Thursday, April 9, 2009
Human eyes
On page 95 Carroll talks about spectral tuning of the human eye in the retina. Explain how spectral tuning works, and the different wavelenghts it applies to. Also explain why it was a selective advantage for humans not to be able to detect UV lighting, but for other animals a selective advantage.
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Spectral tuning is the result of the interaction between the opsin proteins and the chromophores. This interaction allows each of the 3 visual pigments (SWS, MWS, LWS) to be tuned to a specific wavelength of light. The SWS opsin detects blue light at 417 nm, the MWS opsin detects green light at 530 nm, and the LWS opsin detects red light at 560 nm. Another pigment, rhodopsin is tuned to 497 nm and is used to see in darkness. To be tuned to a certain wavelength of light means that all other wavelengths of light that strike the visual pigment are reflected away, and only light of the wavelength that the opsin is tuned to will be recognized.
ReplyDeleteOn page 123, Carroll states that if an opsin is no longer needed in an organism, natural selection will become relaxed. When natural selection is relaxed, there is no mechanism that prevents the opsin genes from changing due to mutations. The detection of UV light had no significant contribution to the fitness of the species because it didn’t play a role in human mating or feeding behaviors.
Having UV vision is a selective advantage for many animals because it helps many species in reproduction and foraging. UV vision in many animals, including the jumping spider (Cosmophasis umbratica) is needed for members of a particular species to find mates and reproduce successfully. First of all, UV light is often used in mating rituals. Jumping spiders are able to detect UV light on the bodies of other individuals, as both males and females have ultraviolet-reflective body parts. UV light is used to attract members of the opposite sex, as females can attract males by fluorescing in green light. One experiment showed that UV light is essential to the courtship and mating of jumping spiders. These spiders were placed in chambers that either contained UV light or did not contain UV light. Jumping spiders were able to mate in the chamber with the UV light (the control group) but were not able to mate in the chamber without the UV light (experimental group). This shows that UV light is essential for some organisms to reproduce and therefore pass on their genes to other members of the species.
ReplyDeleteIn addition, UV vision is also used by many species, including bees, to find food and resources. Bees have trichromatic vision just like humans, but they are also able to see UV light. These bees use UV vision to find pollen during the dark in flowers that have UV reflecting zones. Therefore, the UV vision of bees not only helps the foraging of bees, but also the reproductive success of plants: by having more animals distributing their pollen, flower-bearing angiosperms will have greater reproductive success. Also, rough-legged hawks (Buteo lagopus) use UV vision in order to catch more prey. Rough-legged hawks feed on voles, which is a type of rodent. The urine and feces of voles have chemicals that can absorb UV light. Therefore, these hawks can easily see the remains of the voles and can more easily tell where voles are located. As this allows more hawks to survive and reproduce, this reduces the relative fitness of voles.
Humans do not have UV vision because our ancestors have had no need for it. Even though many of our olfactory genes are fossilized and our senses of smell are relative poor compared to other animals, our trichromatic vision has allowed us to find food and resources well enough to survive and reproduce normally. If UV vision would have helped our ancestors survive, natural selection may have selected for individuals with UV vision; however, since no humans have UV vision, our ancestors must not have needed UV vision and were sufficient with trichromatic vision.
Sources:
cbst.ucdavis.edu/education/courses/.../bennettfinaldraft.doc
http://www.highbeam.com/doc/1P2-8867904.html
Spectral tuning is when a visual pigment in the human retina is matched with a certain wavelength of light by the action of visual pigments made of the protein, opsin, and the vitamin A derived- chromophore. Spectral tuning tunes three different visual pigments, SWS, MWS, and LWS, to wavelengths of 417nm (blue), 530nm (green), and 560nm(red), respectively.
ReplyDeleteWith the human adaptation of being active during the day and sleeping during the night (to rest and avoid the clutches of nocturnal predators), the opsin required to discern the lower wavelengths of UV light became unnecessary. As a result, humans do not have as many rods, photoreceptors packed with visual pigments that perceive dim light, as they do cones, which allow humans to perceive color that is very useful during the daytime. Also, the lens of a human eye protects from dangerous high-energy UV wavelengths in sunlight, thus assisting in the protection of the eye from forming cataracts and ultimately losing the ability to see, a major selective disadvantage. In certain cases of Aphakia, where the lens is surgically removed (due to cataracts and other formations), people have reported being able to see UV light.
Visual pigments needed to see UV light are advantageous for nocturnal animals because they all them to have night-vision, enabling them to capture food during the night.
http://www.guardian.co.uk/science/2002/may/30/medicalscience.research
As Ryan says, humans have three visual pigments which are sensitive to different wavelengths of light. Those three visual pigments are SWS, MWS, and LWS. They tune into specific wavelengths of light which gives the ability to see colors. SWS tunes into blue light, MWS tunes into green light, and the LWS tunes into red light. Each color has different wavelengths and our three visual pigments tune into specific wavelengths. This is called Spectral tuning. I would say that the process of Spectral tuning is well covered by Ryan, but I would like to elaborate more about why humans are not evolved to see UV light.
ReplyDeleteAs Ryan and Sean Carroll say, the detection nof UV light has no significant contribution to the fitness of humans. However, wouldn’t be better for us to see UV light because UV light is not always good for us. If we can see UV light, we will be able to avoid them and get less skin cancer or sunburn. To find why humans are not evolved to have UV light vision, we need to know why other animals use UV light.
UV light is used by many fish, amphibians, reptiles, birds, and some mammals. According to the article YongSheng Zhi, they sate that UV light is used for basic activities such as foraging, mate selection, and communication. According to their study, they say that the UV vision is associated strongly with UV-dependent behaviors of organisms and when UV light is not important to organisms, the SWS1 gene can become nonfunctional. They give an example of dolphins and birds that once had UV light vision but as time passed on UV light has become less important and they have lost this ability.
It is reasonable to say that we have evolved to not see UV light because we already have our color vision. We can forage, select mate and communicate just with the color vision. Evolution does not favor the overly complex organism. Evolution gets rid of something that we don’t need and they just modify what we already have to fit better in our capricious environment.
Source: http://www.pnas.org/content/100/14/8308.full.pdf+html
http://en.wikipedia.org/wiki/Ultraviolet#Human_health-related_effects_of_UV_radiation