2018
AP Biology
Sample Student Responses
and Scoring Commentary
Inside:
Free Response Question 1
R Scoring Guideline
R
Student Samples
R
Scoring Commentary
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AP
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BIOLOGY
2018 SCORING GUIDELINES
Question 1
Figure 1. Phylogenetic tree representing the evolutionary relatedness among bear
populations based on mitochondrial DNA sequence comparisons
Polar bears are highly adapted for life in cold climates around the North Pole. Brown bears, black bears,
and pandas are found in warmer environments. Researchers collected complete mitochondrial DNA
sequences from several populations of bears and constructed a phylogenetic tree to represent their
evolutionary relatedness (Figure 1).
A researcher studying adaptation in bears sequenced the nuclear gene encoding a lysosomal trafficking
p
rotein (LYST) in polar bears, brown bears, black bears, and panda bears. There are seven inferred amino
acid substitutions that are found only in polar bears. Mutations that cause similar substitutions in the
human LYST protein are associated with Chediak-Higashi syndrome, an autosomal recessive condition in
which pigment is absent from the hair and eyes. The researcher used the inferred amino acid sequences to
build the distance matrix shown in Table 1.
TABLE 1. AMINO ACID DIFFERENCES IN THE
L
YST PROTEIN AMONG BEAR SPECIES
Panda Black Brown Polar
Panda −
Black 33 −
Brown 34 1 −
Polar 40 7 8 −
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BIOLOGY
2018 SCORING GUIDELINES
Question 1 (continued)
(a) Use the phylogenetic tree in Figure 1 to estimate the age in hundreds of thousands of years of the most
recent common ancestor of all brown bears. Identify the population of brown bears to which polar
bears are most closely related based on the mitochondrial DNA sequence comparison. Identify two
populations whose positions could be switched without affecting the relationships illustrated in the
phylogenetic tree.
Estimate (1 point)
• First two digits of the answer must be between 30 and 35.
Identification (1 point)
• European
Identification (1 point)
• European/Polar OR Asian/Western (North American)
(b) Construct a cladogram on the template to represent a model of the evolutionary relatedness among the
bear species based on the differences in LYST protein sequences (Table 1). Circle the position on the
cladogram that represents the out-group.
Construction (1 point)
• Correctly illustrated evolutionary relationship among the four species
Circling (1 point)
• Correctly circled out-group based on orientation of cladogram
(c) A student claims that mitochondrial DNA sequence comparisons provide a more accurate phylogeny of
b
ear species than do LYST protein sequence comparisons. Provide ONE piece of reasoning to
support the student’s claim.
Reasoning (1 point)
• Genes show more variability (in nucleotide sequence) than proteins do (in amino acid sequences).
• mtDNA genome contains multiple genes vs. one lyst gene.
• The phenotype associated with the lyst gene is under strong selection.
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BIOLOGY
2018 SCORING GUIDELINES
Question 1 (continued)
(d) A researcher genetically engineers a mouse strain by deleting the mouse lyst gene and replacing it with
the polar bear lyst gene. Predict the most likely difference in phenotype of the transgenic mouse strain
compared to the wild-type mouse strain. Justify your prediction.
Prediction (1 point)
• Mouse fur and/or eyes will not have pigment/will have reduced pigment.
• Mouse (fur) will be white/lighter.
Justification (1 point)
• Polar bear lyst gene/LYST protein is associated with a lack of pigment/white hair.
• Mutated human lyst gene/ LYST protein is associated with a lack of pigment in hair and eyes.
(e) Describe how the mutation in the lyst gene became common in the polar bear population. If the lyst
g
ene were the only determinant of fur color, predict the percent of white offspring produced by a
mating between a polar bear and a brown bear.
Description (1 point)
• Natural selection for the white fur phenotype
Prediction (1 point)
• 0%
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AP
®
BIOLOGY
2018 SCORING COMMENTARY
Question 1
Overview
This question is based on recent investigations into the evolutionary history of polar bears. Students were
presented with a phylogenetic tree created from mitochondrial DNA (mtDNA) data for five populations of bears.
Using this phylogenetic tree, students were asked to estimate the age of the most recent common ancestor of all
brown bears, to identify what population of brown bears was most closely related to polar bears, and to identify
two populations of bears whose positions on the phylogenetic tree could be switched without altering the
proposed relationships. Students were presented with a data table indicating the number of amino acid
differences in the lysosomal trafficking protein LYST among four species of bears. Using these data, the students
were asked to construct a cladogram on a provided template and circle the outgroup position on that cladogram.
Students then were asked to provide one piece of reasoning to support a claim that mtDNA sequence
comparisons provide a more accurate prediction of bear phylogeny. The students were then asked to predict the
most likely difference in the phenotype of a mouse that was genetically altered to contain the polar bear-specific
lyst allele. The students were asked to justify their prediction. Finally, students were asked to describe how the
lyst allele became common in the polar bear population and to predict what percentage of the offspring resulting
from a mating between a brown bear and a polar bear would be white.
The key understandings and skills students were expected to demonstrate included the following:
• Basic concepts of evolution were used to evaluate and interpret data and to describe evolutionary
relationships and processes.
• Knowledge of Mendelian and molecular genetics was used to predict the outcomes of a genetic
alteration and a mating based on experimental design.
Sample: 1A
S
core: 10
T
he response earned 1 point in part (a) for estimating that the most recent common ancestor of all brown bears
existed 325,000 years ago. The response earned 1 point in part (a) for identifying that European Brown Bears are
most closely related to Polar Bears. The response earned 1 point in part (a) for identifying that Western North
American Brown Bear and Asian Brown Bear positions could be switched on the phylogenetic tree. The response
earned 1 point in part (b) for constructing a cladogram that correctly orders the taxa, from left to right, Panda,
Polar, Black, and Brown. The response earned 1 point in part (b) for circling a correct outgroup at the leftmost
terminal branch, without including the ancestral node of the cladogram. The response earned 1 point in part (c)
for supporting the student’s claim by reasoning that mtDNA would include more genes that code for more
proteins that could be compared in contrast to one gene coding for a single LYST protein. The response earned
1 point in part (d) for predicting that the mouse’s fur would lack pigment. The response earned 1 point in part (d)
for justifying that humans having the mutant form of the LYST protein lack pigment in hair. The response earned
1 point in part (e) for describing that the lyst gene became common when white individuals had a selective
advantage. The response earned 1 point in part (e) for predicting that zero percent of offspring should be white.
S
ample: 1B
Score: 8
T
he response earned 1 point in part (a) for estimating that the most recent common ancestor existed 325,000
years ago. The response earned 1 point in part (a) for identifying that European Brown Bears are most closely
related to Polar Bears. The response earned 1 point in part (a) for identifying that Polar Bear and European Brown
Bear positions could be switched on the phylogenetic tree. The response earned 1 point in part (b) for circling a
correct outgroup at the leftmost terminal branch, without including the ancestral node of the cladogram. The
response earned 1 point in part (d) for predicting that the mouse would be growing pigmentless hair. The
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AP
®
BIOLOGY
2018 SCORING COMMENTARY
Question 1 (continued)
response earned 1 point in part (d) for the justification that because the lyst gene in Polar Bears encodes for a lack
of pigment in hair, the mouse would begin growing pigmentless hair. The response earned 1 point in part (e) for
describing that the lyst gene became common as a result of natural selection for white Polar Bears that were able
to blend into a snowy white environment. The response earned 1 point in part (e) for predicting that no offspring
should be white.
Sample: 1C
Sc
ore: 6
The response earned 1 point in part (a) for estimating that the most rec
ent common ancestor existed 325,000
years ago. The response earned 1 point in part (a) for identifying that European Brown Bears are most closely
related to Polar Bears. The response earned 1 point in part (a) for identifying that the position of the Polar Bear
and the position of the European Brown Bear could be switched on the phylogenetic tree. The response earned
1 point in part (b) for constructing a cladogram that correctly orders the taxa, from left to right, Panda, Polar,
Black, and Brown. The response earned 1 point in part (b) for circling a correct outgroup at the leftmost terminal
branch, without including the ancestral node of the cladogram. The response earned 1 point in part (e) for
predicting that zero percent of offspring should be white.
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