Tuesday, March 17, 2009
Color Vision
On page 95 it states that "Color vision begins when light of a particular wavelength strikes the visual pigments in our retina. These visual pigments are made up of a protein, called an opsin, and a small molecule called a chromophore." So what happens when a decrease in vision or blindness occurs? Does opsin and chromophores decrease? Have these proteins always been in eyes of all species, or have they evolved through evolution? Are they a natural selection? Does opsin and chromophore play a role in the shape of the retina?
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Opsins and chromophores are not the main aspect involved in vision. Although they are extremely important, they are what makes for colored vision and not vision in general. In humans, there are 3 different visual pigments; SWS, MWS and LWS, which are sensitive to short, medium and longer wavelengths of light. Carroll is not talking about visual in general here, he is specifically discussing colored vision. Blindness can be caused by a plethora of abnormalities such as the disease, optic nerve hypoplasia. The optic nerve is the nerve bundle that sends signals from the eye to the back of the brain, and optic nerve hypoplasia is a disease which affects the optic nerve which can lead to decreased visual accuracy. Likewise, people with diabetes can go blind because of something called glaucoma, which is part of a group of diseases of the optic nerve which involves loss of retinal ganglion cells. Although this does involve a loss of cells, it does not directly discuss the loss of opsins and chromophores. Likewise, proteins do not just disappear; they can be denatured. This most likely happens pretty often, but people have enough of these opsin proteins to be able to still function and see correctly. Blindness is generally due to other factors that don’t always relate to opsins and chromophores. Yes, colored vision is selected for, as Carroll discussed, in monkeys. Specific species of monkeys have evolved color vision in order to see their food and be able to pick the correctly colored food to obtain the correct amount of nutrients in their diet. Carroll specifically mentioned the red leaves that monkeys, without color vision, would not be able to see. The red leaves are higher in protein content and are overall more nutritious for them. Thus, the color vision has provided them with more possibility of survival and reproduction. On the other hand, there are species of fish that live in dark, cave-like parts of the ocean, who have no need for vision. Because it was not selected for, it was not maintained. Therefore, the gene mutated so many times that it has become a fossilized gene. The important thing to note is the “use it or lose it” belief; that if a species does not use a certain gene and maintain it in good condition, the gene, over time, will be completely lost. Likewise, if there is a drastic mutation in a gene that is extremely selected for, the specific organism(s) with that mutation will be exterminated by natural selection. If a monkey were to be born with a mutation that did not allow it to see in color, it would not get the correct nutrition and ultimately, not be able to pass on its genes to offspring, thus ending the mutation with him and not passing that specific trait on. Chromophores do not play a role in the shape of the retina because they are simply a region of a molecule responsible for its color and absorbing different wavelengths. Opsins, on the other hand, may play a role in the shape of a retina because they are a group of light-sensitive membrane-bound G protein-coupled receptors of the retinylidene protein family found in photoreceptor cells of the retina. Because they are membrane-bound, they may play a role in defining the shape. This is because the more of these there are, the bigger the retina membrane would have to be, thus showing that they do, in fact, have the possibility of playing a role in the retina’s shape. Likewise, this connects to the major theme of biology, structure and function. The structure of these proteins will allow the retina to obtain more or less light and ultimately decide how an organism sees. If these opsins are structured one way, they will function in allowing an organism to see in color. If they have a different structure, they will not allow an organism to see in color.
ReplyDeleteI would like to address Jackie’s question about how a decrease in vision or blindness occurs by analyzing color blindness in humans. As Carroll has stated, the human eye works by refracting rays of light through the lens and onto the retina, exciting the visual pigments located on our retinal cones, thus sending nervous signals to the brain which processes what is actually being seen. Humans have trichromatic vision, meaning that our eyes contain three classes of cones: long wave length sensitive, middle wave length sensitive, and short wavelength sensitive cones. These cones detect the three basic colors which are combined in varying intensities in the brain to form the images we see. Those three colors are red, green, and blue. This makes sense because red and blue are on the two ends of the visible color spectrum, as they have the longest and shortest wavelengths that can be picked up by our visual cones. Green would be the middle wave length sensitive cone as it is located somewhere in between. Color blindness occurs when one class of cones is not functioning. Most of the time, this is through a genetic error, meaning a mutation. This is not to say that one class of cones has gone missing all together, rather, the genes coding for a specific cone have been switched in such a way that the eye has two sets of similar sized cones and then a third, non-similar set (Ex: short set, short set, long set). The long and middle cone photopigments are encoded by genes that reside in a head-to-tail tandem array on the X chromosome. Two categories of mutations of these genes have been found to be associated with dichromacy. In one category of mutations, the gene(s) for a spectral class of pigment have been deleted or replaced with a functional gene for a different spectral class. In the other genetic category, a normal gene is replaced by a mutant one encoding a photopigment that does not function properly. It was similar genetic mutations that brought about the evolution of trichromatic vision in humans in the first place. Except this time it is easy to see that color blindness is not a selective advantage, and it would not be favorable to pass along that trait. However, since humans have developed many methods of addressing or even fixing color blindness, the genes are allowed to pass on from generation to generation, thus spreading a malevolent trait. One must stop to wonder: what other traits are arising from human intervention in evolution?
ReplyDelete1. Joseph Carroll, Maureen Neitz, Heidi Hofer, Jay Neitz, and David R. Williams. Functional photoreceptor loss revealed with adaptive optics: An alternate cause of color blindness. PNAS 2004 101: 8461-8466.
Vision of color is the reflection and absorption of certain wavelengths of light by the human eye. While white light is the mixture of all colors, black light is the complete absence of color. As the prompt stated, color vision occurs when different wavelengths of light hit the “visual pigments in our retina” (95).
ReplyDeleteIn humans, the retina contains cones and rods. There are three types of cones that each have a different pigment. One is best at absorbing short-wavelength, one medium and one long. Each can be activated to see color when light comes into the eye. Different colors are seen when each cone is activated to a certain extent. For example, when red light is seen, it activates the long wavelength cones more than any others, thus giving a vision of red.
Color blindness is most often a genetic disease. It is thought to be located on the x chromosome and is more often seen in males than females. It is caused by a decreased pigmentation in one or more of the three cones. Also, it can occur if one or more of the cones is completely absent.
It is a common misconception that color blindness if when a person only can see in black and white. This is very rare and most people who are “color blind” have a certain color that they cannot differentiate from others. For example, Protanomaly is a condition when a person is “red weak.” Therefore, they don’t see in black and white, but rather have a hard time seeing red. When looking at the color purple, it looks much more like blue because their retina has a hard time seeing the red and only sees the blue component.
In some situations, this is actually a selective advantage. In the army, many have noticed that color blind people can differentiate camouflage easier than those that can see color perfectly. Because they see colors differently, the camouflage color may not be as similar to the background as it appears to people with normal vision.
However, even though it was a selective advantage in that one particular case, color blindness causes hardships for many. Pilots, for example, cannot be color blind. Also, when driving, caution lights could be dangerous because color blind people can often not differentiate the colors. There are numerous other examples where being color blind is not a selective advantage. It has been shown to be a genetic “disease” that passes through the x chromosome, so many females are carriers and males are color blind more often.
http://www.toledo-bend.com/colorblind/aboutCB.asp
http://en.wikipedia.org/wiki/Colorblind
http://www.allaboutvision.com/conditions/colordeficiency.htm