3.24 d: variation and natural selection quiz proctor · pdf filewhich of these best explains...

1. Scientists believe that Han Chinese who live in low-altitude regions of China and Tibetans who

live at high altitudes in the Himalayan mountains became genetically separated about 3,000 to

6,000 years ago. When Han Chinese try to live at high altitudes, their blood thickens as their

bodies try to counteract the low oxygen levels by producing more red blood cells. Some

individuals have thinner blood and do not suffer from altitude sickness or infertility at high

elevations. Biologists have identified that many Tibetans have an allele for a gene known as

hypoxia-inducible factor 2-alpha (HIF2a) that is rare in Han Chinese, and these individuals who

possess the HIF2a allele have thinner blood and do not suffer from altitude sickness or infertility

at high elevations. The percent of each population that possesses the HIF2a allele is shown in

the table below.

Which of these best explains the difference in the frequency of the HIF2a gene between the Han

Chinese and Tibetan populations?

(A) The lack of oxygen at high elevations has caused more mutations in the DNA of the Tibetan

population, and this has resulted in Tibetans becoming fitter than the Han Chinese because

they possess the HIF2a allele.

Distractor Rationale:

This answer suggests the student may understand that environmental factors can cause

mutations and that some mutations can be beneficial, but does not understand that the

lack of oxygen would not cause a mutation producing the HIF2a allele because a low

oxygen level is not a mutagen.

(B) Both populations possessed the HIF2a allele at a high frequency, but when the Han Chinese

moved to lower elevations, the allele was lost, since it no longer provided a survival benefit

to individuals who expressed the trait.

3.24 D: Variation and Natural Selection Quiz

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This answer suggests the student may understand that the presence of genes that are no

longer beneficial in a new environment may decrease in populations over time, but does not

understand that HIF2a provides no benefit when expressed at lower altitudes, so the gene

frequency should remain relatively unchanged because there is no selective force selecting

against the gene.

(C) Among Tibetans who are exposed to lower levels of oxygen at high elevations, those who

possess the HIF2a allele are better able to survive and produce more offspring than those

who lack the allele.

Rationale:

This answer suggests the student understands that if a gene increases reproductive

success in a new environment in individuals with the gene, the gene frequency will increase

in the population due to natural selection; individuals with the favorable gene will survive and

reproduce with more success, passing their genes (including the gene with the HIF2a allele)

on to their offspring and increasing the frequency of the allele in the population.

(D) There is a higher frequency of the allele among the Tibetan population because the

population is smaller and genetic drift has produced rapid change in the HIF2a gene

frequency.


This answer suggests the student may understand that in small populations, random events

can produce rapid changes in gene frequencies, but does not understand that the low-

oxygen environment is a strong selective factor, which will select for the HIF2a allele

because it increases an individual’s chance of survival and reproduction.

Aligned to: LO 3.24 CA 3.24: Predict Impact of Change in Genotype


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2. To attract females, male grasshoppers rub their hind legs over a vein on their wings, producing a

buzzing call that females use to locate and select their mates. There is variation in wing vein

thickness: grasshoppers with a thinner vein produce a higher-pitched treble chirp, while those

with a thicker vein produce a lower-pitched bass chirp. Wing vein thickness is under genetic

control, and the bass allele (B) codes for a thicker vein, while the treble allele (T) codes for a

thinner vein.

Scientists want to see if noisy highways are changing the allele frequency in the grasshopper

population. To find out if grasshopper alleles have changed due to a noisy environment,

scientists collect grasshoppers living at eight sites along German highways. The scientists also

collect grasshoppers living in quiet areas a few kilometers from each of the sites. The bass and

treble alleles are then analyzed in each population, and the data are recorded in the table below.

Based on the data in the table, which statement best explains the relationship between the

locations of grasshoppers and the frequencies of vein thickness alleles?

(A) More females prefer higher-pitched chirps across both areas, because the high pitch

signifies greater male fitness in the populations, and therefore, females have evolved to

always prefer males with the treble allele.


This answer suggests the student may understand that different-frequency chirping may

convey a selective advantage to certain males in attracting females, but does not

understand that this is not a factor in this situation because the higher-pitched chirp is not

favored by females in the quiet areas.

(B) The higher-pitched chirps are easier to hear near the highways, so the treble allele has a

selective advantage in areas near highways; however, there is no advantage for either allele

in quiet areas.


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Rationale:

This answer suggests the student understands that the treble allele conveys a selective

advantage near highways because it allows for the chirp to be heard among the low-pitched

traffic noise, and that there is no advantage in the quiet areas because both chirps can be

heard.

(C) The higher-pitched chirps are easier to hear near highways, so fewer vehicles kill

grasshoppers with the treble allele; however, there is no advantage for either allele in quiet

areas because of the lack of traffic.


This answer suggests the student may understand that alleles that allow animals to be more

easily heard may convey a selective advantage around highways and other urban areas, but

does not understand that this is an unlikely scenario because there is no evidence that

grasshoppers are being killed on the highway in significant numbers.

(D) There is less genetic variation near highways due to geographic isolation, because the

grasshoppers are unable to easily move across the pavement, causing a shift in allele

frequency in the environment and an increase in the frequency of the treble allele.


This answer suggests the student may understand that a decrease in genetic diversity (a

genetic bottleneck) and geographic isolation can produce changes in allele frequency, but

does not understand that alleles that code for a chirp that can be heard by females convey

a strong selective advantage because they increase the chance that a male will be able to

reproduce and pass on his genes.



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3. Scientists are studying a population of the mosquito Wyeomyia smithii in New Jersey. They

record the average date on which the mosquito larvae pupate (that is, slow down their normal

physical functions and become dormant pupae for the winter). The diagram below shows the life

cycle of the mosquito.

The life cycle of the mosquito is controlled by a genetic switch linked to length of day. Some

mosquitoes lay their eggs earlier in the fall season to ensure that their offspring pupate before

winter. Mosquitoes that lay their eggs later in the season are able to produce more larvae, but if

the larvae do not pupate before the first hard freeze, they die. Data on the change in pupation

timing in the mosquito population and weather data are shown in the table below.


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Based on the results shown in the table, which statement best explains the change in the

average time of pupation in the mosquito population?

(A) Due to warmer weather later in the year, mosquitoes with genetic variations that trigger

mating later in the fall season have a selective advantage because they contribute more

offspring to the next generation than mosquitoes with alleles that trigger them to mate earlier

in the season.

Rationale:

This answer suggests the student understands that warmer fall weather is selecting for

behavioral phenotypes that cause mosquitoes to lay eggs later in the season, because

these phenotypes allow an individual to produce more offspring in the next generation.

(B) Mosquitoes have learned that days continue to be warmer later in the year, and adapt by

mating later in the fall season, because this allows them to take advantage of the warmer

weather and produce more offspring before freezing temperatures arrive.


This answer suggests the student may understand that the mosquito population is adapting

to the warming weather patterns, but does not understand that this is not a learned

behavior because the response to lay eggs is genetically determined and subject to natural

selection.

(C) Over the past 40 years, mosquitoes have accumulated new mutations that have changed

their behavior, as shown by the fact that they are now producing offspring that go dormant

later as a result of the new mutations.


This answer suggests the student may understand that new mutations can arise that change

the behavior of mosquitoes in the population, but does not understand that natural

selection changes the frequencies of these variations as the climate changes, because the

variations in behavior are already present in the population.

(D) The warmer weather is activating dormant genes (genes that can remain inactive until

triggered by a change in the environment) that are usually not expressed in the mosquitoes,

and causing the mosquitoes to lay their eggs later in the season.


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This answer suggests the student may understand that dormant genes can be activated by

changes in the environment, but does not understand that the evidence does not support

this, because genetic variation is already present in the population and that the behavior is

changing as a result of natural selection on the existing variation.



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4. Mummichog fish are found along the east coast of the United States. Like many other animals,

these fish produce an enzyme, lactose dehydrogenase (Ldh), that is involved in lactic acid

fermentation in muscle cells. In the mummichog population, there is significant variation in the

allele that codes for Ldh. The Ldh-b allele codes for a form of the enzyme that is a more effective

catalyst in cold water than other forms, allowing mummichog fish that have the allele to perform

anaerobic respiration more efficiently. The graph below shows the frequency of the Ldh-b allele in

mummichog at different latitudes extending from Maine to Georgia.

The graph below shows three possible changes in Ldh-b allele frequency that could result from

environmental changes.

If climate change results in a gradual warming of water temperatures of approximately 4°C along

the habitat range, which statement best predicts the resulting change in the Ldh-b allele

frequency in mummichog?


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(A) The change shown for allele frequency I will occur, because individuals in mummichog

populations will gain a selective advantage if they express the gene, and the Ldh-b allele will

become more frequent in populations farther south.


This answer suggests the student may understand that a change in water temperature will

cause a change in the allele frequency, but does not understand that an environmental

change such as warmer waters would cause a shift in the trend line (the line that expresses

the average frequencies at each latitude) to the left, where the water is cooler, because the

change would reduce the selective advantage of the Ldh-b allele in mummichog and lower

the allele’s frequency at all latitudes.

(B) The change shown for allele frequency II will occur, because the Ldh-b allele will not convey

a selective advantage in mummichog in warmer waters, and this will result in a decrease in

the Ldh-b allele in all areas.

Rationale:

This answer suggests the student understands that the warmer water temperatures would

reduce the frequency of the Ldh-b allele in mummichog because the allele would convey a

weaker selective advantage in the warmer waters, and in response, the frequency of the

Ldh-b phenotype would decrease at any given latitude due to natural selection, shifting the

trend line (the line that expresses the average frequencies at each latitude) to the left.

(C) The change shown for allele frequency I will occur, because the Ldh-b allele in mummichog

will provide a selective advantage in warmer waters since warmer water temperatures will

require more anaerobic respiration.


This answer suggests the student may understand that increased water temperatures may

decrease dissolved oxygen levels and result in greater rates of anaerobic fermentation, but

does not understand that the change would reduce the fitness of the Ldh-b gene because

the allele conveys an advantage in colder water.

(D) The change shown for allele frequency II will occur, because warmer waters will result in

gene flow of the Ldh-b allele as the mummichog migrate north to warmer waters.


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This answer suggests the student may understand that environmental changes can cause

gene flow due to movement of individuals, but does not understand that if this gene flow

occurred there would be an increase in the Ldh-b allele frequency in colder waters, not a

decrease.



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3.24 d: variation and natural selection quiz proctor · pdf filewhich of these best explains...

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