Evolution in the dark: unifying our understanding of eye loss

The evolution of eye loss in subterranean, deep sea, and nocturnal habitats has fascinated biologists since Darwin wrestled with it in On the Origin of Species. This phenomenon appears consistently throughout the animal kingdom, in groups as diverse as crustaceans, salamanders, gastropods, spiders, and the well-known Mexican cave fish, but the nature, extent, and evolutionary processes behind eye loss remain elusive. With the advantage of new imaging, molecular, and developmental tools, eye loss has once again become the subject of intense research focus. To advance our understanding of eye loss as a taxonomically widespread and repeated evolutionary trajectory, we organized a cross-disciplinary group of researchers working on the historic question, "how does eye loss evolve in the dark?." The resulting set of papers showcase new progress made in understanding eye loss from the diverse fields of molecular biology, phylogenetics, development, comparative anatomy, paleontology, ecology, and behavior in a wide range of study organisms and habitats. Through the integration of these approaches, methods, and results, common themes begin to emerge across the field. For the first time, we hope researchers can exploit this new synthesis to identify the broader challenges and key evolutionary questions surrounding eye evolution and so-called regressive evolution and collectively work to address them in future research.