Mechanisms of Evolution and Speciation

How can changes in a population result in the formation of a new species? Directions: 1. Have 1 member of your team make a copy of this file, adjust the share settings so that it is “editable” by anyone with the link, and SHARE the link with the rest of the group in the chat. *If you have any concerns about your group, let me know ASAP. 2. Each group member will choose a font color to respond in. Each person should write his/her name in their chosen font color here: 3. As your team reads the passages, highlight any important info. 4. As you read each question aloud, DISCUSS what the question is asking (or what you need clarification on), BRAINSTORM potential ideas for each question & GENERATE the best potential response as a team. Each team member should alternate writing responses to each question (in your corresponding font color). a. Many questions say “JUSTIFY”-- this means to explain in complete sentences with supporting evidence & reasoning. b. Remember, as this is a shared document, I am able to see who wrote what to verify that everyone is contributing (but color coding makes it easier for YOU all to see it too). 5. DO NOT SKIP QUESTIONS. If you all get stuck & can’t figure it out-- send a team member to the mainroom to ask me clarifying questions. 6. When you reach a STOP, come check in with me before moving on. 7. Your team member will turn this in for the team when your group is done. The rest of the team will “mark as done.” Why? Have you ever wondered how the great diversity of life on Earth has come about or how a single new species forms? Environmental pressures may cause populations to change over time or evolve. This is because an organism’s ability to live to adulthood in its current environment will determine its reproductive success and ability to pass on its genes. But changes within a population can occur without creating a new species. At what point do scientists start thinking of a new name for a species? 1. What variables do the graphs in Model 1 compare? Body size in grams 2. What are the three types of selection illustrated in the graphs in Model 1 Directional, Disruptive, and Stabilizing Selection 3. According to the graphs in Model 1, there is variation in the body mass in the original population. Using your knowledge of genetics, describe how this variation is possible. This variation is possible because of the different types of selection 4. Refer to graph A of Model 1. a. How is the population that has experienced selection different from the original population? The population that has experienced selection is different because their average body size has shifted, having more individuals weighing between 100 and 125 grams. b. Fitness is defined as the relative ability of an individual (or population) to survive, reproduce, and pass on genes. Which individuals in the original population appear to display better fitness? Body size 100 has the best fitness in graph A. c. As a group, propose some characteristics of the environment that could lead to the population changes illustrated in graph A. Good amount of food available 5. Refer to graph B of Model 1. a. How is the population that has experienced selection different from the original population? It differs because there is now more diversity/variety in body size because instead of one peak, there are two. b. Which individuals in the original population appear to display better fitness? Body size 37.5 and 100 appear to have the best fitness. c. As a group, propose some characteristics of the environment that could lead to the population changes illustrated in graph B. Those in the middle size range don’t do as well since they aren’t small enough or large enough to benefit from their environment’s shelter or hunting options 6. Refer to graph C of Model 1. a. How is the population that has experienced selection different from the original population? The population’s body size has narrowed down, making more individuals weigh at the key size of 75 grams. b. Which individuals in the original population appear to display better fitness? Body size 75 displays better fitness. c. As a group, propose some characteristics of the environment that could lead to the population changes illustrated in graph C. Small amounts of food available 7. As a group, define the following terms in grammatically correct sentences. Each definition should contain the following words: population, selection, fitness, and environment. a. Directional selection The population’s selection shifts from either increasing or decreasing due to having an easier or harder environment, which as a result, would increase or decrease the species’ fitness. b. Disruptive selection Disruptive selection is a natural selection that selects the average of both extremes. The environment of the selection determines the fitness of the population and results in the two extremes which consists of most individuals. c. Stabilizing selection Stabilizing selection is a descriptive term for what happens to an individual trait when the extremes of the trait are selected against in a population Check in with the instructor before moving on. 8. In each of the following examples, describe the likely outcome due to the environmental pressure and state the type of selection. Justify your choice. a. Finches with a small beak cannot crack open seeds. More finches will have bigger beaks in the future since smaller beaked finches are less likely to survive and produce offspring. b. Human babies with very high or very low birth weights have a higher mortality rate. Many living babies will have an average weight. I think it is stabilizing selection. c. A population of seed-cracker finches feeds on seeds available in two sizes, small or large. The seed cracker finches will evolve to have two shapes of beak, a small one to eat the small seeds and a big beak for the large seeds d. Overfishing occurs in two rivers in British Columbia, Canada, where larger salmon are preferentially caught. Because larger salmon are being killed, smaller salmon will increase within the population. Read This! Natural selection, the improved fitness of certain individuals in the population that allows for survival and reproduction, is the primary mechanism by which populations change over time. Other mechanisms include the introduction of a mutation in the population and artificial selection (the effect of humans purposefully breeding animals to select for desirable traits). But at what point does a change in the population develop a new species? Data has been collected showing that the average height of an American adult is increasing, but that does not mean the American population has evolved into a new species. 9. Refer to Model 2. Identify the pairs of organisms that are able to produce offspring. Organisms A & B, C & D, E & F 10. Which pair of organisms in Model 2 are members of the same species? Organisms A and B are members of the same species 11. Consider all of the pairs of organisms in Model 2 that are not of the same species. What criteria are missing in all cases that could be used to define a species? Physical traits, ability to reproduce, location Read This! The primary criteria for animals to be classified as different species is that there must be reproductive isolation, meaning for some reason organisms from the two populations cannot pass on their genetic code through reproduction for several generations. Other criteria such as differing morphology (appearance and body structure) and how much DNA the organisms share are also used to make a final determination when comparing two similar organisms. 12. A common farming practice is to breed a female horse with a male donkey. The result is a very robust animal – the mule. Most mules however are sterile, and therefore cannot reproduce. Are horses and donkeys members of the same species? Justify your answer with a specific example from Model 2. No they are not members of the same species as they are an example of model species C & D where mating is possible but the offspring cannot reproduce. 13. Many species of birds have elaborate mating rituals that include bird calls, nest construction, and courtship displays. A researcher is comparing two populations of birds with similar morphology that live in similar niches. Male birds in one population build a nest before attempting to court a female, while males in the other population build the nest in cooperation with the female. Is it likely the researcher will classify these birds as the same species? Justify your reasoning. Yes, because if they have similar mating rituals, it is likely that both the males and females will mate with each other and produce offspring. 14. Could directional selection lead to the creation of a new species? Justify your reasoning using what you’ve learned from Models 1 and 2. Yes because this shift of favored phenotypes could be cause by geological isolation which will also lead to them having different food sources and these are the signs of a different species 15. Could disruptive selection lead to the creation of a new species? Justify your reasoning using what you’ve learned from Model 2. Yes, disruptive selection could lead to the creation of a new species because if the two separate peaks decrease in mating with each other, they could evolve differently. Thus, over time, not being able to produce viable offspring with each other. 16. Could stabilizing selection lead to the creation of a new species? Justify your reasoning using what you’ve learned from Model 2. No, because the majority of the original population is still the majority of the new population. Check in with instructor before moving on. Extension Questions 17. Refer to Model 3. Fill in the table below with the number of alleles of each type in each generation. 1st Generation 2nd Generation 3rd Generation 4th Generation Light 4 2 1 0 Medium 4 5 5 6 Dark 4 5 6 6 18. Compare the frequency of the three alleles in the 1st generation of the small population illustrated in Model 3. The frequency of the three alleles in the 1st generation of the small population are equal. There is the same amount of light, medium and dark alleles in the population. 19. In the scenario illustrated in Model 3 two of the light alleles were lost (through the death of the individual who had these two alleles) before reproduction occurred in the 1st generation. How did this affect the distribution of the three alleles in this small population? It caused the frequency of the light allele to decrease, so in the fourth generation there were none. 20. The phenomenon illustrated in Model 3 is called genetic drift. It mainly occurs in small, isolated populations. Propose an explanation for why the light allele disappears from the population by the fourth generation. The light allele provides no benefit for the species, and since they live in small and isolated populations, there isn’t much genetic variation causing certain alleles to “die out” through the generations 21. Will the light allele ever reappear in this population? If yes, describe the circumstances that would need to occur for the light allele to reappear. No, because the light allele populations have died out in the 4th generation making it impossible for them to reappear. 22. If the population in Model 3 had been very large (hundreds of organisms), would the loss of two alleles from that population have led to the disappearance of that allele? Justify your reasoning. No, because there would be higher allele frequency. Thus, a the chance of its disappearance would be very unlikely.