Tuesday, March 31, 2009
The Source Of All Variety Is Mutation
On page 57, Carroll starts his discussion of how traits are worked into genes because of their selective advantage to the species. According to Carroll, selective advantages become dominant because of mutations that occur sometimes one in a billion in one organism's DNA. This mutation can lead to a successful trait that gave the organism an advantage over others. Therefore, over time, the gene became dominant in the species. Explain this process. How does one single mutation turn into a dominant trait that every member of the species has? How long does it take for these traits to spread? How many mutations must occur before the gene is seen regularly? Finally, discuss the spread of different traits and the varying ways they can spread. For example, is there a difference between the way hair color (for hiding from predators) versus immunity (to a deathly disease) spreads? Does one take longer to catch on than the other? Do they become regular traits in different ways?
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I am only going to address the first part of this question. All mutations happen by chance. According to Carroll, “there are an estimated 175 new mutations among the 7 billion DNA letters in every individual” (Carroll 58). Most mutations either occur in regions of DNA that don’t convey meaningful information, don’t change how a gene works, are compensated by our carrying two separate copies of the gene, or affect the gene in such a way that produces an effect within a tolerable range of variation. The last scenario is when the mutation either goes or stays. Let’s say that the mutation is beneficial, and will stay within the population. How long will it take? That depends on two factors once it has arisen in the species: how beneficial it is, and the generation time/reproduction habits of the species. If the animal produces a lot of babies in a short period of time, then the gene is surely going to be passed on throughout the population really fast.
ReplyDeleteTake the example that Carroll posed in the book, of the black fur mutation in white furred mice. The calculation is that in 1 million years, the mutation will occur independently 1000 times in a population of 10000. Once the trait occurs, two things will determine its spread: the advantage, and the population size. The two go against each other; greater advantage leads to faster spreading while greater population leads to a longer amount of time before the gene spreads to every member of the population. An advantage of .001 will spread over 10000 individuals in 19807 generations while an advantage of .2 will spread over 99 generations.
The big picture is this: a mutation changes a gene, which changes an individual. That individual animal has a greater chance of reproduction if that mutation is beneficial. Depending on how beneficial that mutation is, the mutation will be passed on at different rates (assuming no bad luck; the individual is not guaranteed to survive). Depending on population size, it will take a different amount of time for the trait to spread across the entire population.
The process of mutations and developing selective adaptations is very important for all organisms and allows them to adapt to the changing environment much quicker and with a higher success rate. The process goes like this: First, a mutation will occur. Technically, the odds sound low, but considering the fact that there is more than one individual in every species, the rate of mutations is much higher. Therefore, it is important to understand that mutations happen extremely often, just considering the fact that there are so many individuals in a species. Next, this mutation goes one of two ways. It will either be a selective advantage and allow that specific individual to survive better or longer than the others of it’s species. In this case the individual will survive to reproduce and will pass on its beneficial traits if nothing out of the ordinary happens, bad luck wise). On the other hand, the mutation might not be a selective advantage and that individual will die before it has time to pass on its mutated genes. If by chance the individual does survive to reproduce, that disadvantage will kill them off eventually.
ReplyDeleteA single mutation will turn into a dominant trait in a species if it is beneficial for them to have that trait. How beneficial it is, is determined by how well that individual survives in the environment. Carroll mentions, “The fittest is a relative, transient status, not an absolute or permanent state”(Carroll, 65). This is an important fact to remember because, as in his example, the black mice versus the white mice had advantages at different times. When they lived in dark environments, the black mice were less predated upon, meaning they were “the fittest”, while when the mice lived in light areas, with sand, they were “the fittest”. Theodore discussed this example in more detail above. The rate of the spread of this mutation depends on the interplay between selection and time. If a gene is selected for, it will spread faster than if it is selected against.
This discussion is directly related to the theme of evolution. We are discussing how and why different species evolve and why it is important for them to do so. If a species stays in one place on an evolutionary scale and does not mutate or change to fit its ever-changing environment, it will die off or go extinct. If a species does not have the power to change quick enough for the environment, such as the dinosaurs, then that species will go extinct. It is the way of life and therefore, mutations must occur or all species will eventually die. This means that mutations are not always bad, generally, they tend to be good, or not change enough in the DNA sequence to change anything in an individual’s body.