Question
Drs. Peter and Rosemary Grant have studied the rate of evolutionary change in the finch populations of the Galapagos Islands. Beak size in these finches determines which types of seeds the finch populations feed on. Finches with larger beaks eat thick-walled seeds, while finches with smaller beaks eat thin-walled seeds. During a drought from 1981 to 1987, the number of plants that produced thin-walled seeds decreased. It was determined that the average beak size (both length and mass) of finches increased dramaticallyduring the drought.
Which type of selection most likely led to the change in beak size during the drought?
(A) directional selection
(B) disruptive selection
(C) stabilizing selection
(D) sexual selection
▶️Answer/Explanation
Ans:
(A) Directional selection occurs when one end, or extreme, of the
range of phenotypes has a survival advantage. In this example, birds
with larger beaks were more likely to survive. Disruptive selection
occurs when both extremes of the range of phenotypes increase in
frequency; since only large beaks increased in frequency and small
beaks decreased in frequency, choice (B) is incorrect. Choice (C) is
incorrect because stabilizing selection favors the middle range of the
phenotype, which did not occur in this case. There is no evidence in the
question that suggests that mate preference that is dependent on beak
size, so choice (D) is incorrect.
Question
Drs. Peter and Rosemary Grant have studied the rate of evolutionary change in the finch populations of the Galapagos Islands. Beak size in these finches determines which types of seeds the finch populations feed on. Finches with larger beaks eat thick-walled seeds, while finches with smaller beaks eat thin-walled seeds. During a drought from 1981 to 1987, the number of plants that produced thin-walled seeds decreased. It was determined that the average beak size (both length and mass) of finches increased dramatically during the drought. During the years 1988 to 1995, the average beak size of the finches decreased.
Which of the following is the most likely explanation for this change?
(A) Female finches prefer to mate with males with smaller beaks.
(B) The drought ended, and the average rainfall returned to
predrought levels.
(C) Predator populations increased.
(D) Finches with larger beaks were more susceptible to diseases.
▶️Answer/Explanation
Ans:
(B) Before the drought, there were more thin-walled seeds and
therefore more food available for finches with smaller beaks.
Therefore, the end of the drought would increase food availability for
finches with smaller beaks and allow more finches with smaller beaks
to survive. There is no evidence in the question that suggests females
have a mate preference that is dependent on beak size, so choice (A) is
incorrect. Choice (C) is incorrect because there is no evidence for
predators killing more finches of either beak size. The question does
not give any information about disease susceptibility or resistance, so
choice (D) is incorrect.
Question
Snakes feed on toads. Cane toads (Rhinella marina) excrete a toxic substance on their skin that is poisonous to many, but not all, snake
species. If cane toads are introduced to a new environment, predict the most likely effect on the snake species in that environment.
(A) All snake species in the environment will die out due to the cane
toads’ toxin.
(B) Snake species that are resistant to the cane toads’ toxin will
increase in numbers.
(C) Snakes that are susceptible to the cane toads’ toxin will acquire
resistance to the toxin.
(D) All snake species will learn to avoid eating cane toads.
▶️Answer/Explanation
Ans:
(B) Snakes that are resistant to the cane toads’ toxin would be more
likely to survive and reproduce, so their relative numbers would be
expected to increase over time. Choice (A) is incorrect because not all
snakes are susceptible to the cane toads’ toxin, so not all the snakes
would die. Individual snakes cannot acquire resistance to the cane
toads’ toxin, so choice (C) is incorrect. Choice (D) is incorrect because
there is no evidence in the question that snakes would learn to avoid
eating cane toads.
Question
The human TAS2R38 gene encodes a cell membrane protein that influences the ability to taste bitter compounds. Individuals who
possess at least one TAS2R38 allele have the “taster” phenotype and can taste certain types of bitter compounds. It is estimated that about
70% of humans have the taster phenotype. Which of the following best explains the frequency of the taster phenotype?
(A) Many toxic compounds have a bitter taste, so the TAS2R38 allele
provided a survival advantage in ancestral humans.
(B) Ancestral humans with the TAS2R38 allele were more likely to
consume bitter-tasting foods.
(C) Bitter-tasting foods have a higher nutrient content and were more
likely to be consumed by ancestral humans who did not have the
TAS2R38 allele.
(D) A lack of the TAS2R38 allele provided a survival advantage in
ancestral humans.
▶️Answer/Explanation
Ans:
(A) Since TAS2R38 is present in the population at a higher frequency, it
probably did provide a survival advantage and allowed individuals who
possessed this allele to survive and reproduce at a greater rate than
individuals who did not possess the allele. Choice (B) is incorrect
because TAS2R38 does not cause individuals to consume bitter foods; it
just influences their ability to taste bitter foods. Bitter foods do not
necessarily have a higher nutrient content than other foods, so choice
(C) is incorrect. There is no evidence in the question that the TAS2R38
allele lowered an individual’s chance of survival, so choice (D) is
incorrect.
Question
Which of the following assertions (about how evolution by natural selection occurs) is incorrect?
(A) There are variations among individuals of a species.
(B) Some variations provide a survival advantage.
(C) Variations acquired during an individual’s lifetime are passed on
to the individual’s offspring.
(D) Over time, the frequency of individuals with variations that
provide a survival advantage will increase.
▶️Answer/Explanation
Ans:
(C) Variations that individuals acquire during their lifetime are called
acquired characteristics and are not passed on to the individual’s
offspring. There are variations among individuals of a species, so
choice (A) is not the answer. Choice (B) is not the answer because
some variations do provide a survival advantage. Over time, natural
selection will increase the frequency of individuals who possess
variations that give them a survival advantage, so choice (D) also not
the answer.