Population genetics questions (2)

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Stevenson University *

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115L

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Feb 20, 2024

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BIO 115L – E COLOGY & E VOLUTION 1 P OPULATION G ENETICS S IMULATION & M ODELING Procedure 1: Modeling Genetic Drift Copy your graph of genetic drift results here. 1. In horses, the genes for white coat color and red coat color are codominant. Heterozygotes have a light red coloration, called roan. If you located a population of wild mustangs in a valley that had 476 red horses, 323 roan horses, and 51 white horses, could you say the population is in Hardy-Weinberg equilibrium? First calculate p and q, then use the Hardy-Weinberg formula to calculate expected genotypic frequencies. 2. If we look at Rh factor genes in the United States, the dominant Rh+ allele makes up about 60% of the gene pool and the recessive Rh- allele makes up 40%. If the U.S. population were in Hardy-Weinberg equilibrium, what portion of people in the country would you expect to have Rh positive blood? What factors might result in departures from expected frequencies in this example? 3. Based on this exercise, can you explain why zoos go to so much trouble to exchange rare animals such as tigers or gorillas rather than maintaining small separate breeding populations? 4. In a classic study of blood types in Italy, Dr . Luigi Cavalli-Sforza (1969) found that small, isolated towns in mountain regions had reduced genetic diversity within populations, but showed significant genetic differences from one town to the next. In valley regions with more movement of people from place to place, he found more diversity within the population of any given town, and fewer differences between towns. If you consider your genetic simulation to represent one mountain town, and other lab groups to represent different mountain towns, can you explain Cavalli-Sforza's results? How could you alter this lab procedure to simulate what happens in the valley towns? 5. Gene frequencies for the ABO alleles vary geographically. For example the IA allele makes up nearly 50% of the genes for blood type among Australian Aborigines, about 25% in Central Asian populations, and 0% in peoples native to South America. No blood type has sufficient selective advantage over another to explain these regional differences. From these facts, what can you infer about the history of our species? Did humanity develop as a large and continuous population spreading out across the planet, or did we disperse while still living in small scattered groups? Procedure 2: Modeling Selection Copy your graph of natural selection results here. 1. Most ser i ous genetic diseases are caused by recessive alleles rather than dominant alleles. Based on this exerc i se, can you expla i n why a recessive lethal gene could persist in a population, wh il e a dominant lethal gene could not? 2. Suppose your model of selection represents the change in frequency of plain white coquina clams because predatory birds see and remove them more quickly from a beach with dark·colored sand . If larval offspring from this population drift to a different beach made of light-colored sand, could selection go the other way? What does this say about fitness of a particular gene? 3. In this simulation, you established the fitness of the aa allele as 0, removing all aa individuals from the breeding population. This may be realistic in the case of a serious genetic disease, but selection is not always so drastic. In the case of coquinas on the beach, white shells may suffer a measurable disadvantage due to higher bird predation, but their fitness is obviously not zero. Some white clams do survive to reproductive age. Suppose predation rates on white clams were not total, but did occur at twice the rate of predation on purple-rayed clams. How would you alter this procedure to simulate a 50 % survival rate of the aa genotype? How would you expect this change in the simulation rules to affect the outcome? 4. Many people incorrectly think of evolution as a completely random process. Based on the changes you observed in frequencies of alleles in this simulation, would you say natural selection is totally predictable, totally unpredictable, or something in between? Explain .

BIO 115L – E COLOGY & E VOLUTION 2 5. In this simulation, you sampled the gene pool without replacing beads in the beaker after you drew each one. Thus, f(A) and f(a) in the gene pool changed slightly after each bead was drawn. For example, if you begin with 50 light and 50 dark beads, the probability of drawing a dark bead the first time is 50/100 = 0.500 . The beaker would then contain 49 dark beads and 50 light beads, so the probability of drawing a second dark bead becomes 49/99 = 0.495. Does this make your simulation slightly less realistic? In small natural populations, does one mating change the gene pool available for the next mating, or not? What biological factors must be considered in answering this question?

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