For humans, life is obviously not a life and death struggle (at least for us). Besides diseases, we live with very little selective pressure. Carroll mentioned the fossilized MYH16 gene in humans on page 134. This gene contributes to large fibers in muscles, which explains why human "muscle fibers are much smaller than those in our relatives."
We can't say for certain that this gene fossilized because of declining selective pressure. However, as we are becoming less and less dependent on our own bodies for day-to-day activities, is it possible that many of our own genes will eventually fossilize and lead to the decay of the human body? Could this occur for domesticated/captive zoo animals as well? What might result of this?
I think there's a small misconception here. Gene fossilization is not necessarily a bad thing. Humans with fossilized MYH16 are able to make do with smaller muscles, because we are able to survive with out intelligence. It's not like we are going to go hunting with only our bare hands and climb trees to pick berries. Us humans don't necessarily rely as much on physical strength, thus it makes sense for the MYH16 gene to be fossilized. Evolution is not only involved in adding addition selective advantages through mutation, but also losing unnecessary features that are no longer needed for survival. However, with humans, with very little selective pressure, evolution occurs much more slowly, or not at all, because natural selection no longer occurs. Thus, a baby with the fossilization of a necessary gene would most likely still be able to survive and reproduce, as long as the gene is not completely vital for survival, of course. But over time, mutations will occur, and some genes will fossilize, possibly negatively affecting the person, but the person will still be able to survive most likely, and will pass down this fossilized gene, and eventually, because of the natural occurrence of mutations, fossilized genes may become more common, however this process takes a very long time, and we wouldn't see an actual effect of the fossilized genes for many, many generations. The same goes for domesticated or zoo animals. Bad genes will be passed down because the selective pressure is decreased.
ReplyDeleteI disagree with Paul on many levels. I think that evolution will still occur in humans, and in an extremely negative manner. We know from Carroll that relaxation of natural selection leads to gene fossilization. But there is another phenomenon of greater destructive power: evolutionary drift. Recall from the beginning of the year during our ecology unit that when natural selection is not allowed to function normally, the most fit to not necessarily survive and reproduce. With the extinction spiral this is a result of lowered populations, with humans this is a result of reduced pressures. As a result, the human race will experience a drift in traits in the population.
ReplyDeleteCarroll gives a prime example of these negative effects occurring when he discusses color vision in chapter 4. In humans, "up to 8 percent of Caucasian males are color-blind" whereas in the wild less than 0.1 percent of a population of a trichromatic species is color-blind (Carroll 105). This is only the beginning of the results of relaxed natural selection. Because people nowadays are always looking to be benevolent and inclusive to even those individuals with genetic disorders, those who are genetically weaker are reproducing, thus spreading their genes throughout the human race. Now I'm no friend of eugenics, but I think that it is undeniably that these propensities are weakening our species as a whole. While I don't think this will get to the point where the entire human body has been rendered dysfunctional due to drift and spread of negatively mutated genes, I do believe that we will see the prevalence of bodily disorders, inherited conditions, and poor body types increasing.
I imagine a similar phenomenon COULD occur in domesticated animals, but breeders generally select for the strongest animals to breed. Crops and animals are treated similarly by breeders/farmers; consider the development of modern corn from primitive teosinte. Human selective breeding has led to larger, improved species of corn. The same happens with domesticated animals. Thus, though humans have pressures relaxed and will thus experience genetic decay to an extent, domesticated animals will have their genes preserved if not improved.
The main idea in this topic that many people get confused is that there is a difference between changing and degenerating. I do not believe that the fossilization of the MYH16 gene exactly means that human bodies are degenerating. In fact, I believe that the MYH16 gene fossilized and became extinct in humans exactly for the reason that it could actually be viewed as a liability. The human brain is much more metabolically costly when compared to the brains of close primate relatives, and the steady maintenance of glucose and blood flow to the brain is highly important to human survival. Since the large fibers in muscle that the MYH16 gene codes probably use a substantial amount of oxygen and glucose, so these can be seen as tissues competing with the brain. Also, humans can use their brain to nullify many more selective pressures than can large muscle fiber. Also, the loss of muscle fiber should be expected. As humans use their brains more to evade natural selection, there is less selective pressure to develop larger muscles. A human with very little muscle can survive and reproduce just as easily as a human that is muscle-bound. This means that a muscle-creating gene that would be essential for survival in primates such as MYH16 would not have a great impact on the fitness of a human with a mutation in that specific gene. Even though the loss of large muscle fibers may seem to be physically degenerating the human body, this loss was just a tradeoff so humans could maintain their brains now and allow for even further brain growth in the future. The growth of the human brain is definitely a sign of the human body changing, but not degenerating.
ReplyDeleteI disagree with Paul, Eric and Aaron’s perspective that the human body is weakened by the fossilization of genes due to the relaxation of selective pressure. While it is true that the growth of industries have put a less rigid physical requirement on the body, I don’t think that the human body has degenerated in any way. On the contrary, some evidence suggests that the human body is actually getting bigger and stronger as the human race become technologically advance and face less of a life and death struggle. One example that illustrates this point is the increase average height in Japanese males from 17,000 B.C. to the present. The average height in 17,000 B. C. was 152 cm. It increased to 158cm during the late Jornon Period (8000 B.C. – 300 B.C.) and to 164 cm during Yayoi Period (300 B.C. - 300 A.D). Currently the average Japanese male stands at 170 cm. The results shown here is opposite to what you would expect of a declining selection pressure. Two possibilities could account for this abnormal result: selection pressure is not relaxed or some other factor offsets the effect of a selection pressure. I think that the first possibility is unlikely. Carroll mentioned that “up to 8 percent of Caucasian males are color-blind whereas in the wild less than 0.1 percent of a population of a trichromatic species is color-blind” (Carroll 105). This shows that color vision is not selected for in modern day society. Thus we can establish the fact that selection pressure is indeed relaxed. This will lead us to possibility that a second factor is countering the relaxation of selection pressure. I presume that this factor is our enlarged brains. The human brain have allowed use to develop tools to facilitate our lives. The human body is influenced by both nature and nurture. In modern society, humans can nurture their body to grow healthy and strong by eating a diet filled with the essential amino acids, fatty acids, and monosaccharides. For example, calcium promotes bone growth and ascorbic acid strengths the immune system. Although there is less selection pressure on human genes, it will still take eons for the “mutations can accumulate in them [genes] genes” (Carroll 123) and render the body fallible. Currently, the immediate effects of nurture override the negligible, long term effects of gene fossilization.
ReplyDeleteSource: http://www.sumitomo.gr.jp/english/discoveries/special/84_02.html
I do not think that a relaxation in selection pressure will cause a decline in the physical fitness of domestic/ zoo animals. Animals that reside in the zoo receive better nurture than those that live in the wild. Domestic pigs are fatter than wild pigs and domestic chickens lay more eggs than wide chickens. Human strengthens the selection pressure by manually picking the fittest organism and allowing them to reproduce. Thus domestic animals often represent the fittest genetic variation of their species because. Humans act as selecting agents and speeding up the evolutionary process for domestic animals.