Wednesday, April 14, 2010
Once Useful, now harmful
Coyne mentioned that "genes that were once useful may, due to cultural change, now have destructive effects" (218). Coyne provides the example of how our love for sweets and fats resulkt in obesity, tooth decay and heart problems. Why do you think this happens? Provide other examples where once useful genes, now have many side effects.
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The affinity for fats and sweets, once upon a time, allowed humans to survive for long periods of time by consuming food that provides large amounts of energy. However, then, each civilization underwent an agricultural revolution, and scarcity of food no longer became an issue. In the post-modern first world, the majority of people are now prosperous enough for starvation to not be an issue. However, people are hungry nonetheless, and still crave the sugars and fats that helped their ancient ancestors survive, despite the fact that, for the fortunate inhabitants of the first world, the age of hunting and gathering is long over.
ReplyDeleteAnother example of an adaptation that is no longer useful is whales' legs. Coyne points out that "Whales still contain some genetic information for making legs" (2009, p. 65), despite the fact that whales now live in marine ecosystems.
The theme of continuity and change states that genetic information is passed from parent to offspring. Therefore, the genes that make humans crave sugars and fats are passed from parent to offspring. Long ago, the humans that did not crave sugars and fats died from starvation before they could reproduce, so the genes that did not make humans crave sugars and fats never had a chance to be passed from parent to offspring. In other words, natural selection did away with genes that did not make humans crave sugar and fat.
While excessive sugar and fat is detrimental to human health, sugar and fat generally does not kill a human before he or she can reproduce. Therefore, genes that make humans crave sugar and fat are still passed from parent to offspring. Natural selection does not act on these genes because the genes have not prevented reproduction.
I agree with Yiding that the desire for sugar and fat, which was important to our hunter-gatherer ancestors, is now more harmful. However, I do not believe that it is completely detrimental. Glucose is an important part of cells' functions - mainly glycolysis in cellular respiration, which Campbell describes in his chapter on cellular respiration (p167). Animals still also store glucose, but in the form of glycogen - a polysaccharide of glucose. (http://en.wikipedia.org/wiki/Glycogen and Campbell 72)
ReplyDeleteFats are important because cell membranes are composed of phospholipids. In order to form these lipids and maintain the integrity of the cell membrane, cells require triglycerides. (Campbell 74).
Clearly, these substances are not useless. It's just that the evolution of society has greatly increased the supply of these substances, which means that humans' genes are just leading to a greater desire than that there should be for fats and sweets.
This relates to the theme of continuity and change. The continuity of genes has remained, still causing humans to stay on the lookout for sugar and fats. However, the environment itself has changed.
Another example of a formerly useful genes have been rendered much less useful by evolution are the genes that code for the development of the appendix in humans. The appendix is perhaps the most famous example of a vestigial feature, a feature that has either changed or lost all use over time. Coyne explains that the appendix used to be very useful for our plant-eating ancestors for processing food. Though in humans, there may be some minor uses, those uses definitely do not outweigh the risks of infection or inflammation of the narrow, easily clogged appendix (Coyne 61). All vestigial features are really caused by genes that are no longer useful because of change in either the organism or environment
I agree with Yiding and and Dennis that our species' affinity for fats and sweets allowed us to store energy for long periods of time. According to one source obese people "can live from 3-weeks up to 25-weeks and more without food depending upon initial health and amount of fat" (http://www.survivaltopics.com/survival/how-long-can-you-live-without-food/). At this stage in human evolution though, I do not think this desire to eat so many fatty and sugary foods is beneficial. If anything wanting to eat that entire box of girl scout cookies is bad design. A better design would be for the human body to only crave "junk" foods when we need more fat in our diets. Kind of like when we, or any other living thing for that matter, need water we get thirsty and get a drink. Fats are important because cell membranes are composed of phospholipids. In order to form these lipids and maintain the integrity of the cell membrane, cells require triglycerides. (Campbell 74). Another option would be that if humans had to much fat in their system the pancreas would secrete fewer hormones into the duodenum through a system of exocrine ducts in response to the negative feedback of small intestine hormones secretin and cholecystokinin. The digestive enzyme pancreatic lipase, which is produced and secreted by acinar cells of the exocrine pancreas, would be inhibited and we would not crave fatty foods (http://en.wikipedia.org/wiki/Pancreas).
ReplyDeleteAnother example of a destructive gene would be the gene that results in the huge claw of the male fiddler crab. The huge claw on the male fiddler crab seems to have evolved for two purposes: mate attraction and ceremonial fighting. “The claw is so large and cumbersome, that it is of little use to the crab in getting food” (http://www.nccoastalreserve.net/uploads/FiddlerFacts8.20.09.pdf). This is an example of sexual selection. Sexual selection occurs when an organism develops a trait that appears to be in opposition to survival but is advantageous to getting a mate. Over time the claw may become to big for the male crab to function properly and any food.
This theme relates to structure and function. The structure of the large claw allows the male fiddler crab to find a mate and fight, however it's size renders it useless for grabbing food. As a result the males take almost twice as long to feed themselves.
Eric Hennings
I agree with Dennis in the first part of his response; for each glucose molecule we consume, we have the ability of making 36 ATP. Such rapid production of energy would not be possible without some sugary food in your diet (Campbell 163).
ReplyDeleteIn addition to Dennis's points on fat, fat also, specifically polyunsaturated fat, allows the body to avoid disease by lowering total cholesterol (http://www.dietbites.com/article1097.html).
This concept relates to the theme science, technology, and society. Continuing Coyne's example on page 218, the reason for the shift from desiring a rare food source to being accustomed to the "local food abundance" and thus making the love of sweets and fats more useless now. In contrast, the opposite may also take place. A gene that was once detrimental may now have a lessed effect. Coyne provides an illustration of this in which "bad" sight genes are not as harmful now due to new technologies (eyeglasses, optometric care). This is one more way that modern technology and the culture of our society help decide the evolutionary pathway of human genes (Coyne 218).
Jerry Coyne summarizes and clarifies the idea of vestigal traits nicely in the Remnants chapter: "A trait can be vestigial and functional at the same time. It is vestigial not because it's functionless, but becaue it no longer performs the function for which it evolved" (Coyne 58).
Another example of desirable and functional genes being rendered useless is found in the genes that instruct the development of wings in the flightless kakapo and kiwi. In ancestors, the wings were fully functional for flight, which is obviously very useful. The vestigial wings don't disappear entirely so that a new function may one day evolve, though today the wings are just nubs (Coyne 58).