At the edge of darkness: A framework for the evolution of visual systems in dim light

Abstract Light is an important biological cue for all living things. For animals, where many behaviours are driven by sensory input from the eyes, light is also a critical environmental feature shaping animal eye evolution, with the intensity of light (i.e. brightness) imposing the largest constraints on visual function. Many studies have investigated the diversity of eyes evolved to function in different light environments, including the almost universal eye reduction and loss in animals living in the absence of light. Within the environmental light gradient from full sun to complete dark, dim light habitats are an interesting ecological niche containing animals with an excess of extreme eye forms ranging from extremely enlarged eyes to complete loss yet have not explicitly been considered in the context of the selective landscapes leading to this diversity in evolutionary outcomes. In this review, a previously published cost–benefit model for unifying understanding of eye loss in the dark is modified to specifically address the diversity of evolutionary outcomes—eye elaboration, functional reduction, or loss—observed in the eyes of animals living at the edge of darkness. Read the free Plain Language Summary for this article on the Journal blog.