Wednesday, April 14, 2010
Natural Selection vs Sexual reproduction
Coyne said, "...NAtural Selection isn't the only process of evolutionary change. Most biologists define evolution as a change in the proportion of alleles (different forms of gene) in a population" (122). Compare and contrast evolution involving genes and evolution involving natural selection. Which process do you think provides the better results and why?? Explain by providing effective examples and make sure to relate ur respose to the biological themes and the AP Biology course.
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Evolution via genes (genetic drift) and evolution via natural selections share one common trait, the two processes both produce changes within a population that is enough to warrant labeling it as evolution. The comparisons stop here, however. One of the greatest differences between natural selection and genetic drift is that genetic drift is completely random. There is a 50% chance of getting one allele from a parent, that means that the other allele is effectively lost. Coyne describes it like flipping a coin, “Although there is a 50 percent chance of getting heads on any given toss, if you make only a few tosses there is a substantial chance that you’ll deviate from this expectation (in four tosses, for example, you have a 12 percent chance of getting either all heads or all tails)” (122). Natural selection, on the other hand, is crafted via selective pressures in the environment, that force organisms to either adapt, migrate, or die.
ReplyDeleteBecause of this, I believe natural selection is superior, because only natural selection can produce adaptations while genetic drift produces features that are neither useful or harmful to the organism.
An example of genetic drift is given in the Campbell book on page 476. A wildflower can only pass on one of it’s pair of alleles: Cw, or CR. Whether or not an allele is passed on is entirely do to random chance during chromosome crossing overs or chance events such as a moose stepping on some of the homozygous plants, reducing their chance of that allele proliferating in future generations. It is because of this random selection that genetic drift is the only way for useless, junk DNA to change, as “Any mutations in these genes have no effect on the organism and therefore can evolve only by genetic drift” (124).
Genetic drift relates to Continuity and Change. Genetic drift explains why species that should have no significant selective pressures affecting their development can still evolve. The reason the species is changing and not remaining homogenous is because of random selection of alleles during sexual reproduction. Conversely, genetic drift can also explain why organisms can show no change in a certain phenotype, as enough random genetic drift allowed the elimination of genes of all other phenotypes.
Coyne mentions the concept of genetic drift, the process of "random change in the frequency of genes over time," as a "legitimate type of evolution" (2009, p. 123). Ultimately, all DNA mutations are random, as no cell intentionally changes its genome during DNA cloning. The process of crossing over in meiosis, the process in which "genetic rearrangement between nonsister chromatids" occurs (Campbell et al., 2008, p. 257), serves to create offspring that are not genetically identical to either of the parents. Crossing over, then, is a way for sexually reproducing species to facilitate genetic drift.
ReplyDeleteGenetic drift does not facilitate the proliferation of any useful genes. Therefore, genetic drift does not promote the evolution of adaptations, unlike natural selection. In natural selection, individuals with less fit adaptations die, and individuals with more fit adaptations survive and proliferate the adaptations. Thus, although genetic drift does not favor any useful genes (though genes that inhibit survival and reproduction will disappear within one generation due to natural selection), unlike natural selection, genetic drift creates new, unique genomes. Kevin said that natural selection can produce adaptations, but I disagree. Natural selection only works to proliferate existing adaptations, not to create new genes to adapt to new environments. Genetic drift, on the other hand, creates new genomes and increases the genetic diversity of a population, preparing the population for future problems.
One example illustrating the importance of genetic diversity was the Irish potato famine. As the entire population of Ireland survived on a single crop, the potato, the combined gene pool for all the crops of Ireland was small and relatively homogenous. When the potato crop failed, none of the potatoes in Ireland had the correct genes to adapt to the attack on the potato. If the people of Ireland had grown an additional crop that had a gene that could adapt to the attack, or if at least one of the potato plants had had that gene, the potato crop would have, ultimately, been safe (Kim, 2007). Genes that create new adaptations occur randomly; the more diverse a gene pool, the higher the probability of the correct gene being present.
Therefore, I don't that natural selection can be inferior or superior to genetic drift. Rather, natural selection and genetic drift are two parts of a single evolutionary process. Without genetic drift, millions of species would become extinct, as natural selection will kill unfit species without those species having a chance to adapt. Without natural selection, millions of species would also become extinct, as unfit genes would continue to plague populations. Natural selection and genetic drift are two equally important components of evolution.