allosteric coupling of two pairs of source and target nodes that respond to an input strain in-phase or out-phase with each other. I study the complete fitness landscapes for ensembles of networks, and find that high-fitness pathways between these functions nearly always exist. At the largest fitness threshold for viable evolution, the functional transitions occur via a "jumper" mutation: a single bond addition or deletion that connects distinct fitness peaks with opposite functions. These mutations can be viewed as a mechanical switch, which I find can switch between incompatible functions with minimal perturbation to the system. In some cases, the mere presence of a bond, regardless of stiffness, constrains the deformation mode and determines function. However, bond formation or breaking is not always necessary: subtle geometric deformations that conserve connectivity can be sufficient to induce sharp functional transitions. The study of this physical system suggests that the single mutation function switches often found in biological systems may be fundamentally mechanical in origin.