This process of capturing a photon and transforming it into a physiological response is known as phototransduction.
One opsin allows vision in just a short stretch of wavelength. This is equivalent to seeing in only one colour. Two opsins allow vision in two colours, and is usual in mammals. Four opsins allows vision in full colour, and is usual in teleost fish, reptiles and birds. In mammals, only Old World monkeys, apes and humans have trichromacy, full colour vision.
- "...two cone opsin gene families appear in contemporary eutherian mammals and, with the exception of some primates, none of these animals derive more than a single photopigment type from each of their two gene families".
References[change | change source]
- Plachetzki, D.; Fong, C.; Oakley T. 2010. "The evolution of phototransduction from an ancestral cyclic nucleotide gated pathway". Proceedings of the Royal Society / Biological sciences 277 (1690): 1963–1969. doi:10.1098/rspb.2009.1797. PMID 20219739.
- Shichida Y.; Matsuyama T. 2009. "Evolution of opsins and phototransduction".  Philosophical transactions of the Royal Society of London. Series B, Biological sciences. 364 (1531): 2881–2895. doi:10.1098/rstb.2009.0051. PMID 19720651
- Lamb T.D; Collin S.P. & Pugh E.N. Jr. 2007. Evolution of the vertebrate eye: opsins, photoreceptors, retina and eye cup. Nature Reviews Neuroscience. 8 (12): 960–976. 
- Kemp T.S. 2005. The origin and evolution of mammals. Oxford University Press.
- Bowmaker J.K. 1998. Evolution of colour vision in vertebrates. Eye 12 (3b): 541–547. 
- Jacobs G.H. 2009. Phil Trans Roy Soc B. 364 (1531) 2957-2967.