Chapter 6
List the five conditions that must be true for a population to be in HWE. Why is it so useful to know the conditions that prevent evolution? For each condition, specify whether violation of that assumption results in changes in genotype frequencies, allele frequencies, or both.
1. there is no selection, 2. there is no mutation, 3. there is no migration, 4. there are no chance events, 5. individuals choose their mates at random; violation of these can change allele and genotype frequencies in a population; random mating doesn't change allel frequencies
16. What happens when you rear fruit flies on food with 1-6% NaCl? Explain the experiment in Fig. 6.28.
A few larvae survived to adulthood at concentrations up to 4%, but all the larvae died at 5%; in the experiment, the researchers began with a stock; continue (219)
2. What is a population?
A population is a group of interbreeding individuals and their offspring (181)
20. What causes cystic fibrosis? What is the function of the protein CFTR? How common is cystic fibrosis? How can cystic fibrosis be so common if for most of human history individuals infected did not survive to reproductive age? Discuss the findings in Fig. 6.31.
Cystic fibrosis is caused by recessive loss-of-function mutations in a locus on chromosome 7 that encodes the protein CFTR; one of CFTR's key functions is to enable cells of the lung lining to ingest and destroy Pseudomonas aeruginosa bacteria; the alleles that cause cystic fibrosis have a collective frequency of approximately 2% among people of European ancestry; they think that maybe it is maintained by mutation-selection balance; homozygous F508 cells were almost totally resistant to infiltration by S. typhi, while homozygous wild-type cells were highly vulnerable. Heterozygous cells were partially resistant because they accumulated 86% fewer bacteria than did the wild-type cells (223-224)
18. How can deleterious alleles persist in a population?
Deleterious alleles persist because they are continually created anew. When the rate at which copies of a deleterious allele are being eliminated by selection is exactly equal to the rate at which new copies are being created by mutation, the frequency of the allele is at equilibrium (221)
14. To Henry Goddard, what was the genetic basis for 'feeblemindedness'? Besides the obvious ethical issues, what were three scientific problems with Goddard's arguments in support of eugenics?
Feeblemindedness was recessive; first, the individuals whose case studies he reports are a highly diverse group. Second, his methods for collecting data were prone to distortion, and third, his method of analysis stacked the cards in favor of his conclusion (215-216)
3. How is Fig. 6.6a a Punnett square? How does Fig. 6.6a differ from a traditional Punnett square?
Has the gametes listed along the top and left side and then shows all of the combinations/offspring we can get; has a lot more gametes and combinations because we are looking at a lot more individuals than we would see in a single-family Punnett square (184)
Black color in horses is governed primarily by a recessive allele at the A locus. AA and Aa horses are nonblack colors such as bay, while aa horses are black all over. In an online conversation, one person asked why there are relatively few black horses of the Arabian breed. One response was, "Black is a rare color because it is recessive. More Arabians are bay or gray because those colors are dominant." Discuss the merits and/or problems with this argument. Generally, what does the HW model show us about the impact that an allele's dominance or recessiveness has on its frequency?
Horses with the dominant allele is not always going to be the most dominant/more common horse
10. How do the data in Fig. 6.21 compare to the data in Fig. 6.19? How were the experiments similar? How could the results differ?
In both, the evolution is rapid at first and then slows as the experiment proceeds, both started with flies
8. What happened to the frequency of the l allele when you start with +/l heterozygote fruit flies (see Fig. 6.19)? How did the experimental results compare to the predicted results? Is this surprising?
Individuals with genotype l/l do not survive, because l/l individuals have zero fitness, Dawson expected his populations to evolve toward ever lower frequencies of the l allele; the experimental results match the theoretical predictions closely; it is astonishing (203); if start with heterozygotes, frequency of lethal allele dropped
21. Explain how the artificial Medea gene affects fruit fly offspring. If we can put a man on the moon, what can't we predict the evolution of fruit flies?
Medea includes two sets of instructions. One causes mothers that carry the element to infuse their eggs with a poison. The other allows embryos that carry the element to make an antidote. If mother carries the gene but the baby does not, the baby dies; (224-)
15. Does mutation alone cause significant changes to allele frequency?
Mutation alone is generally not a rapid mechanism of evolution, in the single generation in Fig 6.26, mutation produced no change in allele frequency (217)
17. What is the ultimate source of genetic variation? Why?
Mutation; it is critical for providing raw material to be further modified and selected for (219)
9. Do dominant alleles increase fitness? Are dominant alleles the most common?
No; No, it is pretty even. When a recessive allele is common, evolution by natural selection is rapid. When a dominant allele is common, evolution by natural selection is slow (205)
Do question 6 on page 228
OKAY
How can a theory be evaluated?
Scientists evaluate theories by using them to make predictions, then checking whether the predictions come true (180)
12. How are the two colors of Elderflower orchids maintained? How did the data shown in Fig. 6.24 help support the hypothesis?
Selection by bumblebees favors yellow until it becomes too common, then it favors purple; shows how the yellow-flowered orchids enjoyed a higher reproductive success than the purple-flowered plants when the yellow was rare and suffered lower reproductive success when the yellow orchids were more common (213)
19. What is spinal muscular atrophy? How common is the disease? Why is the disease this common? Why is the disease not selected against?
Spinal muscular atrophy is a neurodegenerative disease characterized by weakness and wasting of the muscles that control voluntary movement; it is the second most common lethal autosomal recessive disease in Caucasians; the loss-of function alleles have a frequency of about 0.01 in the Caucasian population and the researchers estimate that the selection coefficient is about 0.09; they are, it states that there is a strong selection against them (221)
5. What are the assumptions of the HWE? Are these assumptions ever met in nature? Then, what use is the HWE? Ok, what important information have you obtained based on the assumptions of the HWE?
The assumptions are that there is no selection, there is no mutation, there is no migration, there are no chance events, and individuals choose their mates at random; these assumptions are not really ever met in nature, it would be rare; the use is that it rests on a specific set of simple assumptions so when one or more of these assumptions is violated, the HWE conclusions no longer hold; if a population is not following HWE, then it is not evolving; IMPORTANT PART OF CHAPTER: species evolve by the opposite of these assumptions (189-190)
Most animal populations have a 50:50 ratio of males to females. This does not have to be so; it is theoretically possible for parents to produce predominantly male offspring or predominantly female offspring. Imagine a population with a male-biased sex ratio, say, 70% males and 30% females. Which sex will have an easier time finding a mate? As a result, which sex will probably have higher average fitness? Which parents will have higher fitness --- those that produce mostly males or those that produce mostly females? Now imagine the same population with a female-biased sex ratio, and answer the same questions. What sort of selection is probably maintaining the 50:50 sex ration seen in most populations?
The female will probably be able to find a mate easier if there are less females; the females will have a higher average fitness; the parents that produce mostly females; when males are less common, the males will find mates easier; the males will have the higher average fitness; the parents that produce mostly males; this is all an example of frequency-dependent selection
11. How does Fig. 6.23 (Foster et al. fly experiment) show heterozygote inferiority?
The heterozygotes are represented by the lower lines in the graphs, showing inferiority (210)
13. What was the eugenics sterilization program? Were individuals sterilized in Nebraska as part of eugenics? (see https://www.uvm.edu/~lkaelber/eugenics/NE/NE.html)
The proponents of eugenic sterilization sought to reduce the fitness of particular genotypes to zero and thereby to reduce the frequency of alleles responsible for undesirable phenotypes; yes, 902 individuals were sterilized (214)
6. Why is Fig. 6.14 an example of selection? Why did the researchers examine the Adh genes? Do the researchers know that the population is evolving due to selection?
The two researchers set up different environments for their different populations, but they did not themselves directly manipulate the survival or reproductive success of individual flies; so that they could determine their ADH genotypes and it was easy for them to manipulate; no, they can tell that the HWE conclusion 1 holds true in the control populations but is not valid in the experimental populations. They do not know whether flies with the Adh F allele have higher fitness because they have higher rates of survival or because they produce more offspring (195)
4. In the Hardy-Weinberg equilibrium (HWE) for two alleles, why does p +q = 1? What is the HWE for 2 alleles? What do each of the components of the HWE represent? What are the two conclusions of the HWE principle?
There are only 2 alleles and they have to add up to one; p2 + 2pq + q2 = 1; they represent the genotypes; Conclusion 1: the allele frequencies in a population will not change, generation after generation. Conclusion 2: if the allele frequencies in a population are given by p and q, the genotype frequencies will be given by p2, 2pq, and q2 (187-189)
7. What evidence was used by researchers to determine SL infants are selected for if their mother contracts malaria?
They calculated the genotype frequences expected under HWE and the expected frequencis among the SL infants were the highest (.50). Also the expected number of infants of each genotype under HWE was highest (38) (198) They violated the HWE, heterozygotes more dominant in malarial condition and they had a potential advantage which is shown in HWE
