A morphological basis for path dependent evolution of visual systems

Path dependence influences macroevolutionary predictability by constraining potential outcomes after stochastic evolutionary events. Although demonstrated in laboratory experiments, the basis of path dependence is difficult to demonstrate in natural systems because of a lack of independent replicates. Here we show two types of complex distributed visual systems each recently evolved twice within chiton mollusks, demonstrating rapid and path dependent evolution. The type of visual system a chiton lineage evolves is constrained by the number of openings for optic nerves in its shell plates: lineages with more openings evolve visual systems with thousands of eyespots, whereas those with fewer 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. One-Sentence Summary Multiple convergent origins of visual systems show macroevolution of complex traits can be rapid and contingent upon pre-existing structures