CHAPTER 6 POPULATION GENETICS SELECTION
1. Which of the following options factually completes the statement, "If a population is in Hardy-Weinberg equilibrium..."?

a. There can be no more than two alleles.

b. The two alleles will be present at equal frequency.

c. Allele frequencies will not change from one generation to the next.

d. The dominant allele will be more common.

|Correct Answer: |C, Allele frequencies will not change from one generation to the next. |

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2. If allele frequencies do not change from one generation to the next, is the population definitely in Hardy-Weinberg equilibrium? Why or why not? No, it might not be in Hardy-Weinberg equilibrium. Any process that selectively targets heterozygotes can affect genotype frequencies without necessarily changing allele frequencies in the next generation. Examples are nonrandom mating, overdominance, and underdominance.
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3. The Hardy-Weinberg equilibrium principle yields which of the following conclusions?
a. If the allele frequencies in a population are given by p and q, the genotype frequencies are given by p2, 2 pq and q2.

b. The allele frequencies in a population will not change over time.

c. If the allele frequencies in a population are given by p and q, the genotype frequencies are given by p2 and q2. d. The first and third answers are correct.

e. The first and second choices are correct.

|Correct Answer: | E, The first and third answers are correct. | |

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4.
Briefly describe all processes that can affect allele frequencies.
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The processes are mutation, which introduces new alleles into the population; selection, which occurs when genotypes have different reproductive success; migration, the movement of new individuals into or out of the population; and genetic drift, the random change in allele frequencies due to chance. (A fifth process, nonrandom mating, can affect genotype frequencies but will not affect allele frequencies unless it is accompanied by selection, e.g. nonrandom mating that results in certain individuals attracting more mates.)
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5. Match the key terms in this chapter listed below with the phrase that is the best match for it.
|[pic] |[pic] |Option |Your Answer |Correct Answer |
|5.1 |[pic] |Genetic drift |A. A state in which genotype |E. Random changes in allele frequencies that |
| | | |frequencies are given by p2, 2pq, |occur due to chance |
| | | |and q2 | |
|5.2 |[pic] |Selection (can be natural or |A. A state in which genotype |D. Nonrandom changes in allele frequencies |
| | |artificial) |frequencies are given by p2, 2pq, |that occur due to differing reproductive |
| | | |and q2 |success |
|5.3 |[pic] |Hardy-Weinberg equilibrium |A. A state in which genotype |A. A state in which genotype frequencies are |
| | | |frequencies are given by p2, 2pq, |given by p2, 2pq, and q2 |
| | | |and q2 | |
|5.4 |[pic] |Gene pool |A. A state in which genotype |F. The total of all copies of all genes in a |
| | | |frequencies are given by p2, 2pq, |population |
| | | |and q2 | |
|5.5 |[pic] |Overdominance, or heterozygote |A. A state in which genotype |B. Selection that favors heterozygotes |
| | |superiority |frequencies are given by p2, 2pq, | |
| | | |and q2 | |
|5.6 |[pic] |Underdominance |A. A state in which genotype |C. Selection that favors homozygotes |
| | | |frequencies are given by p2, 2pq, | |
| | | |and q2 | |

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6. The HIV epidemic is unlikely to lead to an increase in the CCR5 delta-32 allele over the short term because ___________________.
a. In populations with high selection pressure, the allele frequency is low.

b. the allele is recessive and deleterious.

c. the allele is recessive and deleterious.

d. The first and second choices are correct.

e. in populations with a high frequency of the allele, selection pressure is relatively low.

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|Correct Answer: | D, The first and second choices are correct. | |

Selection will eventually cause an advantageous allele to become more common, but it's a slow process. For selection to cause a rapid increase in an allele's frequency, the selection must be strong and the allele must already be somewhat common. The delta-32 allele of CCR5 is most common in European populations, but HIV infection is low there. Selection due to HIV is strongest in Africa, but the delta-32 allele is very rare there.
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7. Eastern grey squirrels (the common urban squirrel of most of the United States) sometimes can be completely black. Black squirrels usually are seen in certain isolated squirrel populations in city parks of the northeastern U.S., and are particularly common around the campus of Princeton University, in New Jersey. Black color appears to be caused by a single dominent allele, and squirrel litters can include both black and grey pups.
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In 1986, 1987, and 1994, three different Princeton biology classes walked the campus and counted the number of black and grey squirrels. Here are their results. (Data from Ken Nicholson, Woodrow Wilson Biology Institute, 1994).

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Part I: Referring to the information about Eastern grey squirrels, calculate the percentage of black squirrels in each year. Have phenotype frequencies changed over time in this population, and if so, can you tell why? Can you determine if the population is in, or close to, Hardy-Weinberg equilibrium?

Part II: Referring again to the information about Eastern grey squirrels, assuming the population is in Hardy-Weinberg equilibrium, calculate the frequencies of the black and grey alleles in each year.
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Part I: The observed frequency of black squirrels was 16% in 1986, 26% in 1987, and 17% in 1994. The data show fluctuation in black squirrel frequency, but it is unclear whether this fluctuation is "real" - it may be due to small sample size or observational technique, and if real, there is not enough information to tell whether drift, selection, or migration, or nonrandom mating are the causes. (Mutation is unlikely to produce allele frequency changes this rapidly.) Since black squirrels may be homozygotes or heterozygotes, we cannot determine actual genotype frequencies, and thus we cannot determine if the population is in Hardy-Weinberg equilibrium.

Part II: If the population is in Hardy-Weinberg equilibrium, genotype frequencies can be calculated by knowing that the frequency of grey squirrels will equal q2, solving for q, and then calculating p = 1-q. Frequencies for the grey allele and black allele were:

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Black coat color in squirrels is theorized to be advantageous in cold climates due to greater absorption of solar radiation, but disadvantageous where natural predators are numerous due to the conspicuousness of black squirrels. This may explain the greater frequency of the black color morph in northeastern city parks, which have cold winters but few natural predators.
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8. Over the long term, selection favoring the rare phenotype in a polymorphic population (i.e., negative frequency dependent selection), will __________ genetic diversity in the population.
a. decrease
b. increase
c. maintain
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|Correct Answer: |C, maintain | |

Negative frequency-dependent selection maintains rare alleles in the gene pool. It can even keep alleles that are slightly deleterious in the population, if the alleles gain a strong advantage when their phenotype becomes rare. (Remember that another process that can maintain rare or deleterious alleles is heterozygote superiority.)
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9. Which of these statements is true for underdominance?
a. Allele frequencies will tend to move towards fixation or loss.
b. Allele frequencies will tend to move toward a stable equilibrium.
c. Allele frequencies may initially hover at an unstable equilibrium, but will eventually change.
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|Correct Answers: |Allele frequencies may initially hover at an unstable equilibrium, but will eventually change. | |
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| |Allele frequencies will tend to move towards fixation or loss. | |

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10. When selection acts against a recessive allele that is initially at high frequency in a population, ____________________.
a. the frequency of the allele will stay high for a long time, then decline slowly.
b. the allele declines in frequency until it is eliminated.
c. the frequency of the allele will be unchanged;
b. the population will remain in Hardy-Weinberg equilibrium. the frequency of the allele will decline rapidly, and then stabilize at very low frequency.
|Correct Answer: |the frequency of the allele will decline rapidly, and then stabilize at very low frequency. |

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11. When selection favors heterozygotes over homozygotes, ____________________.
a. both alleles are maintained at a frequency different from that predicted by Hardy-Weinberg principles based on the strength of selection against the recessive allele.
b. the selective advantage enjoyed by the deleterious allele in the heterozygote exactly
c. genetic diversity in the population can be maintained indefinitely, in spite of selection acting against the allele. balances the selective disadvantage suffered by homozygous recessive individuals.
d. The first two choices are both correct.
e. The first three choices are correct.
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|Correct Answer: |The first three choices are correct. | |

This phenomenon is called overdominance or heterozygote superiority, and it can maintain deleterious alleles in populations. It may explain the puzzle of why natural populations have surprisingly high genetic diversity.
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12. When selection favors homozygotes over heterozygotes ____________________.
a. genetic variation among populations will decline. [pic]
b. the most common outcome will be that the most common allele will become fixed (will increase to a frequency of 1) in the population. [pic]
c. both alleles are maintained in the population at frequencies different from those predicted by Hardy-Weinberg principles. [pic]
d. the genetic variation within a population is maintained over time
|Correct Answer: |the most common outcome will be that the most common allele will become fixed (will increase to a frequency of 1) in | |
| |the population. | |

This is a case of underdominance. Allele frequencies may initially hover at an unstable equillibrium, but inevitably, sooner or later, it will drift from that equilibrium. As soon as it starts to drift, the rarer allele will be found more frequently in heterozygotes, where it will be selected against, and it will quickly be lost.
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13. Pier and his colleagues have suggested that cystic fibrosis is maintained at relatively high levels in people of European ancestry because of heterozygote superiority; specifically, he proposposes that heterozygotes are more resistant to typhoid fever than are the dominant homozygotes. Evidence in favor of this hypothesis includes which of the following findings?
a. The mutation rate for new loss-of-function mutations in the CFTR gene is too low for the prevalence of the disease to be explained by mutation/selection balance. [pic]
b. Pier and his colleagues have found, in 11 European countries, an association between the severity of typhoid fever outbreaks and the frequency of the delta-F508 allele (the most common loss-of-function mutation) a generation later. [pic]
c. Pier et al. engineered cells homozygous for functional CFTR alleles, homozygous for a common loss-of-function allele, and heterozygous for the two. The loss-of-function homozygotes were virtually impervious to invasion by typhoid fever-causing bacteria; heterozygotes were more vulnerable, but accumulated 86% fewer bacteria than did the dominant homozygotes. [pic]
d. All of the choices above are correct. [pic]
e. Salmonella typhi bacteria manipulate their host cells, causing them to express more CFTR protein on their membranes.
|Correct Answer: |All of the choices above are correct. | |

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14. According to the basic model of mutation/selection balance, if selection is strong and mutation rate is low, the equilibrium frequency of a deleterious allele will be __________; when selection is weak and the mutation rate is high, the frequency of the allele will be relatively __________. low/low [pic] low/high [pic] high/low [pic] high/high
|Correct Answer: |low/high |

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15. Which of the following statements about mutation-selection balance is true?
A model that predicts an equilibrium frequency that is equal to the square root of selection strength over mutation rate. [pic]
The process that currently maintains the high frequency of the cystic fibrosis phenotype. [pic]
The process that maintains two flower colors in elderflower populations. [pic]
Positive selection of an allele at the same rate that it is introduced by mutation. [pic] A model that only applies to deleterious alleles.
|Your Answer: |The process that currently maintains the high frequency of the cystic fibrosis phenotype. | |
|Correct Answer: |A model that only applies to deleterious alleles. | |

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16. What are compound chromosomes, and how do they affect fertility? Why are they uniquely suited for studies of underdominance?
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Compound chromosomes are chromosomes that have swapped entire arms. Animals with compound chromosomes can produce fertile offspring only if they mate with other individuals carrying the same compound chromosome (and even then, 3/4 of zygotes are inviable). Therefore, a population in which some individuals have one kind of compound chromosome, and some have a different compound chromosome, will be analogous to a case of underdominance, because some purebred zygotes (analogous to homozygotes) will survive, but all hybrid zygotes (analogous to heterozygotes) will die.
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17. Three of the following conditions are necessary for selection of one phenotype over another to lead to evolutionary change. Which statement is not?
|The phenotypes differ in reproductive success. |[|
| |p|
|[pic] |i|
|Selection is strong enough to counteract any opposing effects of drift, migration, and mutation. |c|
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|The phenotypes are not affected by environment. | |
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|The phenotypes are caused by heritable variation in genotype. | |
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|[pic] | |[|
| | |p|
| | |i|
| | |c|
| | |]|

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|Correct Answer: |The phenotypes are not affected by environment. | |

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18. Describe at least two major biological flaws in logic of the forced sterilization program for eliminating feeblemindedness.
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Two of the biggest flaws were, first, "feeblemindedness" had a very strong environmental component and was not clearly related to heritable genetic variation as had been proposed; the estimates of heritability were biased due to circular reasoning in data collection. Second, even if "feeblemindedness" were mostly due to a rare recessive allele, even strong selection would not appreciably reduce the frequency of the allele.
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19. There are two common alleles for the human muscle enzyme ACE (angiotensin-converting enzyme)-a shorter D allele, and a longer I allele that has an insertion of 287 base pairs.The ACE coded by the I allele has lower activity, but it is also associated with superior muscular performance after physical training. One study (Williams et al. 2000) of 35 II and 23 DD men found that though they didn't differ in muscular efficiency before training, after 11 weeks of aerobic training, the II homozygotes had 8% greater muscular efficiency.The I allele is also associated with greater endurance and greater muscular growth after strength training.

Speculate on why the D allele still remains at relatively high frequency in the human population. How could you test your ideas?
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There are many possible answers. The I allele might have some costs, such as (say) lowered speed of muscle contraction; the D allele might have unknown benefits; there might be heterozygote superiority. Also, genetic drift, mutation or migration might have caused the D allele to be at higher frequency than would be predicted by selection alone, particularly since the strength advantage for II homozygotes appears relatively minor. Some obvious research paths are: testing other measures of fitness; testing heterozygotes; investigating mutation rate; examining allele frequencies in various different human populations.
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20. Consider what makes a new mutant allele dominant or recessive. To guide your thinking, imagine an enzyme that changes substance A to substance B. If B is a nutrient that is needed only in minimal amounts, will a loss-of-function mutation be dominant or recessive? If A is an abundant toxin that must be entirely broken down, will a loss-of-function mutation be dominant or recessive? How about a new mutant allele that results in a form of the protein that can catalyze an entirely new reaction (say, from A to new substance C?) Can you think of other examples of protein function that will affect whether a new allele is dominant or recessive?
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Whether an allele is dominant or recessive depends on what happens in heterozygotes, which in turn often depends on whether the dose of normal gene product is more important than its mere presence. Consider a heterozygote with one normal allele and one loss-of-function allele. The heterozygote's cells will only produce half of the normal amount of gene product. If the only thing that matters for the trait is whether the gene product is produced at all, then the heterozygote will appear normal - the normal allele will be dominant over loss-of-function allele. But if the amount of the gene product is very important, then the heterozygote will have an altered phenotype, because it is only making half the gene product. in this case, loss-of-function alleles will be either dominant or codominant. In the examples given, a loss-of-function allele will usually be recessive for production of a minimally needed nutrient; dominant for breaking down an abundant toxin; and dominant for catalyzing a new reaction.
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21. West Nile virus is an emerging disease that has rapidly spread across North America in recent years, infecting birds, humans and horses. It can cause severe flu-like symptoms and fatal encephalitis in humans. Glass and colleageus recently sequenced CCR5 genotypes in several hundred West Nile virus patients in Arizona and Colorado, and compared them to another group of patients that also had flu-like illnesses, but did not have West Nile. Here are the distributions of genotypes in patients with and without West Nile:
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Part I: Referring to the West Nile virus information given above, what was the frequency of the CCR5-delta32 allele in both of these patient cohorts? The CCR5-delta32 allele is at Hardy-Weinberg equilibrium in the North American population at large. Does either patient population appear to be in Hardy-Weinberg equilibrium? (You may assess your data "by eye," without statistical tests, if you wish.)

Part II: The percentage of CCR5-delta homozygotes in North America is usually 1%. How does this compare to both populations?

Part III: Propose an explanation for the data. How does this compare with the situation of CCR5-delta in HIV susceptibility? What are the selective forces of the two diseases on this locus?
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Part I: Genotype frequencies in the West Nile population were 0.88 for the normal CCR5 allele, and 0.12 for the delta-32 allele. For the control population, frequencies were 0.93 for the normal CCR5 allele, and 0.07 for the delta-32 allele. The West Nile patients appear to not be in Hardy-Weinberg equilibrium (actual percentages 0.80, 0.16, 0.04; compared to expected Hardy-Weinberg genotype frequencies of 0.77, 0.21, and 0.01. (The deviation from Hardy-Weinberg equilibrium is statistically significant.) The control patients are in virtually perfect Hardy-Weinberg equilibrium (frequencies of 0.86, 0.13, and 0.01).

Reference: Glass, W.G., D.H. McDermott, J.K. Lim, S. Lekhong, S.F. Yu, W.A. Frank, J. Pape, R.C. Cheshier, and P.M. Murphy. 2006. CCR5 deficiency increases risk of symptomatic West Nile virus infection. J. Exp. Med. 203(1):35-40.

Part II: CCR5-delta32 homozygotes are present at the normal frequency in the control patients, but are nearly four times more common than usual in the West Nile patients.

Reference: Glass, W.G., D.H. McDermott, J.K. Lim, S. Lekhong, S.F. Yu, W.A. Frank, J. Pape, R.C. Cheshier, and P.M. Murphy. 2006. CCR5 deficiency increases risk of symptomatic West Nile virus infection. J. Exp. Med. 203(1):35-40.

Part III: Several explanations are possible. CCR5-delta32 is suspected to increase susceptibility to West Nile virus infection, and/or increase risk of progression to the severe form of the disease. (It also increased risk of mortality in this study.) This is in sharp contrast to the situation in HIV, in which CCR5-delta32 confers protection against infection by HThe two diseases are exerting opposing selection on the CCR5 locus, West Nile selecting against the delta32 allele, and HIV selecting for it. The opposing effects of these two diseases make more sense if we consider West Nile's type of viral attack is quite typical for viruses, while HIV is very unusual in its specific targeting of immune cells. In other words, West Nile is the kind of virus that the immune system probably evolved to defend against, but HIV is not.

Alert readers will also have noticed a deficit of heterozygotes in the West Nile group. It is possible that there might be a heterozygote advantage.

Reference: Glass, W.G., D.H. McDermott, J.K. Lim, S. Lekhong, S.F. Yu, W.A. Frank, J. Pape, R.C. Cheshier, and P.M. Murphy. 2006. CCR5 deficiency increases risk of symptomatic West Nile virus infection. J. Exp. Med. 203(1):35-40.
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22. Left-handedness in humans is associated with a variety of subtle health and fitness costs, yet remains at high frequency in some human populations. Left-handers often have an advantage in baseball, tennis, fencing and other sports in which pairs of individuals face off against each other. Since most players train against right-handers, they are less able to defend themselves against a left-handed opponent. Based on this phenomenon in sports, it has been proposed that left-handedness may be maintained in human populations due to an advantage in hand-to-hand combat.

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Part I: What type of selection is being proposed?
Part II: Geneticists Charlotte Faurie and Michel Raymond recently studied the frequency of left-handers in traditional societies, compared to the level of "violence" in those societies. They used homicide rate as an index of violence. The figure below shows their data. Are the data consistent with the hypothesis that left-handedness gives an advantage in combat?

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Part III: Suppose the combat hypothesis is true. In extremely violent societies, would you predict that left-handedness would increase to fixation?

Part IV: If left-handedness had no costs, would you expect left-handedness to change over time in any of these societies? Explain.
|Your Answer: |
|v |

Part I: frequency-dependent selection

Source: Faurie C, Raymond M. 2004. Handedness, homicide, and negative frequency-dependent selection. Proc. R. Soc. B. 272:25-28.

Part II: The data are consistent with the combat hypothesis.

Source: Faurie C, Raymond M. 2004. Handedness, homicide, and negative frequency-dependent selection. Proc. R. Soc. B. 272:25-28.

Part III: No. First, left-handers would lose their combat advantage if the frequency of left-handedness rose above 0.50. In addition, recall that left-handedness also carries several costs that right-handedness does not carry. This should keep left-handedness below 0.50, even in a population with a large amount of hand-to-hand combat. (The Yanomamo may represent the maximum frequency for left-handedness.)

Source: Faurie C, Raymond M. 2004. Handedness, homicide, and negative frequency-dependent selection. Proc. R. Soc. B. 272:25-28.

Part IV: If left-handedness carried no costs, we would expect left-handedness to increase to 0.50 (half the population) in all societies. This should occur more quickly in the more violent societies. Once at 0.50, left-handedness and right-handedness should remain at equal frequency. (Any time that one phenotype declined below 0.50, it would begin to enjoy a selective advantage in combat, and should come back up to 0.50 again.)

CHAPTER SEVEN: MIGRATION AND DRIFT

1. In evolutionary biology, migration is synonymous with _____________.

a. gene flow

b. evolution

c. selection

d. mutation

|Correct Answer: |A gene flow |

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2. Banded water snakes migrate from the mainland of Lake Eerie to various islands. Unbanded water snakes persist at relatively high frequency on the islands in spite of migration because _____________.
a. as juveniles, unbanded morphs have higher survivorship on the islands than do banded morphs
b. heterosis
c. negative frequency-dependent selection
d. mutation/selection balance maintains polymorphism in island populations.
|Correct Answer: |as juveniles, unbanded morphs have higher survivorship on the islands than do banded morphs | |

The effect of selection against banded water snakes on the islands is balanced by constant migration of banded water snakes from the mainland.
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3. If migration proceeds unopposed by any other evolutionary process, the result will be _____________.
a. homogenization of allele frequencies among populations.
b. the loss of one or more alleles in one or more population.
c. an increase in genetic diversity among populations
d. an increase in the frequency of homozygotes in all populations
|Correct answer: |A homogenization of allele frequencies among populations |

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4. Goules and Goudet supported their hypothesis that migration (gene flow) in red bladder campions decreased genetic variation among populations by showing that genetic variation was lowest in the _____________ populations.
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a. intermediate aged
b. older
c. younger
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|Correct Answer: | A intermediate-aged | |

The intermediate-aged populations have had enough time for migration to homogenize allele frequencies across populations. Younger and older populations had fewer individuals, and were dominated by genetic drift.
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5. Which of the following statements regarding the founder effect is false?
a. Explains the low frequency of genetic diseases in some island populations of humans. b. Tends to reduce genetic variability in the founder population compared to the source population. c. Tends to lead to lowered heterozygosity. d. Is a process that randomly affects allele frequencies.
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|Correct Answer: |Tends to reduce genetic variability in the founder population compared to the source population. | |

The smaller the population, the stronger and faster the effects of genetic drift.
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6. Each of the figures (A, B, and C) illustrates the results of a series of computer simulations of changes in allele frequency in a group of populations due to chance alone. Figure _____ most likely represents the simulations performed on the smallest populations; Figure _____ most likely represents the simulations performed on the largest populations.
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a. B; C
b. A; B
c. B; C
d. A; C
|Correct Answer: |A; C | |

The smaller the population, the faster drift will act, and the faster alleles will move to fixation or loss.
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7. The computer models of genetic drift shown in Figure 7.15 demonstrate that three of the statements below are true. Which statement is false?

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a. genetic drift causes allele frequencies to wander between 0 and 1. b. Genetic drift strongly affects only populations that are very small. c. Genetic drift tends to eventually lead to fixation or loss of any given allele. d. Genetic drift proceeds randomly in any given population.
|Correct Answer: |Genetic drift only affects populations that are very small. | |

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8. In Buri's classic experiment on genetic drift in flies, what was surprising about the change in average heterozygosity over time? What is a likely explanation?

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Average heterozygosity declined faster than predicted. In Figure 7.17, the predicted result is shown by the dashed grey line (for populations of 16 individuals); but the experimental data match the solid grey line, which is the expected result for populations of 9 individuals. This indicates that not all individuals reproduced. Some flies may have died before mating; others may have been rejected as mates. Another way to state this is that though the actual population size was 16, the effective population size was 9.
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9. Consider a population of 100 mice on an island, with allele frequencies B = 0.20 for black coat color, and b = 0.80 for white coat color, Black, B, is dominant to white, b, and the population is in Hardy-Weinberg equilibrium. Twenty-five homozygous black mice from the mainland float to the island on an uprooted tree after a storm. What are the allele frequencies now? What were the genotype frequencies before and after the migration event? Is the population in Hardy-Weinberg equilibrium? How long will it take it to get back into Hardy-Weinberg equilibrium if no more mice arrive, and if there are no other forces affecting allele frequencies?
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Originally there were 100 mice, and the gene pool contained100 copies of the black allele and 100 copies of the white allele. The Hardy-Weinberg genotype frequencies were BB= 0.25, Bb = 0.50, and bb = 0.25; that is, there were 25 homozygous black mice, 50 heterozygous black mice, and 25 white mice. After the storm, there are now 125 mice, and the gene pool contains 150 copies of the black allele (the 100 old copies plus 50 new ones) and 100 copies of the white allele. These are distributed in 50 homozygous black mice (25 old + 25 new), 50 heterozygous black mice, and 25 white mice. The new allele frequencies are B= 0.60 (150/250) and b = 0.40 (100/250), and the genotype frequencies are BB = 0.40 (50/125), Bb = 0.40 (50/125), and bb = 0.20 (25/125). These genotype frequencies do not match Hardy-Weinberg equilibrium frequencies (0.36, 0.48, and 0.16, respectively). The population is not in Hardy-Weinberg equilibrium. In theory, a single bout of random mating will put the population back in Hardy-Weinberg equilibrium in the next mouse generation. (Note, though, that in reality, parental mice do not disappear instantly. Until the first-generation mice have all died, the population will not be in Hardy-Weinberg equilibrium. In mice, this will take about a year and a half.)
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10. Refer to the information presented in the previous question. Now, suppose the twenty-five black mice float away again on another tree without breeding, and the island is back to its original state. Allele frequencies on the island are back to B=0.20, b=0.80. On the continent, there is a large population of many thousands of mice, with allele frequencies B=0.80, b=0.20. One year, human ships begin moving back and forth between the island and the continent, and occasionally a mouse comes along for the ride, and stays and breeds. Equal numbers of mice ride in each direction. The shipping trade continues indefinitely. Where will the allele frequencies ultimately stabilize? (Assume that no other forces are affecting allele frequencies.)
a. Both populations will end up at B=0.80, b=0.20.
b. The island and continent populations will both end up at a stable equilibrium of B=0.50, b=0.50.
c. Black will move to fixation in both populations, because it is dominant.
d. Both populations will end up at B=0.20, b=0.80. e The continent's allele frequencies will not change; the island will settle at an equilibrium that is somewhere between the continent's and the island's original states.
|Correct Answer: |Both populations will end up at B=0.80, b=0.20. | |

Migration tends to drive island populations to the allele frequencies of the continent. (Notice that in this case, unlike the example of the banded water snakes, there is no selection counteracting the effect of migration.)
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11. Small relictual populations of collared lizards remain in glades of desert-like habitat scattered throughout the oak-hickory forests in parts of the Ozark Mountains. Templeton and colleagues supported the hypothesis that drift had been acting in these populations by documenting that _____________.
a. most populations were fixed for a single genotype, but genotypes varied among populations b. all populations were fixed for the same allele at each locus studied. c. average heterozygosity was declining steadily over time
d. individual populations were polymorphic for several loci and most populations were genetically similar to one another.
|Correct Answer: |most populations were fixed for a single genotype, but genotypes varied among populations | |

This is not what we would predict if selection had been acting, but is exactly what we would predict if drift were the major evolutionary force. Random fixation of alleles due to genetic drift has caused different genotypes to become fixed in different populations, even though they all live in very similar environments.
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12. Briefly describe the fundamental tenets of Kimura's neutral theory, and its two surprising implications. What data was this theory based on? Is the theory supported by recent data?
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Kimura's neutral theory holds that virtually all mutations are selectively neutral, that the substitution rate is equal to the mutation rate, and that the evolution of DNA sequences is dominated by genetic drift. The two surprising implications were that neither population size nor natural selection have an effect on the rate of DNA sequence change. The theory was based on amino acid substitutions in a fairly small number of genes that had been studied at the time. Recent data shows that though the neutral theory is applicable to noncoding regions and some coding regions, a large fraction of coding regions have been affected by selection.
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13. What were the two early observations about amino acid substitutions that led to development of the neutral theory?
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The first was Zuckerkandl and Pauling's discovery that amino acid substitutions in vertebrates appeared to occur at a steady, clock-like rate. The second was Kimura's calculations showing that amino acid substitutions occur every two years on average in the vertebrates (when substitution rates for well-studied genes are extrapolated to the whole genome). These substitution rates seemed too fast and too steady to be explainable by selection.
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14. What was the key finding of McDonald and Kreitman's comparison of the Adh gene in three different fly species? What is the interpretation of the finding? What is the logic of comparing sequences both between and within species?
| |
| |

McDonald and Kreitman found that 29% of fixed differences between their fly species were synonymous substitutions, but that only 5% of polymorphic differences within species were synonymous. They reasoned that if there were no selection (i.e. if the neutral theory were true), then the ratio of synonymous to nonsynonymous substitutions should be constant both within and between species. However, if there are differences in this ratio between vs. within species, the within-species ratio should represent the neutral replacement rate, and the higher between-species ratio is thought to be evidence of selection.
[pic]
15. Genes associated with which of these traits have shown evidence of recent positive selection?
a. Parasite-host interactions
b. Symbiont interactions c. Histone proteins d. Increased brain size in humans
e. Color vision in humans
f. Recently duplicated genes
|Correct Answers: |Increased brain size in humans | |
| | | |
| |Recently duplicated genes | |
| | | |
| |Symbiont interactions | |
| | | |
| |Color vision in humans | |

Histone proteins show very little variation across species, and appear to be under negative selection to eliminate new variants. Color vision in humans is similar to color vision of other Old World primates, and is not likely to have been under recent selection.
[pic]
16.
[pic]Match the key terms in this chapter listed below with the phrase that is the best match for it.
|[pic] |Option |Phrase |Your answer |
|16.1 |gene flow, or migration|A. The apparent number of breeding individuals in a | |
| | |population, as reflected in the genetic and evolutionary | |
| | |trends of a population; usually lower than the actual | |
| | |number of individuals | |
|16.2 |inbreeding depression |B. A change in allele frequencies of a new population due | |
| | |to its being started by a small random sample from a large| |
| | |population | |
|16.3 |sampling error, or |C. Random discrepancy between observations and expected | |
| |sampling bias |results, due to selection of just a small part of the | |
| | |population (not the whole population) | |
|16.4 |founder effect |D. Lowered fitness due to high levels of mating between | |
| | |close relatives | |
|16.5 |hitchhiking, or |E. An increase in frequency of a neutral allele due to | |
| |selective sweep |close linkage to a beneficial allele | |
|16.6 |effective population |F. The movement of alleles between populations. | |
| |size | | |

[pic]
17. The major genetic effect of inbreeding is to _______________.
a. The first and third choices are both correct
b. increase the number of loci at which the average individual is homozygous
c. increase the number of loci at which the average individual is heterozygous d. increase the number of recessive alleles in a population over time
e. The second and third choices are both correct.
|Correct Answer: |increase the number of loci at which the average individual is homozygous | |

Inbreeding decreases heterozygosity, exposing deleterious alleles that formerly were hidden in heterozygotes. This can reduce average fitness, a phenomenon called "inbreeding depression". (Note that inbreeding does not increase the number of recessive alleles, but simply reveals those recessive alleles that were already there.)
[pic]
18. According to Westemeier et al., the "extiction vortex" in Illinois greater prairie chickens was due, in effect, to a positive feedback loop between ____________.
a. mutation and selection b. drift and inbreeding c. drift and heterosis
d. migration and drift.
|Correct Answer: |drift and inbreeding |

[pic]
19. Westemeier and others supported their hypothesis for the decline of the Illinois greater prairie chicken by demonstrating which of the following?
a. A comparison of DNA collected from prairie chickens in the 1990's with DNA from museum specimens collected in the 1930's and 1960's showed a loss of genetic diversity over time.
b. Hatching rates increased when prairie chickens from other, more genetically diverse, populations were transplanted to the Jasper County populations. c. Illinois prairie chickens had less diversity at six selectively neutral loci than did prairie chickens from populations in other states.
d. All choices are correct.
|Correct Answer: |All choices are correct. |

[pic]
20. Sunflowers were domesticated fairly recently from a wild sunflower native to North America, and have been under intense artificial selection for increased number of seeds, size of seeds, and sunflower-oil content. Domestic sunflower strains have lower genetic diversity at most loci than do the wild populations. On one chromosome, there is a cluster of loci called LG06 that all show signs of recent positive selection in domestic sunflowers. Some of the common alleles at certain of these loci increase seed size and oil content. However, at other loci in this cluster, the most common allele (in domestic sunflowers) is associated with maladaptive traits.

Why do domestic sunflowers have lower genetic diversity than wild sunflowers? Why did the LG06 maladaptive alleles become more widespread during the domestication of sunflowers?
| |
| |

Domestic sunflowers probably have lower genetic diversity due both to a founder effect and probably to strong selection that has eliminated undesirable alleles. (It appears that sunflowers passed through several "genetic bottlenecks" during domestication.) There are several reasons that maladaptive traits could have become common, including inbreeding depression, genetic drift, etc., but in this case, they seem to have hitchhiked to fixation in a selective sweep, due to being physically linked to desirable alleles that increase seed size and oil content. (The LG in LG06 stands for "linkage group.")

Reference: J.M. Burke, S.J. Knapp, and L.H. Rieseberg. 2005. Genetic consequences of selection during the evolution of cultivated sunflower. Genetics 171:1933-1940.
[pic]
21. A recent study of 170 threatened species found that 77% of the threatened taxa have lower heterozygosity (measured at many loci) than closely related taxa that are not threatened. What does this imply for the long-term future of these threatened taxa?
| |
| |

Many answers are possible. Low heterozygosity, in and of itself, indicates that the populations probably have low allelic diversity, which will make them less able to evolve in response to changing environmental conditions. In addition, the lowered heterozygosity is likely to have resulted from small population size and associated genetic drift (including founder effects) and/or inbreeding. Small population size is always cause for concern, and inbreeding can lower fitness via inbreeding depression. In either case the prognosis for long-term survival is not good.

Reference for this data: D. Spielman, B.W. Brook, and R. Frankham. 2004. Most species are not driven to extinction before genetic factors impact them. Proc. Natl. Acad. ScUSA 101(42):15261-15264.
[pic]
22. An important tool for testing whether selection or drift is acting on a particular gene is to calculate the ratio of nonsynonymous substitutions per site (dN) to synonymous substitutions per site (dS). Correctly match each of the following ratios to its correct interpretation.

|[pic] |Option |Interpretation |Your answer |Correct |Comments |
| | | | |answer | |
|22.1 |dN/dS < 1 |A. replacements are neutral| |C |If the rate of nonsynonymous replacements is less|
| | | | | |than the rate of of synonymous replacements, the |
| | | | | |implication is that selection is eliminating the |
| | | | | |nonsynonymous mutations. |
|22.2 |dN/dS = 1 |B. replacements are | |A |Correct! |
| | |advantageous | | | |
|22.3 |dN/dS > 1 | C. replacements are | |B |If nonsynonymous replacements exceed silent |
| | |deleterious | | |replacements, it must be because they're favored |
| | | | | |by selection. |

CHAPTER EIGHT: EVOLUTION AT MULTIPLE LOCI

1. A haplotype is best defined as the ________________.

a. ABO blood type conferred by an individual gamete [pic]

b. genotype of either the paternal or maternal chromosomal complement

c. haploid genotypes of all the gametes produced by a diploid individual [pic]

d. multilocus genotype of a chromosome or gamete

| | | |
|Correct Answer: |multilocus genotype of a chromosome or gamete | |

Haplotype is a contraction of "haploid genotype," and thus refers to haploid entities such as single chromosome or a single gamete.
[pic]
2. Which of the following statements regarding linkage disequilibrium is true?
a. is reduced by sexual reproduction
b. Is reduced by selection that favors certain combinations of genotypes
c. Is increased by any random sampling error that happens to create or destroy certain chromosome genotypes but not others
d. Is increased by crossing-over during meiosis
e. Exists when D is less than zero, but not when it is greater than zero.
| | | |
|Correct Answers: |Is reduced by sexual reproduction | |
| | | |
| |Is increased by any random sampling error that happens to create or destroy certain chromosome genotypes but not others| |

[pic]
3.When two loci are in linkage equilibrium, the allele carried on a chromosome at one locus is independent of the allele carried on the chromosome at another locus. When this is the case,
a. the frequency of each haplotype can be calculated by multiplying the frequency of its two alleles allele frequencies will not change over time b. all possible haplotypes occur at equal frequency
c. the two loci (or, more specifically, their protein products) do not interact.
|Correct Answer: |the frequency of each haplotype can be calculated by multiplying the frequency of its two alleles |

[pic]
4. Selection on multilocus genotypes in random-mating populations leads to linkage disequilibrium when _______________.
a. it prevents crossing-over during meiosis b. it eliminates all haplotypes from the population
c. it reduces mutation rates d. some allele combinations confer greater fitness than do others
|Correct Answer: | D some allele combinations confer greater fitness than do others |

[pic]
5. In Clegg et al.'s experiment on linkage disequilibrium in freely mating fruit flies, why did linkage disequilibrium decline faster than expected? Why did the blue line overshoot the zero-disequilibrium point on the graph shown in Figure 8.7?

[pic] The most likely explanation is that linkage disequilibrium declined faster than expected because of heterozygote superiority. This would have resulted in selection for heterozygotes, causing an overrepresenation of flies that were heterozygous for both loci, and fewer flies that were homozygous for both locCrossing-over in heterozygous flies creates new chromosome genotypes (reducing linkage disequilibrium); crossing-over in homozygous flies does not. If this was the case, more genetic recombination would have occurred, per generation, than was expected.

The blue line overshot the zero point simply due to sampling error in a finite population of flies. Notice that this is an example of how genetic drift can create linkage disequilibrium in a population that was in linkage equilibrium.
[pic]
6. By breaking up overrepresented haplotypes and creating new ones, _______________ reduces linkage disequilibrium.
a. genetic recombination
b. crossing over
c. sexual reproduction
d. all of these
| | | |
|Correct Answer: |D all of these | |

Crossing-over during meiosis breaks up "old" combinations; because the diploid genotype consists of haplotypes from two different parents, crossing-over will create new allelic combinations (or haplotypes).
[pic]
7. Match the key terms in this chapter listed below with the phrase that is the best match for it.
|[pic] |Correct |Option |Your Answer |Prase |
|7.1 |A |Haplotype | |A. All of the alleles present on a single chromosome or in a |
| | | | |single haploid gamete. |
|7.2 |B |Linkage equilibrium | |B. A state in which the probability of an allele being |
| | | | |present at one locus is independent of the allele present at |
| | | | |another locus on the same chromosome. |
|7.3 |E |Genetic load | |C. The blending of gene pools that occurs when two different |
| | | | |populations meet. |
|7.4 |D |Genetic recombination (crossing-over can | |D. Creation of new combinations of alleles during sexual |
| | |also be accepted as an answer) | |reproduction. |
|7.5 |C |Population admixture | | E. The fitness burden caused by the accumulation of |
| | | | |deleterious mutations in a population. |

[pic]
8. Summarize the logic behind Diaz et al.'s effort to establish the age of the 84GG allele in the Ashkenazim. What are the three possible causes of linkage disequilibrium, and how did Diaz et al. rule out two of them in this case? Could either of those two actually be a factor? What information did Diaz et al. have to know to calculate the age of the 84GG allele?
| |
| |

The logic is that if linkage disequilibrium is introduced by genetic drift, the two loci should gradually come back into equilibrium at a rate determined by the amount of crossing-over between the two locThe three possible causes of linkage disequilibrium are selection on multilocus genotypes, population admixture, and genetic drift. Selection was ruled out because one of the loci is selectively neutral, and population admixture was ruled out because the 84GG allele is not known to occur in other populations. However, it is possible that one of these two causes might be a factor - there might be a third linked locus, as yet unidentified that selection is acting on, or a now-vanished population might have contributed the 84GG allele. However, if selection is not a factor, the origin of 84GG can be calculated using the known rate of crossing-over to estimate the rate of decay of linkage disequilibrium.
[pic]
9. Which of the following statements regarding linkage disequilibrium between Ashkenazi disease alleles and nearby marker loci are true?
a. Appears due to genetic drift, associated with three periods of founder effects
b. Decays at a rate determined by crossing-over between the two loci.
c. Can be used to estimate the age of the most recent common ancestor of each allele.
d. Appears due to three periods of population admixture.
e. Is generally due to selection on multi-locus genotypes.
f. the first three options are correct
| | | |
|Correct Answers: |Appears due to genetic drift, associated with three periods of founder effects | |
| | | |
| |Decays at a rate determined by crossing-over between the two loci. | |
| | | |
| |Can be used to estimate the age of the most recent common ancestor of each allele. | |

Recall that selection on multilocus was ruled out because the marker loci are selectively neutral. Population admixture was ruled out because, in the cases discussed, the Ashkenazi are already the population with the highest known frequency of these alleles; i.e. there is no known source population with a higher frequency.
[pic]
10. The genetic signature of recent positive selection on a locus is _______________.
a. high frequency and high linkage disequilibrium
b. low frequency and low linkage disequilibrium
c. high frequency and low linkage disequilibrium
d. high frequency and low linkage disequilibrium
e. low frequency and high linkage disequilibrium
| | | |
|Correct Answer: |high frequency and high linkage disequilibrium | |

An allele that is in linkage disequilibrium with nearby markers is a young allele. A young allele that is at high frequency must have been driven rapidly to high frequency - and that means it has been under recent positive selection. The higher the frequency, and the further the disequilibrium extends away from the locus, the stronger the selection.
[pic]
11. Which of the following statements about CCR5-delta32 allele is true?
a. Probably was driven to its current frequency by the Black Death.
b. Is the most common CCR5 allele in European populations.
c. Is at higher frequency among African than among European populations.
d. Probably was driven to its current frequency in European populations by selection.
e. Has been recovered from Stone Age skeletons that are approximately 20,000 years old.
| | | |
|Correct Answer: |Probably was driven to its current frequency in European populations by selection. | |

CCR5-delta32's highest frequency approaches 20% in some European populations, but even there it is not the most common allele. It has been recovered from Bronze Age skeletons approximately 2900 years old. The Black Death has been suggested as a possible explanation for the rise of the CCR5-delta 32 allele in Europe, but other analyses, including that of the Bronze Age skeletons, indicate that it was fairly common in Europe well before then.
[pic]
12. Why did Maynard-Smith's model of sex present a paradox for evolutionary biology? Why does his model not work in real life?
Correct answer: Maynard-Smith's model showed that if sexual and asexual reproduction result in equal numbers of equally fit young for each female, then asexual reproduction has a large fitness advantage because it results in twice as many grand-offspring. Therefore, most populations should end up composed entirely of asexual reproducers. The paradox is that this has not happened - most populations in the wild reproduce sexually. The solution lies in the fact that the assumptions of his model must be wrong, particularly, the assumption that the offspring of sexual reproduction have the same fitness as the offspring of asexual reproduction.
[pic]
13. Which of the following accurately characterize(s) Muller's ratchet model for the selective advantage of sexual reproduction?
Drift establishes linkage disequilibrium because multilocus genotypes with few mutations are eliminated by chance events. [pic] All of the above correctly characterize Muller's ratchet model for the evolution of sexual reproduction. [pic] Sex is favored selectively because it recreates the zero-mutation genotypes lost due to drift. [pic] In populations without sexual reproduction, deleterious mutations accumulate, eventually imposing such a burden that the population goes extinct.
| | | |
|Correct Answer: |All of the above correctly characterize Muller's ratchet model for the evolution of sexual reproduction. | |

Muller's ratchet is one of the ways in which sexual reproduction can gain an advantage over asexual reproduciton. But notice that because of its reliance on drift and mutation, Muller's ratchet is a very slow process.
[pic]
14. Which of the following statements regarding sexual reproduction are true?
a. Is probably maintained in natural populations mostly because of its effect of breaking Muller's ratchet. b. Is advantageous in changing environments but disadvantageous in constant environments, compared to asexual reproduction.
c. Produces only half as many offspring per female, compared to asexual reproduction.
d. Is advantageous both in changing environments and in constant environments, compared to asexual reproduction.
e. Causes genetic recombination via crossing-over, which combines genotypes from different parents. f. Produces only half as many grand-offspring per female, compared to asexual reproduction.
| | | |
|Correct Answers: |Produces only half as many grand-offspring per female, compared to asexual reproduction. | |
| | | |
| |Causes genetic recombination via crossing-over, which combines genotypes from different parents. | |
| | | |
| |Is advantageous in changing environments but disadvantageous in constant environments, compared to asexual reproduction.| |

[pic]
15. Describe Andersson and Hughes' study of Muller's ratchet in bacteria. What features of their populations might have increased the effect of Muller's ratchet? Approximately how many generations reproduced during the experiment, and how many bottlenecks occurred? Were the results consistent with the Muller's ratchet theory? Did Muller's ratchet affect most populations, and is it thought to be the major reason for the dominance of sexual reproduction.
|Correct Answer: |
|C |

Andersson and Miller allowed 444 cultures of bacteria to grow for two months, with one bottleneck every day during which each culture was reduced to a single individual. The combination of asexual reproduction with frequent population bottlenecks was expected to cause Muller's ratchet to operate, i.e., accumulation of deleterious mutations and consequently reduced fitness. At the end of the two months, representing roughly 1700 generations and roughly 60 bottlenecks, all cultures were compared to the wild-type strain. 5% of the experimental cultures had reduced fitness (reduced growth rate) and none had increased fitness, so the result was consistent with Muller's ratchet. But since, Muller's ratchet takes a very large number of generations to have an appreciable effect - and even then the effect is small - it is not thought to be the major process favoring sexual reproduction.
[pic]
16. Which of the following correctly characterize(s) the "changing environment" hypothesis for the selective advantage of sexual reproduction?
a. Linkage disequilibrium is created because different multilocus genotypes are favored in under different conditions.
b. Sexual reproduction is favored because it can create the new mutlilocus genotypes favored under new environmental conditions.
c. Changing environments will favor asexually reproducing females because of their higher reproductive rate.
d. The last two choices are correct.
e. The first two choices are correct.
|Correct Answer: |The last two choices are correct. | |

Unlike Muller's ratchet, the "changing environment" model is a rapid process; sexual reproduction can gain an immediate advantage if the environment changes.
[pic]
17. From the end of this chapter: "There are two classes of models for sex. In the first class, genetic drift creates linkage disequilibrium. Sex is then favored because it helps to re-create high-fitness genotypes lost by drift. In the second class, natural selection creates linkage disequilibrium. Then the pattern of selection changes, and sex is favored because it helps to re-create the now-favorable genotypes recently selected against."
Refer to the paragraph above. Which of the following models or experimental results is an example of the first class?
a. Muller's ratchet
b. Evolutionary "arms races" between parasites and their hosts
c. Correlation of percentage of males with trematode infections, in freshwater snails
d. Red Queen hypothesis
e. Reduced thermal stability of rRNA in endosymbiotic bacteria
f. Reduced growth rate of bacterial colonies periodically reduced to 1 individual

| | | |
|Correct Answers: |Correlation of percentage of males with trematode infections, in freshwater snails | |
| | | |
| |Muller's ratchet | |
| | | |
| |Reduced thermal stability of rRNA in endosymbiotic bacteria | |
| | | |
| |Reduced growth rate of bacterial colonies periodically reduced to 1 individual | |

[pic]
18. From the end of this chapter: "There are two classes of models for sex. In the first class, genetic drift creates linkage disequilibrium. Sex is then favored because it helps to re-create high-fitness genotypes lost by drift. In the second class, natural selection creates linkage disequilibrium. Then the pattern of selection changes, and sex is favored because it helps to re-create the now-favorable genotypes recently selected against."

Reference the paragraph above in answering this question: By wyat class is sexual reproduction thought to be maintained?
a. the first class
b. the second class
c. both
| | | |
|Correct Answer: |Both | |

The second class is suspected to act more quickly and have greater effects, but both are important.
[pic]
19. Cancer is thought to arise when a cell accumulates an unlucky set of deleterious mutations, by chance, that switches the cell from being a harmonious part of a multicellular body to being a "parasite." Like true parasites, a dividing population of cancer cells uses materials and resources entirely for their own advantage, disrupting the health and integrity of the rest of the body. In addition, cancer cells have high mutation rates, and face a problem of a steady accumulation of deleterious mutations. This can become so severe that some cancer populations die out simply due to their own mutations - a Muller's ratchet effect.

Part I: Refer to the paragraph above in answering this question. In general, do parasites tend to reproduce sexually or asexually? Why?

Part II: Refer to the paragraph above in answering this question. Do cancer cells reproduce sexually or asexually? Would they gain any advantages if they switched from one mode of reproduction to the either?

Part III: Refer to the paragraph above in answering this question. The most dangerous development in a cancer is metastasis - formation of new cancer tumors in other organs. Metastasis requires coordination of many genes involving cell-cell adhesion and cell movement. Recently it has become clear that metastasis is associated with a peculiar tendency of cancer cells to fuse with nearby normal cells, combining its genes with the genes of the other cell. Why does it make sense, evolutionarily, that cancer cells do this? Part I: Parasites tend to use sexual reproduction. Parasites live in the environment of their hosts' bodies, which are constantly evolving to counteract the parasite. Since sexual reproduction gives an advantage in changing environments, the speedily and constantly changing environment of a parasite-host evolutionary arms race gives sexual reproduction a large advantage over asexual reproduction. (This is the "Red Queen hypothesis" of parasite-host coevolution.)

Part II: Cancer cells generally reproduce asexually, by cell division. However, they should gain advantages if they could switch to sexual reproduction. The genetic recombination could allow the cancer cells to act as more successful parasites and evade the host's cancer-killing abilities. In addition, genetic recombination would allow cancer cells to escape the Muller's ratchet caused by their tendency to rapidly accumulate deleterious mutations.

Part III: Cell-cell fusion is analogous to sexual reproduction because it results in genetic recombination. This is particularly useful for cancer cells, which often have such high mutation rates that many of their genes are disabled. Fusion with a normal cell allows a cancer cell to regain functional versions of many of these genes, and to combine them with certain unusual mutations, such as those that allow evasion of cancer-killing processes. Note that cell-cell fusion allows cancer cells both to avoid Muller's ratchet and also to survive as parasites in a changing environment. Hence, both advantages of sexual reproduction are at work.

Interestingly, some work indicates that a fused cancer cell will metastasize not to any organ, but specifically to the "home organ" of the cell that it fused with. This may be because the cancer acquires actively transcribed genes that gave the normal cell the ability to recognize and adhere with other cells of that organ.
[pic]
20. Each graph represents one of Dunbrack et al's experimental populations of Tribolium beetles. Correctly match each graph with the description of its experimental treatment.
|[pic]|[pic] |Option |Your Answer |Correct Answer |Comments |
|20.1 |[pic] |The red |A. [pic] |D. [pic] |This would be |
| | |population | | |expected if |
| | |reproduced | | |sexual |
| | |sexually. | | |reproduction |
| | | | | |were |
| | | | | |disadvantageous|
| | | | | |; is that what |
| | | | | |Dunbrack et al.|
| | | | | |found? |
|20.2 |[pic] |Both |A. [pic] |B. [pic] |Does the |
| | |populations | | |pattern of |
| | |are asexual;| | |Malathion |
| | |black | | |concentration |
| | |beetles have| | |suggest a |
| | |a three-fold| | |selective |
| | |reproductive| | |advantage for |
| | |advantage. | | |the black |
| | | | | |population in |
| | | | | |this graph? Is |
| | | | | |that expected |
| | | | | |under the |
| | | | | |treatment |
| | | | | |specified in |
| | | | | |the question? |
|20.3 |[pic] |The black |A. [pic] |A. [pic] |Correct! |
| | |population | | | |
| | |reproduced | | | |
| | |sexually. | | | |
|20.4 |[pic] |Both |A. [pic] |C. [pic] |Which |
| | |populations | | |population, red|
| | |are asexual;| | |or black, has |
| | |red beetles | | |the advantage |
| | |have a | | |in this graph? |
| | |three-fold | | |Which should |
| | |reproductive| | |have the |
| | |advantage. | | |advantage in |
| | | | | |the treatment |
| | | | | |specified by |
| | | | | |the question? |

Incorrect.