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Name: Marina Dawoud Bio220 – Diversity of Life HW 4: Mechanisms of Evolution Exercises [37 pts] 1. A) Please explain the TWO roles that mutations play in evolution. (hint: recall the ‘big 4’ and then consider what each first needs in order to operate). [2] Mutations in evolution play two key roles: A. They introduce new genetic variation into a population, which is essential for natural selection to occur. B. They can directly cause changes in an organism's traits, which can affect its fitness and survival. B) Apparently, individual yeast rarely experience mutations. A single yeast cell may not show any point mutations in its genome! How can we reconcile the low rate of mutation in each individual with the fact that mutations drive evolution? (hint: consider the difference between Lamarck and Darwin) [2] Evolution occurs over long periods of time and involves entire populations. Unlike Lamarck's theory of inheritance of acquired characteristics, Darwin's theory of natural selection focuses on the accumulation of beneficial mutations over generations. So even if an individual yeast cell may not show many mutations, the cumulative effect of mutations across a population can drive evolutionary change. In other words, it's not just about the mutations in one individual, but rather the collective impact of mutations on the genetic diversity of a population. C) Compare and contrast the sources of genetic variation in eukaryotes vs. bacteria/archaea. [2] Both eukaryotes and bacteria/archaea have genetic variation, but the sources differ: 1. Eukaryotes: Genetic variation primarily comes from sexual reproduction, which mixes up genes. Also, mutations during DNA replication can introduce new variations. 2. Bacteria/Archaea: They mostly reproduce asexually, so most of their genetic variation comes from mutations. They also do something cool called horizontal gene transfer, where they can swap genes with each other. 2. Hundreds of pests have evolved resistance to the pesticides used to eradicate them! Because of resistance, even though we spray larger amounts of chemicals on crops than ever before, we lose more food to pests than in the past. Below is a graph of a rodent population that evolved resistance to a 1

Name: Marina Dawoud pesticide. Please explain how natural selection eventually led to 100% of individuals in the population being resistant. Be sure to include all the conditions/steps necessary for natural selection to work ! Note that time (in years) is on the x-axis. [3] The graph shows how natural selection led to 100% resistance in the rodent population. Here's how it happened: 1.In the beginning, there was a variation in the rodent population's susceptibility to the pesticide. Some individuals were more resistant than others. 2. When the pesticide was sprayed, the susceptible rodents were killed, while the resistant ones survived. 3. The surviving resistant rodents then passed on their resistant genes to their offspring through reproduction. 4. Over time, the resistant genes became more prevalent in the population, as the susceptible individuals were eliminated. 5. As this process continued, generation after generation, eventually, all individuals in the population became resistant to the pesticide. So, through natural selection, the pesticide exerted pressure on the population, favoring the survival and reproduction of resistant individuals. This led to the evolution of 100% resistance in the rodent population. 3. Experiments during the New Synthesis showed that Natural Selection can only operate on phenotypes (i.e., can’t ‘see’ the genotype), even though an outcome of Selection is change in the frequency of genotypes from one generation to the next. Thinking about the conditions necessary for Selection: A) If there was no phenotypic variation in a population, but that phenotype was heritable, what would happen to this population over time? [1] If there was no phenotypic variation in a population, but that phenotype was heritable, the population would not undergo any significant changes over time. Since there is no variation in the phenotypes of 2

Name: Marina Dawoud individuals, natural selection would have no basis for favoring certain traits over others. As a result, the frequency of genotypes would remain relatively stable, leading to little to no evolution occurring in the population. Variation is a key driver for natural selection to act upon and drive evolutionary changes. B) What if there was phenotypic variation, with differences in performance, but the variation was not heritable? [1] If there was phenotypic variation in a population, with differences in performance, but the variation was not heritable, then the population would not experience significant evolutionary changes over time. While there may be differences in performance among individuals, if those differences are not passed on to future generations, they would not contribute to the overall genetic makeup of the population. C) What if there was heritable, phenotypic variation but no ‘struggle’ (i.e., all phenotypes perform equally well)? (hint: could other evolutionary mechanisms still operate?) [1] If there was heritable, phenotypic variation but no "struggle" or differences in performance among the phenotypes, genetic drift could still occur.While natural selection might not occur, genetic drift could still influence the evolution of the population. 4. You are working with fruit flies to study the change in allele frequency of a single allele called Mindflayer . You start with 24 populations; each has an initial frequency for Mindflayer of 0.5. (i.e., 50% of alleles carried in the population are Mindflayer ). You raise flies for 13 generations by transferring the offspring from each generation to a new vial, where they produce the next generation. In trial 1 you take a random sample of 20 flies from the population to transfer, whereas in trial 2 you take a random sample of 200 flies to transfer. After 13 generations you see the following allele frequency data: Treatment 1: 0.55, 0.6, 0.2, 0.9, 0.45, 0.35, 0.1, 0.65, 0.65. 0.55, 0.75, 0.35, 0.60 Treatment 2: 0.85, 0.8, 0.75, 0.8, 0.75, 1.0, 0.8, 0.85, 0.9, 0.8, 0.85, 0.8, 0.8 What was the main cause of the different results in the two trials? (hint: graph the frequencies vs. time [generation #] for each treatment to visualize the changes). Make sure to explain your reasoning. [2] In Treatment 1, where a random sample of 20 flies was transferred each generation, a smaller sample size can lead to greater randomness in allele frequency changes from one generation to the next. In Treatment 2, where a random sample of 200 flies was transferred each generation, a larger sample size, the randomness in allele frequency is likely to be averaged out, resulting in a smoother and more predictable change over time. 3

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