Evolution of tetrapod visual opsins

Evolutionary history of the tetrapod visual pigments known as opsins; each triangle represents a particular visual pigment that facilitates color vision, with colors indicating peak spectral sensitivity. The ancestral vertebrate is thought to have had four opsins. Two were subsequently lost along the lineage leading to early mammals, perhaps because color vision was of limited value in these nocturnal organisms. In the lineage leading to Old World primates, a new opsin was gained because of a gene duplication of the long wave-length opsin. Adapted from Frentiu and Briscoe (2008).