Sunday, March 22, 2009
Natural Selection based from Mutations
The pigeon example mentioned through pages 53-54 helps to show how a mutation can actually help the selection of a certain species over another. The example illustrated is that some pigeons have a white rump that helps in maneuvering so as to evade falcons. What are two other examples through the book where a mutation has actually helped an organism's chance at survival? Also, Sean Carroll states that natural selection picks out the winners and losers of mutated organisms, please add in your response how a mutation, or lack thereof, has determined the status, whether a winner or loser, of that organism.
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One example of a mutation that has helped is with the icefish of Antartica (p 23-25). These fish were able to "invent" an antifreeze protein and mutations caused the organism to have larger hearts and capillaries to make up for their lack of red blood cells. These mutations made the icefish a winner because while many of the other organisms went extinct because of the sudden environmental change, the icefish were able to adapt, through these mutations, to survive and reproduce.
ReplyDeleteAnother example is the colobus monkeys and their ruminant digestive system (p92). This digestive system involves a large stomach with many chambers that allows the organism to consume large amounts of food at one time and store it for later use. This mutation makes this organism a winner because this digestive system allows the colobus monkey to digest cellulose, make protein from nonprotein sources, and upgrade lower quality foods. This is an advantage because if there is a lack of food, these monkeys will be able to store and use their food for a longer period of time than the otehr chimps and apes. Also almost every bit of food will be able to utilized while the food that the other organisms digest may not all be useful to the organism.
According to the 8th theme of biology, evolution, natural selection screens for genetic variations a population so that only the fittest organisms occupy a given niche. For example, rock pocket mice that dwell on sandy back grounds are light-colored. Being light-colored is a selective advantage for the mice because they will less likely be spotted by predators. However, in lava flow regions, populations of the same species of mice are black. This phenomenon is caused because any light colored individuals in the population will standout like an eyesore to any predators. Therefore, those mice “having a black-causing mutation in their MC1R gene” (Carroll 60) will be at a selective advantage because black mice blend better into a black environment than a light colored mice. Birds of prey such as falcons can easily spot and capture the light colored mice. As a result, light colored mice will die off at a faster rate than black mice. This facilitates the transformation from a light colored population to a dark-colored population. A mutation in just one of the 10 sites on the mice’s MC1R gene is responsible for this change in coat color. MSH, a hormone produced in the intermediate lobe of the pituitary gland, “binds to MC1 receptors of melanocytes and stimulate the production of melanin” (Carroll 171) and cause the fur to darken. Darken mice in lave flow areas are the winners over light colored mice.
ReplyDeleteAs discussed in chapter 4, mutations in the opsin gene fine tune an organism’s opsin maximum absorption spectrum to fit the organism’s visual needs. Marine animals live in an environment where only blue light is available. As a result, their rhodpsin is “blue-shifted”, which means that their peak absorption spectrum is 10 to 20 nm shifted towards blue end of the light spectrum. For example, 3 mutations in is the bottlenose dolphin’s rhodopsin “are primarily responsible for the net 10 nm blue shift from the rhodopsin is even further blue-shifted”(Carroll 107). The blue-shift mutation is good instance that exemplifies how a specific genetic variation is selected for in a given population. As you can see from the bottlenose dolphin example, a blue-shift doesn’t require massive genetic deviation; 3 point mutations are enough to provide a 10 nm shift. Although DNA polymerase proofreads the nucleotides it adds to each stands, it still makes a mistake once in every 500 million sites (Carroll 60). Some of these mistakes will inevitably occur on the rhodopsin gene and cause the rhodospin to be either red-shift or blue-shift. This creates genetic diversity in the population of marine animals (7th theme). However, since marine animals live in an environment where only blue light (480 nm) penetrates the water, those individuals with blue-shift mutations will be visually enhanced while those individuals with a red-shift mutation will be visually impaired. As a result, blue-shifted individuals will be more likely to survive and pass on their genes to the next generation. These individuals can be considered winners of the genetic lottery.
Mutations can help an animal in numerous ways – from the loss of hemoglobin in icefish to the addition of a ruminating digestive system of colobus monkeys, as Liz M have mentioned, to a change in fur color in mice to the blue shift of rhodopsin in crustaceans. However, there is a striking difference between these mutations. A change in fur color or a shift in rhodopsin requires only few point mutations on the gene. However, the loss of hemoglobin and the development of a ruminating digestive system require major genetic modifications. While the error in DNA replication can account for point mutations, how do major genetic changes take place? Do accumulations of point mutations over generations gradually change an organism until it no longer resembles its ancestor?
One example of a mutation that would be beneficial can be found on oage 52, with the classic example of the different colored peppered moths in England and North America. Mutations caused some of the moths to be dark-colored rather than light-colored. Normally, this is a disadvantageous mutation, because dark-colored moths are easier to spot and therefore are easier prey for birds that prey on the moths. However, as the Industrial Revolution progressed, pollution made the environment of the moths (the trees and lichen) darker. Because of this, dark-colored moths actually blended in more, and were harder to spot and prey on than light-colored moths. This mutation was then an advantage for the moths, as it helped enhance their survival by increasing the effectiveness of their camouflage, hiding from predators. However, once humans started reducing the amount of pollution allowed to pervade the environments of the moths, the trees and lichen became light-colored again. Thus, the dark-colored moth was once again easier to prey upon, and that mutation was therefore a disadvantage. In the long run, the mutation making the moth darker made the moth a loser, because the lighter color was eventually favored over the darker color, and so the proportion of dark-colored moths to light-colored moths dropped significantly.
ReplyDeleteAnother example is illustrated by Carroll on page 140 with the howler monkey. Mutations caused howler monkeys to have enlarged throats and voiceboxes. These mutations were helpful because they allowed howler monkeys to call out to longer distances. This enhanced communication with other howler monkeys. This is an essential part of the howler monkey lifestyle, as the monkeys communicate to each other for everything from alerting others of danger to telling others of a good spot for food. Therefore, these mutations were naturally selected for, and turned out to be advantageous. The howler mokeys with the mutations were 'winners,' because the mutations gave them abilities which enhanced their chances for survival.