Path dependence is the phenomenon whereby the evolutionary trajectories of complex systems are contingent on historical events. Path dependence occurs when evolutionary trajectories contain “critical junctions,” which are defined as events that commit lineages to one of multiple possible evolutionary pathways, thereby constraining the suite of possible outcomes. Path dependence is well established for the evolution of particular proteins in unicellular laboratory systems but has been very difficult to establish in any system outside of the laboratory. Indeed, demonstrating path dependence in natural systems has been challenging because it requires the identification of critical junctions and elucidation of the constraints that those junctions impose on future evolutionary paths.

In their new study titled "A morphological basis for path-dependent evolution of visual systems", post-doctoral fellow Dr. Rebecca Varney (University of California, Santa Barbara), her mentor Dr. Todd Oakley (University of California, Santa Barbara), and their collaborators Dr. Dan Speiser (our Department), Dr. Johanna Cannon (University of California, Santa Barbara),  Morris Aguilar (University of California, Santa Barbara) and Dr. Douglas Eernisse (California State University) took advantage of two lineages of chitons that have evolved two different visual systems, eye spots and shell eyes, to test the evolutionary role of path dependence. They found that the type of visual system that a chiton lineage can evolve is constrained by the number of openings for sensory nerves in its shell plates. Lineages with more nerve openings in their shell evolved eye spots, whereas those with fewer openings evolved shell eyes. Thus, the evolution of complex visual systems, which are often portrayed as deterministic, is path dependent, with events at specific critical junctions constraining lineages to one of a subset of possible pathways.