Saturday, April 4, 2009
Natural Selection: Change Is Not Necessarily Progess
"Natural selection cannot preserve what is not being used, and it cannot plan for the future." -Carroll pg. 136. Both in "Making of the Fittest" and in AP Biology class, it is always reinforced that evolution doesn't think ahead, or really think at all; natural selection only acts to select what is best for whatever the present situation of a species may be. Using a specific species example, show an instance in shich natural selection has caused a species to move in a direction that could be considered "unprogressive."
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When humans think of evolution, we always think that organisms would get bigger and better. However, this is clearly not the case with the icefish, Champsocephalus gunnari, which is found in the icy waters that surround Tiny Bouvet Island. These icefish lacks red blood cells and the life sustaining, oxygen carrying hemoglobin. Ironically, the loss of red blood cells have actually helped the fish survive in its frigid environment. According to the 5th theme of biology,interaction with the environment, “organisms are open system that exchange material and energy with their surroundings” (Campbell 22). The icefish live in below freezing water and have to sculpt its body to suit this particular condition. The low temperature increases the viscosity of blood and makes it difficult for the heart to pump. Thus the “blood” of icefish “contains just 1 percent cells by volume (all white cells)” (25). Additional adaptations include larger gills and hearts. The fish’s scaleless skin can directly assimilate oxygen from the oxygen rich waters with the help of enlarge capillary beds that lie below the skin. Instead of hemoglobin, the icefish have large amount of myoglobin, which we learned is the oxygen binding molecule found in muscles. Furthermore the ice fish also “invented” the antifreezes proteins to prevent ice crystals from forming in their blood streams. As a result, the ice fish is extremely well adapted to living in the South Atlantic Ocean. For them, these evolutionary changes isn’t a backward move but a move forward that have allowed them to inhabit niches that have been previously off limits. In this extreme environment, the ice fish faces less competition and less danger of eaten by predators. Sometimes you need to take a step backward before you can go forward.
ReplyDeleteThere are many instances of genetic mutations having both positive and negative effects in human populations. For example, when people think of mutations that cause Cystic Fibrosis, they think that the mutations are all bad. However, some of the most common CF mutations make the body more resistant to the bacterium that causes Typhoid Fever (Carroll 180). Another example of a mutation that may seem to be unprogressive now but was actually progressive when the mutation first occurred is the occurrence of sickle cells. As we all know, sickle cell anemia is a horrible disease that affects many regions of the world where malaria is present. Thanks to Anthony Allison, it is known that the sickled shape of the red blood cells in people with sickle cell anemia makes the cells more resistant to malarial infections (Carroll 174-176). At the time when this mutation occurred, it was more beneficial to remain malaria-free than it was to have non-sickled cells. There are numerous other examples of genetic mutations that were progressive at the time of first occurrence, but are know seen as “unprogressive.”
ReplyDeleteAs a human, I value my eyesight as one of my most important senses. Not all animals have full-color vision however. It is interesting that the adaptation of having full color vision would actually disappear in different species over time. Why would full color vision disappear? Wouldn’t that be unprogressive? Genealogic trees and the distribution of opsin genes have showed that opsins that coded for color vision was “one of initial abundance” (103). Color vision disappeared in certain mammals as they adapted to different lifestyles. I agree with Jia in that organisms can lose a trait that can be seen as beneficial as a response to their environment. Like the icefish have lost their ability to make hemoglobin, nocturnal animals such as the owl monkey have a non-functional SWS opsin gene, preventing color vision. In the gene of the owl monkey, a mutation has changed a codon into a premature stop codon, halting the translation of the protein, thus eliminating color vision. Even though color vision seems to be a quintessential asset for humans, the nocturnal lifestyle of the owl monkey eliminated the owl monkey’s need to have a gene that coded for color vision. Animals need light to distinguish color, which is why we cannot see in color in areas with little light. As the owl monkey is active during the period of the day with little or no light, it would not utilize its SWS opsin gene to detect color. Not using the SWS opsin gene would relax natural selection on that gene, allowing mutations to accumulate so the gene is not expressed. Other examples of nocturnal animals losing their color vision would be the bush baby and the slow loris. These animals both had fossil SWS opsin genes. It is interesting to note that both species had the same “big chunk of code missing near the beginning of the gene that obliterates the ability to make the opsin” (124). The same deletion in both species indicated that they shared a common ancestor that lost the ability to make the SWS opsin, and this loss of ability was inherited by the slow loris and the bush baby. The inherited trait of no color vision relates to the theme of heritable information which deals with “the inheritance of biological information in the form of DNA molecules” (Campbell 22). In this case, both species inherited the deletion of part of the SWS opsin gene from their common ancestor, which caused their loss of color vision.
ReplyDeleteAnother example of an animal losing their full color vision is the blind mole rat. The blind mole rat lives underground, and its environment receives little light. Because of its environment, its SWS opsin gene is fossilized. This relates to the theme of interaction with the environment because the environment greatly affected the blind mole rat’s adaptations and development. The mole rat has degenerated eyes and only has two intact opsin genes, “a red-shifted MWS/LWS pigment that is tuned to detect the light received through the subcutaneous eye and a dim-light rhodopsin” (126). The two opsin genes are responsible for maintaining the circadian rhythm of the blind mole rat. As we have learned, the circadian rhythm of an animal is responsible for its behavior such as when the animal eats and when it sleeps. Natural selection have acted on these genes so that the biological clock of the blind mole rat could keep on running.
A great example of how natural selection does not plan for the future and can lead a species in an "unprogressive" direction is the example of melanism in English and North American peppered moths. As Carroll points out on page 52, the peppered moths' predators are birds. The moths use their color to blend in with their surroundings (camouflage) to avoid the birds, enhancing their chances of survival and reproduction. Before the Industrial Revolution, the environment of the moths (trees and lichen) was light-colored. Therefore, natural selection chose for light color in moths, because the light color gave those moths an advantage over dark-colored moths, because light-colored moths were harder to see and therefore were not consumed as much as dark-colored moths, allowing the light-colored moths to have a greater chance of survival, and therefore a greater chance of surviving long enough to reproduce. However, as the prompt states, natural selection did not plan for the future, and actually led the moths in an 'unprogressive' direction. Due to pollution from the Industrial Revolution, the trees and lichen became darker. In this new environment, the dark-colored moths had better camouflage, and the light-colored ones stood out to predators. Therefore, the light color, which used to be a selective advantage, became a disadvantage. The light-colored moths were more easily consumed. This shows how natural selection did not plan for the future when it selected for light color in the moths. This turned out to be an "unprogressive" development because in the future, it became a disadvantage, hindering the individuals' chances of survival and reproduction, rather than remainign an advantage. The example of the peppered moth shows how natural selection does not 'think,' but rather simply chooses for what traits are best in the current environment. Choosing for light color was advantageous in the moths' current environment at the time, but was not advantageous in the future.
ReplyDeleteNatural selection can cause a species to move in a direction that is considered unprogressive when there is no immediate need to preserve certain genes. Natural selection can not tell the future so genes that are not in use at the moment are not selected for. For example, Saccharomyces kudriavzevii, a type of baker’s yeast, is no longer able to convert galactose to glucose as an alternate form of nutrients. This is because the baker’s yeast no longer needed to convert galactose to glucose so natural selection did not preserve those genes. However, it could be in the future that the food source available is galactose and in that case this type of baker’s yeast will not be able to survive. This is unprogressive in terms of evolution, but natural selection can only protect and select for what is needed.
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