Saturday, April 11, 2009
Neutral Theory
On page 66, Caroll introduces Mootoo Kimura's Neutral Theory, saying that "when two versions of a gene differ... the difference may be neutral, of no consequence to fitness." This may seem contradictory to the traditional view of mutations in natural selection, but it is not. Explain how a mutation could result in no change in fitness. Is it possible for a mutation that causes a change in phenotype to be neutral? And is it possible for a mutation that does not change the phenotype to affect fitness? Also, the Neutral Theory states that the majority of genetic variation within populations is due to these neutral mutations, not natural selection. Explain why this is.
Subscribe to:
Post Comments (Atom)
A neutral mutation is a mutation that does not affect the particular phenotype whose expression is under the genetic control of the mutated gene. A neutral mutation can be in junk DNA and therefore not affect the outcome of the gene. For instance, a reason that a mutation may not lead to a change in fitness, is that many codons code for the same amino acid that is called to the ribosome. The code may be altered but still call for the same amino acids and therefore not affecting the gene and being neutral. The key for a mutation to be neutral is that it does not affect the phenotype, but it may affect the future offspring. In a neutral network the genotypes code for the same phenotypes. But, phenotypes may be affected often through mutations. For instance, an amorphic mutation is when there is a null allele and it does not function anymore. For this type of mutation the phenotype cannot be neutral and it is affected. Although many cases of these phenotype mutations are recessive there is a possibility to have a recessive allele that will cause you to have a mutation. On the other hand, mutations that cause the organism to gain a function are mostly dominant and there is a higher chance of an organism of having this mutation because it can be passed on easier. There is a reverse mutation that may cause the mutated phenotype to return to the original phenotype. This is the closest example to how a phenotype can be changed and the mutation would be neutral. But it is not fully neutral because the genotype does not code for the same phenotype, but instead the correct phenotype.
ReplyDeleteThe neutral theory may state that the neutral mutations have caused the majority of genetic variation. This is because over time the genetic material that is mutated is passed on without being noticed, and the more neutral mutations are occurring to that genetic material. It has also been quoted that “neutral mutations prepare the ground for later evolutionary innovation”. This says that a neutral mutation is what leads to evolution and the variation of genes.
http://blog-msb.embo.org/blog/2008/11/the_role_of_neutral_mutations.html
http://www.bookrags.com/research/neutral-mutation-wog/
A mutation can be neutral in two separate ways. The first relates to the concept of redundancy in the genetic code. In the genetic code, there are multiple codons (triplets of nucleotides) for the same amino acid. For example the amino acid Thr is coded for by the codons ACU, ACC, ACA, and ACG. This means that if a mutation occurs there is a high chance that the amino acid sequence coded for by the gene will not change. So it is highly probable that a mutation will neither affect the phenotype or the amino acid sequence of the protein, thus there would be no impact on evolutionary fitness.
ReplyDeleteThe second way in which a mutation could be neutral relates to the inherent properties of proteins. Proteins are polypeptide chains hundreds of amino acids long. Therefore if a mutation were to cause an amino acid to change, and there were no other significant mutations to the gene coding for said protein, the protein might still retain its intended shape, and because of that there would be no change in phenotype. This would occur because the interactions between the R groups of all the amino acids in the protein would be able to hold together, even if one amino acid was different. Also the new amino acid might be chemically similar to the one it replaced and therefore would have no effect on the function.
It is possible for a mutation that changes a phenotype of a gene to be neutral. This ties into the concept of fossil genes. When genes are no longer needed by a cell due to environmental changes, the gene is allowed to die off. For example Carroll talks about the Japanese yeast whose galactose pathway mutated to the point of fossilization, but did not impact evolutionary fitness because the gene was no longer needed.
It is immposible however to have a gene that affects evolutionary fitness but does not impact evolutionary fitness for the sole reason that by definition, for a cell or organism to become more fit due to mutation, the mutation must alter the phenotype in a manner that is beneficial to the organism/cell.
http://en.wikipedia.org/wiki/Neutral_mutation
http://www.bookrags.com/research/neutral-mutation-wog/
In order to learn how mutations affect our genetic code, we need to know 3 different kinds of mutations. There are three kinds of mutations: Nonsense mutations, mis-sense mutations, and silent mutations. Nonsense mutations results in a premature stop codon, missense mutations result in code for a different amino acid and Silent mutations code for the same or different amino acid but there is no functional change in the protein. Majority of these mutations does not affect our phenotypes or even what they make. All these mutations are doing is that they are just adding one base nucleotide to shift the genetic code rightward or leftwards. Sometimes it may result in very bad situation such as the RNA polymerase stops copying DNA because of early stop codons. However, our gene has multiples of repetitions to make one amino acid. This prevents these little mutations from changing the types of proteins that our genes create. Therefore, it is not likely that those mutations will cause to change our phenotype.
ReplyDeleteWhat Motoo Kimura is saying about the neutral theory is that when one compares the genomes of existing species, the vast majority of molecular differences are selectively neutral. The molecular changes represented by these differences do not influence the fitness of the individual organisms. Therefore, this theory regards these genomic features as neither subject to natural selection. It is always possible that these neutral mutations to change our phenotypes because our body system is organized with many different kinds of proteins. Since genetic codes are for making proteins, if there is a serious mutation in our genes, there will be wrong protein coming out of the cells and it will eventually change our phenotypes, such as gray hair, pale skins and even if it does not affect our phenotypes, it will definitely affect our body system. This is why there are diseases like leukemia or sickle cell disease.
Nevertheless, Kimura says that this neutral theory is compatible with natural selection. Without natural selection, there wouldn’t be any forces action on an organism. Mutations will rampage in the wild and organisms will not be able to produce the right protein at the right time in this changing environment.
http://en.wikipedia.org/wiki/Mutation
http://en.wikipedia.org/wiki/Neutral_theory_of_molecular_evolution