Welcome to the discussion group for Why Evolution Is True By Jerry Coyne. Prompts and posts are student generated. This is a collective effort to engage in discussions that connect the theory of evolution with the biological concepts and themes discussed in our course throughout the year.
Tuesday, April 13, 2010
Refuting "Bad Design"
What are some examples many scientists believe are evident of bad design that you disagree with, or that other scientists find to actually be beneficial to the organism? For example, the structure of the Giant Panda's thumb, once thought to be a bad design, has been studied further and now a new study shows that the thumb is “one of the most extraordinary manipulation systems” among mammals. Experiments show that the thumb and the accessory carpal bone “form a double pincer-like apparatus enabling the panda to manipulate objects with great dexterity” (http://www.godandscience.org/evolution/designgonebad.html). Many scientists argue that the thumb of the giant panda is bad because it doesn't function like a primates thumb. However, others argue that this doesn't necessarily mean that the structure is bad design, since the structure allows the Panda to take hold of bamboo shoots just fine. Make sure to relate this theme to structure and function.
For a while one popular argument for bad design was the inverted retina of vertebrates. With an inverted retina, photoreceptors face the inside of the eye as opposed to the more obvious, expected design facing outward (see diagram: http://www.answersingenesis.org/Home/Area/Magazines/tj/images/v13n1retina1.gif). This creates the supposed "backwards wiring" of our eye.
Recently, research has shown that the seemingly twisted structure of the retina makes its function much more efficient and of high quality in the following ways:
First of all, the tips of photoreceptors are in contact with the cells of the RPE, or retinal pigment epithelium. This has three important implications.
The RPE regenerates photorecpetive pigments. When light hits rhodopsin at the inner end of a photoreceptor, the cascade of reactions that eventually forms an image that we see begins. This happens through the isomerization of 11-cis-retinal, a part of rhodopsin. The RPE uses vitamin A to regenerate used retinal after light hits it. Similarly, the RPE recycles old particles of the membrane which may otherwise decrease photoreceptor sensitivity and cause blockage. Lastly, because it contains many pigments, the RPE absorps excess light in order to maintain the high resolution of an image.
Behind the RPE is the choriocapillaris, a capillary bed which must be close to the photoreceptors in order to provide them with the necessary oxygen (through vasodilation and diffusion) in order to function.
Eilrayna's comment about the RPE is a great example of a refutation of "bad design", but also how theories can still be overturned if there is evidence to prove that the belief originally held is false, regardless of how long ago it was. Another example of a refutation against a commonly held belief of "bad design" is the human appendix. This small outgrowth connected to the human cecum is believed to have been completely useless. Evolutionary biologists did note that the appendix was useful to our ancestors that had a completely vegetarian diet, but dismisses their function to modern humans. Biologists dismissed the appendix as a "vestigial organ" in that they could not find any digestive function to the appendix though it is connected to the cecum. Surgeons also see the appendix as a nuisance in that it can develop tumors, but can just be removed without any adverse side effects noted. This dismissal of the appendix as a "vestigial organ" has been recently been countered against by some researchers that found that the appendix may actually have a very useful function. The appendix is said to be a safe house for "good" bacteria that help with the digestive process. For example, if a disease such as cholera comes around and suddenly destroys all the "good" bacteria, the human body needs these bacteria to continue with its digestive process, so the appendix can release its own bacteria that will reboot the digestive system's bacteria. Though this may seem to be a minute detail, it can be very useful if one does receive a disease that eradicates all the "good" bacteria and the body needs the bacteria to be replenished in a short amount of time. Now that such detail has been discovered, we cannot ignore the appendix as some useless outgrowth that has no value. The appendix is related to the theme structure and function in that the location of the appendix is important in that it is located near the end of the digestive process so that no invasive bacteria can destroy the bacteria located in the appendix. This way, there will always be a supply of "good" bacteria within the human system.
For a while one popular argument for bad design was the inverted retina of vertebrates. With an inverted retina, photoreceptors face the inside of the eye as opposed to the more obvious, expected design facing outward (see diagram: http://www.answersingenesis.org/Home/Area/Magazines/tj/images/v13n1retina1.gif). This creates the supposed "backwards wiring" of our eye.
ReplyDeleteRecently, research has shown that the seemingly twisted structure of the retina makes its function much more efficient and of high quality in the following ways:
First of all, the tips of photoreceptors are in contact with the cells of the RPE, or retinal pigment epithelium. This has three important implications.
The RPE regenerates photorecpetive pigments. When light hits rhodopsin at the inner end of a photoreceptor, the cascade of reactions that eventually forms an image that we see begins. This happens through the isomerization of 11-cis-retinal, a part of rhodopsin. The RPE uses vitamin A to regenerate used retinal after light hits it. Similarly, the RPE recycles old particles of the membrane which may otherwise decrease photoreceptor sensitivity and cause blockage. Lastly, because it contains many pigments, the RPE absorps excess light in order to maintain the high resolution of an image.
Behind the RPE is the choriocapillaris, a capillary bed which must be close to the photoreceptors in order to provide them with the necessary oxygen (through vasodilation and diffusion) in order to function.
http://www.arn.org/docs/odesign/od171/retina171.htm
Eilrayna's comment about the RPE is a great example of a refutation of "bad design", but also how theories can still be overturned if there is evidence to prove that the belief originally held is false, regardless of how long ago it was.
ReplyDeleteAnother example of a refutation against a commonly held belief of "bad design" is the human appendix. This small outgrowth connected to the human cecum is believed to have been completely useless. Evolutionary biologists did note that the appendix was useful to our ancestors that had a completely vegetarian diet, but dismisses their function to modern humans. Biologists dismissed the appendix as a "vestigial organ" in that they could not find any digestive function to the appendix though it is connected to the cecum. Surgeons also see the appendix as a nuisance in that it can develop tumors, but can just be removed without any adverse side effects noted.
This dismissal of the appendix as a "vestigial organ" has been recently been countered against by some researchers that found that the appendix may actually have a very useful function. The appendix is said to be a safe house for "good" bacteria that help with the digestive process. For example, if a disease such as cholera comes around and suddenly destroys all the "good" bacteria, the human body needs these bacteria to continue with its digestive process, so the appendix can release its own bacteria that will reboot the digestive system's bacteria. Though this may seem to be a minute detail, it can be very useful if one does receive a disease that eradicates all the "good" bacteria and the body needs the bacteria to be replenished in a short amount of time. Now that such detail has been discovered, we cannot ignore the appendix as some useless outgrowth that has no value.
The appendix is related to the theme structure and function in that the location of the appendix is important in that it is located near the end of the digestive process so that no invasive bacteria can destroy the bacteria located in the appendix. This way, there will always be a supply of "good" bacteria within the human system.
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