Tuesday, April 7, 2009
Opsins
On page 95, Carroll discusses opsins in humans. Humans have three opsins that are sensitive to different wavelengths of light. Birds and fish have four opsins and most mammals only have two. Why would birds and fish need more opsins compared to the three opsins humans have? What is the role of the opsin gene? Could natural selection have affected the numbers of opsins? If so, then how.
Subscribe to:
Post Comments (Atom)
Opsins absorb different wavelengths of light like you said, and different types of opsins are more or less suitable for different circumstances. For example, rhodopsin is receptive of dim wavelengths of light, therefore making it more suitable for a deep sea fish or a cave dwelling animal. Depending on the long term circumstances a species is exposed to, a species will either acquire new or adapted opsins through natural selection when necessary for survival, or lose genes to fossilization if certain opsins are no longer useful to the organism and selection relaxes. This explains the difference in opsins among species as closely related as New World and Old World primates and it also explains the similarities in opsins of species as different as homo sapiens and bony fish.
ReplyDeleteMany birds and insects have four opsins because it gives them a broad range of wavelengths that are visible. Somewhere along the lineage of certain birds, it became beneficial to have UV vision, so the the opsin that received light less than 400 nm was forcefully selected. The same pathway occurred in insects too, very unrelated animals. This brings me to my next point about the proof opsins have for independent convergence. Species might be extremely diverse, but given the same circumstances, or problem, they will develop the same solution, or evolutionary mutation. In a similar manner, if two unrelated species have a common trait that they don't use, it is likely they will both lose it to fossilization. Similar fossilizations should not be considered independent convergences because there never was a “problem” that the species solved, but they are a good demonstration of how similar environmental factors independently produce similar traits in organisms. For example cetaceans and coelacanths have both independently lost the functionality of their SWS opsin because neither species has much use for enhanced colored vision, so natural selection relaxed on the gene for this opsin, and when a mutation occurred it was not removed. To conclude, the amount or type of opsin gene relies on the circumstances of the species and whether certain opsins affect the survival of an organism. This is because only when survival is affected will selection quickly act.
Opsins are part of the visual pigments in an organism's eyes that allow it to see varying wavelengths of light. As proteins, their structure is defined by specific genes in the DNA. Mutations occurring over time allows the the proteins to be modified or new genes to be created for the synthesis of the opsin proteins. The three opsins that humans have allow us to see within certain wavelengths of light. Fish live underwater, and the light visible to humans is obscured as it passes through the water. The evolution of having four opsins would allow the fish to see more light and therefore be able to perform better competitively in an underwater environment. Fish also evolved to have four opsins in order to better fit their aerial environment. Flying well above the land, the adaptation to see a larger portion of the light spectrum would allow birds to see subtleties that humans, having two opsins, could not. This is beneficial since they need to search the land far beneath them in order to find suitable prety. Without the extra opsins, they would not compete as efficiently at finding a food source.
ReplyDeleteNatural Selection could have affected opsin number very directly and simply, in a survival of the fittest scenario. As birds and fish developed opsins, those with 4 lived to survive and reproduce while those with less died off. It was straightforward natural selection.
ReplyDeleteAn often overlooked opsins i called Melanopsin, one not responsible for seeing images, but for regulating body clocks and other non-visual responses to light. These opsins control light sensitive reflexive reactions in the animl body. Birds and fish, living a more "hardcore" (lacking the technology and abilities we humans possess) rely heavily on these clocks to live their lives and survive and reproduce, and therefore would require more of this opsin than humans. As a matter of fact, birds and fish actually possess melanopsin in many body parts which are light-sensitive, and not only the rare ganglion regions of retinal cells as in humans.
http://en.wikipedia.org/wiki/Melanopsin
Opsins are part of the visual pigments in an organism's eyes that allow it to see varying wavelengths of light. As proteins, their structure is defined by specific genes in the DNA. Mutations occurring over time allows the the proteins to be modified or new genes to be created for the synthesis of the opsin proteins. The three opsins that humans have allow us to see within certain wavelengths of light. Fish live underwater, and the light visible to humans is obscured as it passes through the water. The evolution of having four opsins would allow the fish to see more light and therefore be able to perform better competitively in an underwater environment. Fish also evolved to have four opsins in order to better fit their aerial environment. Flying well above the land, the adaptation to see a larger portion of the light spectrum would allow birds to see subtleties that humans, having two opsins, could not. This is beneficial since they need to search the land far beneath them in order to find suitable prety. Without the extra opsins, they would not compete as efficiently at finding a food source.
ReplyDeleteAs Caroll points out, the early mammals were believed to be nocturnals. In this dark setting, a third opsin did not provide a huge selective advantage. Because of this, mutations that caused the third opsin to become useless were did not disadvantage the mammal. Even if this may seem counterintuitive to our idea of "progress," natural selection did not prune mammals with these mutations and allowed the third opsin to become useless. However, since the gene was not completely fossilized, changes in the lifestyle of old world monkeys added selective pressure and caused the third opsin to resurface. Because of trichromatic vision, the monkeys were able to distinguish the younger, more nutritious leaves from the normal ones. The younger leaves had a red hue, so animals that were only dichromatic could not see this difference in color. As a descendent of these old world monkeys, humans have retained this third opsin. Birds, however, developed a fourth opsin because of their lifestyle. This opsin allows them to see ultraviolet light, which plays a significant role in mating and finding food. Because they are flying, they need more precise vision to find prey and survive. The need to survey the area from high altitudes, and only three opsins would not be good enough.