Sunday, March 29, 2009
The organization of life and mutations
One of the major themes of biology is that the properties of life emerge from the hierarchical organization of life: progress is a consequence of more complex organization. However, chapter 2 of the book , which focuses on the “holy trinity” of evolution (chance, selection, and time), makes it seem as though natural selection contradicts this theme because it is based on random mutations to further development. Carroll attempts to reject this contradiction by stating that “Mutation generates random variation, selection sorts out the winners and losers” (57). Explain how the coupling between mutation and selection reinforces the first theme of biology rather than rejects the first theme of biology. What examples does Carroll provide to help expand and explain the correlation presented in his statement on pg. 57? How do LINES and SINES represent the organization of randomness? How does the interplay, or exclusiveness, of fossil and immortal genes reflect organization? How do introns/exons, tandem repeats, and redundancy in the genetic code represent that natural selection harnesses the randomness of mutations to fuel the organization of life?
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As Carroll explains, the common misconception is that evolution is caused by random changes in the DNA, so evolution must be random. Those who believe this fail to understand the role of selection in the process of evolution. If every random mutation stuck to a species, then humans would have so many useless features that we would probably not survive. Since we are obviously surviving, this is not the case, and it is also apparent that most traits of humans serve a purpose, or else they would be lost through the fossilization of the genes that are responsible for the useless traits. Selection is the force that filters the useful traits from the useless.
ReplyDeleteWithout an organizational force like selection, we would see chaos in species. Thankfully, selection is a natural force, so it is impossible for a harmful mutation to stick in a species, because the nature of selection is that a harmful mutation will prevent the bearer from passing on this harmful mutation. Therefore the only mutation that will be passed on is one that increases the chance that the bearer will reproduce and pass it on. This natural mechanism of selection itself organizes the progress of species and makes it so only helpful mutations stick in species, allowing for only good progress. Any example of any species shows how selection maintains good mutations and filters out the bad. For example, take the stickleback fish and their armored plates. When moving into lake habitats, many mutations might have occurred to the fish. Carroll shows how the number of armored plates decreased, which increased maneuverability, when the sticklebacks moved into lake habitats. This helped the fish survive most likely by making it easier for them to catch prey. Because it was beneficial to the stickleback fish, selection kept the gene that produced fewer plates in tact, helping to make sure it stayed. SINES and LINES don’t have much to do with organizing the randomness of mutations, they are simply random chunks of junks that are inserted in genes and serve as markers. The force that organizes the good mutations from the bad mutations is selection that keeps the genes that help survival to survive. SINES and LINES do, however, help monitor genes to keep track of when certain mutations occurred. For example, if humans and chimpanzees have the same SINES or LINES in a certain gene for vision, then the common ancestor of chimps and humans had this SINE or LINE, showing that selection has kept this gene the same since the split into chimps and humans.
Showing proof of a rejected bad mutation can be much harder. Referring back the stickleback fish, DNA analysis can show how genes have changed in the fish to produce less armored plates, but how can we see the bad mutations that were rejected? The simple answer is we can’t. It’s very possible that a stickleback fish was born with a mutation in its DNA that caused it to have 40 armor plates, a very high number. This will not be present in the DNA of the stickleback fish, however, because this was a large disadvantage to the fish, and it most likely did not survive to pass on this trait. The one thing we CAN see in DNA is fossilized genes. These are not bad mutations that were not rejected, but rather genes that were once useful and no longer are, so mutations in these genes are not really filtered because they have no effect on the survival of an organism.
Mutation and selection together teams up to become the force that reinforces the first theme of biology which states that properties of life emerge from the hierarchical organization of life: progress is a consequence of more complex organization. Mutation most likely isn’t even the creation or deletion of new genetic material but rather a gradual change in genetic material. For instance, the hooded rats on page 47 seem to demonstrate incomplete dominance where a form of intermediate inheritance in which heterozygous alleles are both expressed, resulting in a combined phenotype. So with this slight variation in genetic patterns ranging from very black to very gray, selection begins to take its course. First the color scheme begins as a defense mechanism used by animals against predators and other harmful threats that could kill it. Take the Moths, as an example, the ones that were blacker in color in the polluted city were able to camouflage and thus avoid predation because they were able to blend into their polluted surroundings more easily. So, here is where selection takes its role. Selection, through predation, limited food supply, and any other limiting factor selects the variation or trait that would most likely survive in the harsh environment. If one trait seems to be a selective advantage in the upcoming challenges of life which allows the creature to survive and reproduce, then that trait will continue to prosper, and like how the book says “continuously compound” until it becomes a mutation in the entire species. So thus, mutation and selection work in a mutual relationship when it comes to evolution. So the requirements of the environment require specialized more complex processes in order to survive and create more life. So then selection acts as the qualifier for specialized life and if it qualifies then all the species with that trait would qualify survive, reproduce, and the variation in the trait will soon become a permanent trait and permanent mutation.
ReplyDeleteIntrons/ exons, tandem repeats, and redundancy in the genetic code all represent the possibility of randomness in mutations that natural selection can harness. All those these help create a buffer during the translation phase of cell transcription when genes become into proteins that have physical or chemical traits. With these buffers, that means a slight misread during the transcription phase wouldn’t hinder the synthesis of the protein necessary for life. The extra intron and extron could also lead to variability in genetic life where it could have a point mutation where the genetic base is substituted, deleted, or inserted. With this slight variation in genetic structure and code, a mutation or variation in the gene comes forth further spawning new creatures of the light and giving evolution another step.
http://biology.about.com/bldefincomdom.htm