Sunday, April 5, 2009
Vision and Hearing abilities
Caroll discussed how as full color vision developed many olfactory receptor genes were fossilized. So as sight became a better tool for organisms their sense of smell decreased.(128). How does this idea hold true for humans who have lost their ability of sight or hearing. Can a blind person hear better than an average person? If so, is it better in terms of decibels?Hertz? Can a deaf person see better than an average person? How can this idea be proved or disproved by the ideas of evolution and natural selection?
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http://www.sciencedaily.com/releases/2004/07/040723093712.htm
ReplyDeleteBlind people seem to hear better because they learn to. This article claims that their brains (especially those blinded early in life) essentially rewire themselves to put other sensory organs to better use. Their ears hear just as well as any of ours; their brains are just better at interpreting signals from the ears.
If a guy is blinded by an injury, a cataract, or staring at the sun, he may develop better hearing, but his blindness will not be passed down to his children. His kids will be born with normal vision and won't be under pressure to develop super ears.
However, there are genetic disorders that can cause blindness/deafness, such as Bardet-Biedl syndrome that are inheritable. I guess if given time, genetically blind people may evolve to eventually abandon their eyes and evolve better hearing/smelling. If selective pressure were in effect and killed off those with worse hearing, then the rest would be naturally selected to live and evolve better ears/noses/etc.
How would mutations affect this group of blind people? Possible mutations in the TRPC2 gene (p129) may "resurrect" the fossil gene for an individual: if this one survives and passes this trait down for better smelling, his offspring will also have better smelling and thus a higher chance at survival than those without.
http://news.bbc.co.uk/1/hi/health/966864.stm
ReplyDeleteThose who become blind do have better hearing than those who can see. The article above discusses the main finding that the occipital lobe of the brain is taken over by "information processing cells" which then 'rewires' the brain, as Ben put it, by using the lobe for language processing. Although this may seem odd, it is in fact not. The lobe has no "real" function if there is no sensory organ to report to it. To keep its use, the lobe is converted to an outgrowth of the parietal and temporal lobes in a sense - the places where speech and hearing develop (respectively).
"The finding adds weight to the theory that loss of sight can lead to better hearing as the brain tries to compensate for the blindness."
What the finding states is that an average blind person can analyze spoken language with more accuracy in finding mistakes and anomalies in half the time it takes an average person who does recognize the mistakes/anomalies. This is a great finding because it shows that blind people do develop a greater acuity in terms of hearing. (To understand more about the experiment, refer to the article at the top).
http://portal.acm.org/citation.cfm?id=1164015
It would seem to make sense that if relaxed selection upon sight causes an increase in hearing acuity, relaxed selection upon hearing would cause an increase in sight acuity. However, this has not truly been proven yet. Although some people who are deaf have reported to have greater sight, there are still contradictory results (refer to article above).
"On the other hand, there are indications that acoustic deprivation may produce an inferiority in all sensory modalities (Myklebust, 1964). These contradictions may be due to methodological differences: The investigators selected different conditions (e.g. attentive/nonattentive) and various samples of deaf subjects (e.g., different age, language, and aetiology groups)."
These ideas can be proved by natural selection with the concept of "relaxed selection." Relaxed selection is the same concept as selection upon fossil genes - there is not as much pressure upon the genes and they erode away. "This suggests that the evolution of trichromatic vision - which allows these primates to detect food, mates, and danger with visual cues - has reduced their reliance on the sense of smell. Relaxed selection on the olfactory receptor genes in trichromatic species has allowed the genes' codes to decay," (Carroll 128-129). Relaxed selection could eventually cause some genes within those who are blind to become fossilized; however, it would give the blind advantages in adaptation with a greater sense of hearing or smell or touch because those are just a few of the sensory pathways they have. Evolution and natural selection do not create out of necessity. Natural selection allows for the favorable traits to flourish which eventually is termed evolution. (Biological theme of evolution).