Practice Exam Questions

Practice Exam Questions & Responses A. (4 pts) Why are mutations important for evolution? Mutations are important for evolution because they are the ultimate source of genetic variation. Mutations can create new alleles by altering existing alleles and create new genes through duplication events leading to new phenotypes to arise. B. (4 pts) What impact do mutations have on the fitness of a population over time in the presence of natural selection and why? Mutations in combo with natural selection weed out deleterious mutations and keeps the fitness of a population. Meaning it will get rid of the mutations that cause the population to die, and the ones that keep fitness the same will stay. This combo also has a rare chance to increase the fitness of a population but is uncommon. Jan 26 Practice Exam 3 Darwin’s 4 postulates set out the conditions that have to be true for evolution by natural selection to occur. What would happen if each claim was not true in a population? Write one “if… then” sentence for postulates 2, 3, and 4. In the 'if' part of the sentence, describe what the postulate would look like if it wasn't true (what is the opposite condition?). In the 'then' part of the sentence describe why the violation of the postulate would impact evolution by natural selection. If you explanation requires one additional sentence that is acceptable. Example: Postulate 1 is that Individuals within a population differ from one another. So I might write for this question: If individuals in the population are identical to each other, then there are no differences for natural selection to select among so the population could not change in phenotype over time. The purpose of this question is to make sure you understand why each postulate is important. Postulate 2 is that the differences are, at least in part, passed from parent to offspring. If the differences are not heritable, then variation will not be passed on from parents to offspring thus not giving them the benefits from said variation. postulate 3 is that some individuals are more successful at surviving and reproducing than others. If everyone has the same survival success, then variation is unimportant and natural selection wont occur since no trait is better than the other. Postulate 4 is that the successful individuals are not merely lucky; instead, they succeed because of the phenotype variant they inherited (and that they will pass onto their offspring). If individuals did not inherit the beneficial phenotype variant, then the ability for them to survive will be based entirely on luck. Practice Exam 5 A What happens to mean value of phenotype and the variation in the population: Directional: Mean fitness of population increases over time with the spread of the beneficial allele. Reduction of standing variation in population for that gene, meaning beneficial allele will go to fixation at a frequency of 1.0 and the less beneficial allele goes to 0. Disruptive: Mean value of trait does not change but there are fewer individuals at the mean. Inflates variation in population. Stabilizing: mean of trait in population does not change over time. Reduces variation in population. Directional selection is where deviant trait values, traits far from the mean, have the highest fitness and intermediate trait values, traits near the mean, do the worst. Directional selection is often found under environmental changes and it usually occurs when populations migrate to new areas that have different environmental pressures. So theres a fast change in allele frequency and plays a major role in selection. What Happens to the standing allelic variation over time? Directional - reduction in variation of population Purifying – No variation at the gene, just one allele Overdominance – two alleles at a gene are maintained Underdominance – reduction in variation Negative Frequency Dependence – maintained and enhanced What Happens to the frequency of each allele? Directional – beneficial allele goes to fixation to 1.0, not beneficial goes to 0. Purifying – frequency of beneficial allele does not change over time if started at fixation Overdominance – both alleles maintained at the same frequencies over time Underdominance – one allele goes to fixation at 1.0 while the other goes to fixation at 0. The most common allele goes to fixation. Negative Frequency Dependence – continuous change in the frequency of trait or allele Does the same outcome occur across all populations of the same species? Directional - Yes Purifying – Yes Overdominance – No Underdominance – No Negative Frequency Dependence – No B. Overdominance occurs when the heterozygote has higher fitness than either homozygote. It leads to a maintenance of the allele frequencies because in order for heterozygotes to be made you need to have both alleles present in the population. Of course, there are some fitness differences between the phenotypes so frequency of the alleles at equilibrium is dependent on this but are still maintained in the population over time. The reason I think it does not occur in all populations is because it depends on location. Sometimes, the heterozygote could have a higher fitness in this location but have the lowest fitness on an area. So I think it varies with location and does not really occur in all populations of the same species.