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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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Name: Marina Dawoud 5. The graph below plots survival vs. birth weight for human babies. A) ‘Percentage of survival’ could also be written as ‘offspring survival rate’ – what is this variable measureing or representing in terms of the ‘success’ of the parent generation? [1] The variable "offspring survival rate" or "percentage of survival" represents the measure of success of the parent generation in terms of the number or proportion of offspring that survive to the next generation. It indicates how well the parents are able to pass on their genetic traits and ensure the continuation of their lineage. B) Thinking about what you determined in A, which type of selection is operating? [1] (a) stabilizing (b) disruptive (c) directional (d) negative frequency-dependent C) Based on that type of selection, how would you expect the variance in birth weight to change in the next generation of this population? [1] Based on directional selection, we would expect the variance in birth weight to decrease in the next generation of this population. This is because directional selection favors individuals with a specific trait or characteristic. Individuals with other traits have reduced fitness. 6. The graphs below (Grant and Grant, 2003), document the effect of drought on populations of finches. Each graph is a histogram showing the number of individuals that have a particular beak depth. The upper graph shows the population before the drought, and the bottom graph shows the population after. 4

Name: Marina Dawoud A) Which type of selection is operating? [1] (a) stabilizing (b) disruptive (c) directional (d) negative frequency-dependent B) Now that you know the type of selection that is occurring, which trait values (in terms of beak depth) in the population would be predicted to have the highest fitness in the current environment? [1] Based on the type of selection happening, which is directional selection, the trait values of beak depth that are moving towards the direction preferred by the environment would have the highest fitness in the current environment. So, the individuals with beak depths that align with the preferred direction would have an advantage in terms of survival and reproduction. 7. Name the Big 4 mechanisms of Evolution and indicate which ones increase variation in populations and which ones decrease variation. Note which can lead to adaptation and which can generate speciation. [5] mechanism + or - Variation? Adaptation? Speciation? Natural Selection + & - when certain traits are favored by the environment yes Genetic drift - It occurs randomly and can lead to the loss of certain alleles. yes, in small populations Gene flow + Happens when Individuals from different trim different no 5

Name: Marina Dawoud populations exchange genes Mutations + can introduce new alleles and can contribute to adaptation . yes 8. A) Dr. Sime asks: Which of the following is not a behavior that likely evolved because it has a positive effect on the indirect component of inclusive fitness? [1] (a) a female wasp helps an unrelated dominant female reproduce so that she will take over the reproductive role when the dominant female dies (b) a bird remains on its natal territory, helping to rear brothers and sisters (c) a human female loses fertility in middle age, but helps in rearing grandchildren (d) All of the above would evolve through positive effects on indirect fitness B) Now, of course, explain HOW you knew! [2] All of the above behaviors have a positive effect on the indirect component of inclusive fitness. They involve individuals helping others, such as relatives or unrelated individuals, in ways that increase their overall reproductive success. So, in this case, all of the above behaviors would likely evolve through positive effects on indirect fitness. Connect and Review 9. Throw back! Explain why terminal taxa on phylogenetic trees are monophyletic, even if they represent a single species or even a single population. It may help to recall the modern definition of evolution – a change in genotype/phenotype frequency from one generation to the next – as well as the definition of lineage . [2] The reason lies in the definition of evolution as a change in genotype/phenotype frequency from one generation to the next. In phylogenetic trees, terminal taxa represent the endpoints or tips of the branches, and they are often labeled with the names of species or populations. Even if a terminal taxon represents a single species or population, it is still considered monophyletic because it represents a unique lineage that has evolved over time. The concept of a lineage is important here. A lineage represents a sequence of ancestor-descendant relationships, showing how species or populations have changed over generations. Each terminal taxon on a phylogenetic tree represents a distinct lineage that has undergone evolutionary changes, even if it's 6

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Name: Marina Dawoud just within a single species or population. So, even though a terminal taxon may represent a single species or population, it is still considered monophyletic because it represents a unique lineage that has evolved over time. 10. Imagine that you are a disease ecologist looking for additional diseases that could make the jump to humans, and you discover a new pathogen! You sequence the genomes of virus samples taken from different patients and build an evolutionary tree. The tree shows that 4 samples from humans are all closely related to viruses from mice. They are more distantly related to viruses found in possums. However, the human samples do not form a single clade. Instead, they are spread out in multiple places over the tree, with some most closely related to other human viruses, and others more closely related to mouse viruses than they are to other human samples. A) Based on that description, draw a simple tree showing the relationships. [2] B) Looking at your tree, what do you think the source of the new disease is? Why? [2] Based on the information provided, it looks like the source of the new disease could potentially be mice. Since the viruses from the human samples are closely related to viruses from mice, it suggests that there may have been a transmission event from mice to humans. C) How many times do you think the virus jumped to humans from another species? Why? [2] Hrad to conclude. D) Based on this evidence, do you think the virus is spreading from human to human? Why? [2] I think it is spreading from mice to humans. 7

Name: Marina Dawoud Bonus (Optional): Turns out that Thomas Huxley (aka. Darwin’s Bulldog) was right in criticizing his friend Charles over his view on evolution operating slowly, slowly, slowly: You have loaded yourself with an unnecessary difficulty in adopting Natura non facit saltum [Nature takes no leaps] so unreservedly . – Huxley, in letter to Darwin Explain why Huxley’s quote is true, particularly knowing what you do about modern evolutionary biology. Extra point if you can link Darwin’s view back to something he read when he developed his theory! How did Darwin get so stuck on this Latin motto? (hint: the work was about rocks, not life…) Huxley's quote, "Natura non facit saltum," which means "Nature takes no leaps," highlights the idea that evolutionary changes usually occur gradually rather than in sudden leaps. This concept aligns with our understanding of modern evolutionary biology. Darwin initially adopted this view because he was influenced by the geological work of Charles Lyell, particularly his book "Principles of Geology." Lyell's work emphasized gradual geological processes, such as erosion and sedimentation, which shaped the Earth's surface over long periods of time. Darwin applied this principle of gradual change to the biological realm, suggesting that species evolved through a slow and continuous process of natural selection. While there can be instances of rapid evolution or speciation, the overall pattern tends to be one of gradual transformation over time. 8

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