A morphological basis for path-dependent evolution of visual systems

Abstract

Path dependence influences macroevolutionary predictability by constraining potential outcomes after critical evolutionary junctions. Although it has been demonstrated in laboratory experiments, path dependence is difficult to demonstrate in natural systems because of a lack of independent replicates. Here, we show that two types of distributed visual systems recently evolved twice within chitons, demonstrating rapid and path-dependent evolution of a complex trait. 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 openings evolve visual systems with thousands of eyespots, whereas those with fewer openings evolve visual systems with hundreds of shell eyes. These macroevolutionary outcomes shaped by path dependence are both deterministic and stochastic because possibilities are restricted yet not entirely predictable.

Editor’s summary

Established morphological traits can direct trait evolution along particular trajectories in a process known as path dependence. Varney et al . explored this process in two lineages of chitons that have evolved two different visual systems, eye spots and shell eyes (see the Perspective by Sumner-Rooney). They found that lineages with more nerve openings in their shell evolved eye spots, whereas those with fewer openings evolved shell eyes. —Sacha Vignieri

Abstract

Multiple convergent origins of visual systems show that macroevolution of complex traits can be rapid and contingent upon preexisting traits.