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Honors Biology
Term 2 Final Exam
Name ______________________________________
Date ____________________________ Per _______
Directions: Pick the one best answer for each multiple-choice question. Write complete and legible
answers for each short answer question.
______1. Lung cancer cells do not respond to the signals that regulate the growth of normal
lung cells. Which of the following processes is not appropriately regulated in the cancerous
cells?
A. mitosis
B. meiosis
C. transcription
D. transpiration
______2. Which of the following occurs in meiosis but not in mitosis?
A. Chromosomes coil and condense.
B. Spindle fibers form across the cell.
C. The nuclear membrane breaks down.
D. Pairs of homologous chromosomes are separated.
______3. The illustration below shows a phase of mitosis. Which of the following statements
describes what is occurring in this phase?
A. The chromosomes are duplicating their DNA.
B. The copies of each chromosome are separating.
C. The chromosomes are moving toward the center
of the cell.
D. The homologous chromosomes are preparing for
crossing over.
______4. In the synthesis phase (S phase) of the cell cycle, a body cell copies its DNA. This
DNA replication occurs in preparation for which of the following processes?
A. cell growth
B. mitosis
C. transcription
D. translation
______5. Domestic horses have 64 chromosomes. How many chromosomes should be in an
egg cell of a female horse?
A. 16
B. 32
C. 64
D. 128
______6. Two chromosome pairs from a diploid organism are
shown below. Assuming meiosis and fertilization occur
normally, which of the following pairs of alleles can an
offspring receive from this parent?
A. A and A
B. A and a
C. A and f
D. F and F
______7. Crossing over is one event that can cause an offspring to have a phenotype that is
different from its parents. Which of the following statements describes another event that
can sometimes cause an offspring to have a phenotype that is different from its parents?
A. The offspring is produced by asexual reproduction.
B. Natural selection favors the offspring with specific genetic traits.
C. Dominant alleles are passed from the parents and expressed in the offspring.
D. A genetic mutation occurs in one of the parent gametes and is passed to the
offspring.
______8. Which of the following encodes the genetic information for the earlobe trait?
A. ATP
B. DNA
C. amino acids
D. carbohydrates
______9. In some pea plant experiments, Mendel studied the inheritance patterns of two
characteristics at once, such as seed shape and seed color. He did this to determine which
of the following?
A. the process by which mutations occur
B. where genes are located within chromosomes
C. whether characteristics are inherited together or separately
D. the number of crosses necessary to cause physical changes in inheritance
patterns
______10. A woman with green eyes and a man with blue eyes have one biological child.
Which of the following statements describes the child’s inheritance of her genes for eye
color?
A. The child inherited genes for eye color from her father only.
B. The child inherited genes for eye color from her mother only.
C. The child inherited more genes for eye color from her mother than her father.
D. The child inherited an equal number of genes for eye color from her mother and
her father.
______11. A gene in horses controls whether the horse has a white coat or a colored coat. A
white female horse and a white male horse are the parents of a total of five female
offspring. Three of these offspring have white coats. The other two offspring have colored
coats. The phenotypes of the horses suggest which of the following as the most likely
pattern of inheritance for coat color?
A. The allele for a white coat is dominant.
B. The allele for a white coat is recessive.
C. The allele for a white coat is sex-linked.
D. The allele for a white coat is codominant.
______12. In cats, the allele for long hair (H) is dominant to the allele for short hair (h). If a
heterozygous long-hair cat is crossed with a short-hair cat, what percentage of the
offspring is expected to be heterozygous for hair length?
A. 0%
B. 25%
C. 50%
D. 75%
______13. In pea plants, smooth pods are dominant to wrinkled pods, and green pods are
dominant to yellow pods. Two pea plants with smooth green pods are crossed. Both plants
are heterozygous for pod texture and pod color. Which phenotype ratio in the offspring
would result from this cross?
A. 6 smooth green : 6 smooth yellow : 2 wrinkled green : 2 wrinkled yellow
B. 8 smooth green : 0 smooth yellow : 0 wrinkled green : 8 wrinkled yellow
C. 9 smooth green : 3 smooth yellow : 3 wrinkled green : 1 wrinkled yellow
D. 16 smooth green : 0 smooth yellow : 0 wrinkled green : 0 wrinkled yellow
______14. A hereditary muscular disease in horses causes abnormal opening and closing of
the sodium ion channels in the muscle cells. Which of the following statements describes
the most likely origin of this disease?
A. A virus evolved specifically to attack the muscle cells of horses.
B. Motor neurons near some of the muscle cells degenerated over time.
C. High levels of sodium in the blood irreversibly damaged the ion channels.
D. A mutation occurred in the gene coding for the sodium ion channel protein.
______15. In garden pea plants, the tall allele (T) is dominant to the short allele (t), and the
round seed allele (R) is dominantto the wrinkled seed allele (r). Which of the following
crosses could produce short pea plants with wrinkled seeds?
A. TtRr x TTRR
B. TtRr x Ttrr
C. TTRr x TTRr
D. TtRR x ttRR
______16. In snapdragons, the allele for red flower color (R) is incompletely dominant to the
allele for white flower color (r). Snapdragons that are heterozygous for flower color have a
pink phenotype. Which of the following crosses could yield all three flower color
phenotypes (red, white, and pink) among the offspring?
A. RR x rr
B. RR x Rr
C. Rr x Rr
D. Rr x rr
______17. Which of the following helps to explain why X-linked recessive traits in humans
occur more frequently in males than in females?
A. Transcription of the X chromosome occurs often in males but rarely in females.
B. Only one copy of the X chromosome is found in cells of males, but two copies are
found in cells of females.
C. Males reach physical maturity more slowly than females, giving recessive traits
more time to appear.
D. Males and females have different sets of hormones that regulate the expression of
traits.
______18. Which of the following describes the function of DNA?
A. encoding genetic information
B. storing energy in chemical bonds
C. speeding up biochemical reactions
D. destroying substances that enter the cell
______19. Which of the following units are repeatedly joined together to form a protein
molecule?
A. amino acids
B. fatty acids
C. nucleotides
D. polysaccharides
______20. The diagram below represents a process that occurs in the nucleus of a eukaryotic
cell. Which of the following statements describes what is happening in this process?
A. Molecules of RNA are forming chromosomes.
B. A molecule of mRNA is directing the production of
tRNA.
C. Molecules of DNA are being packaged for export to
the ribosomes.
D. A molecule of DNA is being replicated to make two
identical copies.
______21. Which of the following statements describes each new molecule of DNA produced
when DNA replicates?
A. Each new molecule is half the length of the original molecule.
B. Each new molecule has only the coding portions of the original molecule in its
sequence.
C. Each new molecule contains one strand from the original molecule and one newly
synthesized strand.
D. Each new molecule retains the A, C, and G bases in the DNA sequence but replaces
the T base with U.
______22. In a sample of double-stranded DNA, 30% of the nitrogenous bases are thymine.
What percentage of the nitrogenous bases in the sample are adenine?
A. 20%
B. 30%
C. 60%
D. 70%
The diagram below shows a process that occurs in cells.
______23. Which process is shown in the diagram?
A. DNA replication
B. transcription
C. translation
D. crossing-over
The box below contains a statement about mutations.
In many cases throughout geologic history, if mutations in the genetic
material of existing species had not occurred, new species would not have
appeared.
______24. Which of the following conclusions about mutations in the DNA sequence of a gene
is most consistent with the statement?
A. Mutations are always rapidly occurring.
B. Mutations are always beneficial.
C. Mutations are the only way new species arise.
D. Mutations are an important mechanism for the evolution of new species.
______25. The diagram to the right represents the beginning
and end products of a process that occurs in the nucleus of
a cell. Which process does the diagram represent?
A. recombination
B. replication
C. transcription
D. translation
An mRNA sequence is shown below. Use this mRNA sequence to answer questions
26-28.
AUGCCGAAACGU
______26. Which of the following statements describes how the RNA sequence specifies the
production of an amino acid chain?
A. Each individual RNA base codes for a single amino acid.
B. Each group of three RNA bases codes for a single amino acid.
C. Each group of three RNA bases codes for an enzyme that helps join amino acids
together.
D. Each individual RNA base codes for the ribosome location where amino acids are
assembled.
______27. Using the mRNA sequence above, what would the tRNA anticodons be?
A. TACGGCTTTGCA
B. UACCCCAAACGA
C. UACGGCUUUGCA
D. UAGGGCUUUGCA
______28. Using the genetic code, identify the amino acids that would be called for during
translation of the above mRNA molecule.
A. Met-Pro-Lys-Arg
B. Met-Lys-Arg-Cys
C. Tyr- Gly-Phe-Val
D. Tyr-Pro-Lys-Arg
______29. Some scientists use molecular evidence to study evolution. One type of molecular
evidence is the amino acid sequence of particular proteins in various species. Which of the
following best describes what the study of these sequences reveals about the species?
A. The more similar the sequences are, the faster the species will coevolve.
B. The more similar the sequences are, the more closely related the species are.
C. The longer the sequences are, the earlier the species evolved in geologic history.
D. The more similar the sequences are, the more adaptations the species have.
______30. The illustration below shows two snakes of the same species that have different
striping. California king snakes may exhibit different patterns
of stripes. According to evolution by natural selection, which
of the following is the most likely result if a snake-eating
predator can more easily detect the snakes with the
crosswise stripes?
A. The percentages of snakes born of each type will
not change.
B. Snakes with lengthwise stripes will become more
common.
C. Snakes with crosswise stripes will learn to move faster.
D. Snakes with crosswise stripes will become snakes with lengthwise stripes.
______31. Penguins are powerful swimmers. Scientists have concluded that penguins
evolved adaptations for powerful swimming early in their evolutionary history.
Which of the following is the most dependable type of evidence scientists used to make this
conclusion?
A. DNA sequences of different penguin species
B. maps of the global distribution of penguin species
C. fossils showing the shape of ancient penguins’ wings and feathers
D. x-rays of the bone structure in modern penguins’ wings
______32. Two groups of very similar-looking finches are found on the Galápagos Islands.
One group of finches is found only on Floreana Island. The other group of finches is found
on several of the islands, including Floreana Island. On Floreana Island, these two groups of
inches do not interbreed. Which of the following conclusions is best supported by this
information?
A. Speciation of the two groups of finches occurred through temporal isolation.
B. Speciation of the two groups of finches occurred through behavioral isolation.
C. Speciation of the two groups of finches occurred through geographic isolation.
D. Speciation of the two groups of finches did not occur.
______33. Fish, frogs, birds, and monkeys all have an embryonic stage in which structures
called pharyngeal slits appear on the sides of the throat. In fish, these slits develop into
gills, but in frogs, birds, and monkeys, these slits develop into other structures. Based on
this information, which of the following is the best conclusion about the evolutionary
histories of fish, frogs, birds, and monkeys?
A. The animals evolved from a common ancestor.
B. The animals did not develop lungs for breathing.
C. The animals are not adapted to eat the same kinds of food.
D. The animals have evolved similar developmental rates from fertilization to birth.
______34. Which of the following examples best illustrates the process of evolution by
natural selection?
A. A person with asthma has trouble breathing.
B. A group of species has common DNA sequences.
C. A man and a woman have 10 children over the course of their lifetime.
D. A population becomes immune to a lethal disease over many generations.
______35. Some areas of a forest contain rich soil, while in other areas the soil is poor. Plants
of a certain species grow taller in the rich soil than in the poor soil. The taller plants receive
more sunlight and are able to produce more offspring than the shorter plants. If these
offspring grow in rich soil, they are tall, but if they grow in poor soil, they are short. Which
of the following statements best explains why this situation is not an example of evolution
by natural selection?
A. The observed differences in plant height are not due to genetics.
B. The differences in soil conditions do not affect all plants equally.
C. The short plants and tall plants are not isolated enough from each other.
D. The short plants are not accumulating gene mutations as quickly as the tall plants.
______36. Which of the following is the best scientific evidence that mammals evolved from
reptiles?
A. similarities in the diets of extinct reptiles and modern mammals
B. fossils that show gradual changes in skull shape from reptile-like organisms to
mammal-like organisms
C. fossils of ancient reptiles and mammals that appear together in the same layers of
rock
D. similarities in the average lifespans of modern reptiles and modern mammals
Use the information below to answer questions 37-38.
Walking sticks are insects that live on plants. In one walking stick species, the insects may
have either a striped body or a body without stripes. In one region where these walking
sticks live, the dominant vegetation is a plant with needle-like leaves that resemble the
walking stick’s striped body.
______37. According to natural selection, which of the following is the most likely result if
there are large numbers of walking stick predators in the region?
A. Walking sticks without stripes will reproduce faster.
B. Walking sticks with stripes will become more common.
C. Walking sticks will develop a new, spotted pattern for their bodies.
D. Walking sticks will live on rare plant species rather than the dominant species.
______38. Walking sticks with stripes exhibit what type of adaptation?
A. Behavioral
B. Coloration
C. Physiological
D. Structural
Insects that are camouflaged in their environment are less likely to be eaten by
birds. The graph below shows the distribution of body color in a population of an
insect species
______39. The insects live on trees. A black fungus begins to grow on the trees where the
insects live. Which of the following graphs shows the most likely distribution of body color
in the insect population after several years of fungus growth?
______40. What type of selection does the above situation show?
A. Directional selection
B. Stabilizing selection
C. Disruptive selection
D. Sexual selection
______41. Which of the following is most likely to lead to the
greatest decrease in the deer mouse population?
A. an increase in the owl population
B. an increase in the grass population
C. an increase in the pine tree population
D. an increase in the cottontail population
______42. Within a prey population, which of the following is most immediately affected by
the arrival of a new predator?
A. death rate
B. evolution rate
C. immigration rate
D. maturation rate
______43. Which of the following best describes the producers in a terrestrial food web?
A. They are at the highest trophic level.
B. They are not affected by decomposers.
C. They convert solar energy to chemical energy.
D. They obtain all their nutrients and energy from consumers.
______44. Part of a food web for a marine kelp forest is shown below.
Which of the following statements correctly describes the
transfer of energy that initially enters this system?
A. The sea urchin gets energy from the sea otter.
B. The shark receives most of the energy that enters
the ecosystem.
C. The crab transfers less energy to
the next trophic level than does the rockfish.
D. The kelp converts energy into
a form that can be used by other organisms.
______45. Seals and sea birds are native wildlife on Macquarie Island, located south of
Australia. In the 1800s, humans introduced rats, rabbits, and cats to the island. The rabbits
fed on the native plant species. The cats fed on the rats, rabbits, and sea birds. In the 1980s,
the Australian government decided to remove all the cats from the island. Which of the
following was a direct, immediate effect of removing all the cats from the island?
A. a decrease in the seal populations
B. an increase in the plant populations
C. an increase in the rabbit populations
D. a decrease in the sea bird populations
______46. Ticks carry bacteria that cause Lyme disease. Ticks do not get Lyme disease, but
they can transfer the bacteria to humans, who can get the disease. Which of the following
statements best describes the relationships among the bacteria, the ticks, and the humans?
A. The relationship between the bacteria and the ticks is competition, and the
relationship between the ticks and the humans is predation.
B. The relationship between the bacteria and the ticks is competition, and the
relationship between the ticks and the humans is parasitism.
C. The relationship between the bacteria and the ticks is commensalism, and the
relationship between the ticks and the humans is parasitism.
D. The relationship between the bacteria and the ticks is commensalism, and the
relationship between the ticks and the humans is predation.
______47. In a population of rodents, birth rate plus immigration rate is greater than death
rate plus emigration rate. Which of the following occurs under these conditions?
A. The size of the population always increases.
B. The size of the population always decreases.
C. The size of the population never changes.
D. The size of the population never reaches its carrying capacity.
______48. Brown tree snakes were accidentally carried to the island of Guam in the cargo
bays of military planes after World War II. Brown tree snakes prey upon birds. There are
no natural predators of brown tree snakes on Guam. Which of the following most likely
happened as a result of the arrival of the brown tree snakes on Guam?
A. Many bird populations increased.
B. Forest biodiversity decreased.
C. Immigration of new species of birds decreased.
D. All reptile species experienced increases in population size.
______49. Which of the following is an example of emigration?
A. A small number of hummingbirds leave an island.
B. A large number of hummingbirds die in a hurricane.
C. A species of hummingbird lays its eggs in the summer months.
D. A species of hummingbird adapts to its environment over time.
The graph below shows population growth for paramecia kept under laboratory
conditions for 18 days.
______50. Which of the following statements explains what is happening in the region of the
curve labeled “X”?
A. The population’s birthrate is zero.
B. The paramecia are in water that is too warm.
C. The paramecia have used up their food supply.
D. The paramecia have reached carrying capacity.
______51. The ecological relationship between a hawk and a rabbit is the same type of
relationship as that between
A. a tick and a deer.
B. a frog and an insect.
C. a mouse and a chipmunk.
D. a bee and a flowering plant.
______52. Which invertebrates exhibit radial symmetry?
A. Cnidarians and echinoderms
B. Sponges and flatworms
C. Roundworms and annelids
D. Mollusks and arthropods
______53. Which of the following invertebrates has a closed circulatory system, nephridia,
and a hydrostatic skeleton?
A. an echinoderm
B. an annelid
C. a flatworm
D. a sponge
______54. Which invertebrate has a gastrovascular cavity?
A. arthropod
B. cnidarian
C. mollusk
D. roundworm
______55. The distinguishing feature of a closed circulatory system is that
A. it does not include a heart.
B. blood is contained within vessels that extend throughout the body.
C. blood is kept at low pressure.
D. blood is circulated less efficiently than in an open circulatory system.
______56. An example of an animal with an open circulatory system is a(an)
A. sponge
B. cnidarian
C. arthropod
D. annelid
______57. The eyespots of flatworms can
A. detect the presence of light.
B. detect motion.
C. detect color.
D. form images.
______58. An endoskeleton is a
A. shell of a mollusk.
B. fluid-filled body cavity that supports the muscles.
C. structural support located inside the body.
D. hard body covering made of chitin.
______59. Vertebrae are characteristic of
A. worms.
B. all chordates.
C. vertebrates only
D. reptiles only.
______60. The main difference between ectotherms and endotherms is
A. the source of their body heat.
B. how they obtain food to provide for their metabolism
C. whether they control their body temperature
D. whether they conserve or eliminate body heat.
______61. What is the general rule regarding respiratory organs in vertebrates?
A. Aquatic vertebrates use lungs, and land vertebrates use gills.
B. Aquatic vertebrates lungs and gills, and land vertebrates use gills.
C. Aquatic vertebrates use gills, and land vertebrates use lungs.
D. Aquatic vertebrates use gills, and land vertebrates use lungs and gills.
______62. A single-loop circulatory system is characteristic of
A. fishes
B. amphibians
C. most reptiles.
D. crocodiles.
______63. Which of the following vertebrates has a heart with four chambers?
A. a salamander
B. a lizard
C. a goldfish
D. a cow
Read the following excerpt from Scientific American and answer questions 64-66
that follow.
Altered Virus Calls Out Hidden Cancer Cells–and Might Help Fight Them,
Too
By Katherine Harmon Courage | May 11, 2011
Most forms of cancer still must be spotted visually to be
diagnosed. But if a newly devised virus can do the job, it
could track down cancer cells too small or well hidden to
be seen in scans. It might also help shrink tumors, too.
The virus in question is a herpes virus, modified
genetically to act only on cancer cells.
Using mice, researchers injected the altered virus, which then "deliver[s] genetic
information that induces a known blood biomarker for cancer to be secreted by cancer
cells," Timothy Cripe, a physician in the Division of Oncology at Cincinnati Children’s
Hospital Medical Center and co-author of a new paper outlining the technique, said in a
prepared statement. The biomarker, Gaussia luciferase, is easy for researchers to spot
via urine or blood tests. The virus might also help to diminish any existing tumors, the
study authors reported, making it a so-called theragnostic compound. Altered versions
of smallpox and herpes viruses are already in clinical trials to help battle back cancer.
The virus made cancer cells produce detectable levels of the biomarker in 90 percent of
subject mice with bone cancer, muscle cancer, Ewing’s sarcoma and other forms of
malignant tumors. It even raised glowing flags in mice that had microscopic amounts of
cancer cells hiding in their kidneys, suggesting that in humans it might work to root out
tumors that are just millimeters across.
The researchers do raise concerns about possible immune responses to the virus, and
even cancer-free mice showed low levels of the biomarker. Cripe noted that "there is
certainly room for further refinement." But, he added, "If ultimately validated in human
trials, it could have implications for people with known cancer risk or who have had a
history of cancer." It could also be used to help track the progress of treatment.
Except for the few types of cancer for which there are already known biomarkers, such
as prostate and some liver cancers, broader cancer screening has been hampered by
high costs and limited access to sophisticated equipment. But given the viral injection
method’s simplicity, it could help make cancer screening more universal.
It could also prove especially helpful in improving cancer diagnosis in areas that lack
advanced imaging equipment, such as remote or developing regions. And that could
help stem the costs of cancer care, which are expected to rise to exceed $157 billion—and
be as high as $207 billion—in the U.S. alone by 2020, by reducing the rates of expensive
established cancers that escape early-stage diagnosis. "Early cancer detection is vital to
improve cure rates," Cripe said. "Cancer stage predicts prognosis."
______64. Which of the following was the author’s purpose in writing this article?
A. To inform readers of the bias often shown in cancer research studies.
B. To persuade readers to get early screenings for cancer.
C. To entertain readers with new scientific discoveries.
D. To inform the public of new scientific methods found to identify cancer.
______65. In the article, Timothy Cripe states, “If ultimately validated in human trials, it could
have implications for people with known cancer risk or who have had a history of cancer.”
What kind of “implications” is he referring to when saying this?
A. Individuals with cancer risks can be screened earlier, therefore allowing them to
start treatment earlier.
B. Individuals with cancer risks can be screened more often, which will increase the
total cost of cancer care.
C. Individuals with cancer risks can be screened earlier, therefore allowing reducing
the rates of expensive established cancers that escape early-stage diagnosis.
D. Individuals with cancer risks cannot be screened early, and therefore may have to
pay billions of dollars to get the care they need.
______66. Which of the following statements supports the main idea of the article?
A. “Most forms of cancer still must be spotted visually to be diagnosed.”
B. “It even raised glowing flags in mice that had microscopic amounts of cancer
cells hiding in their kidneys…”
C. “Using mice, researchers injected the altered virus, which then “delivers genetic
information that induces a known blood biomarker for cancer…””
D. “Cancer stage predicts prognosis.”
Read the following articles. Use TWO pieces of textual evidence (FROM EACH
ARTICLE) to explain, how natural selection leads to evolution in populations. Make
sure to cite each piece of evidence in your essay.
“The Peppered Moths of Britain”
The peppered moth (Biston betularia) is a moth, which flies during the night and rests on
trees during the day, where it is camouflaged to hide from birds. The two most common
forms are called typicall, a pale speckled moth which is well-disguised on light-colored
lichens growing on trees, and carbonaria, a black moth which is easy to see on the same
background. Both are different forms of the same species, like humans with blonde or
brown hair. Before the Industrial Revolution, when there was much less pollution, many
trees were covered in lichen and the typical form was well camouflaged when resting on
them. Because the carbonaria form stood out against this pale background, birds
found carbonaria moths much more easily, so carbonaria were more likely to be eaten, and
so less likely to survive to pass on their genes. This meant that the carbonaria form was
rare.
As coal-burning factories were built, air pollution increased significantly, which killed off
the lichens and blackened the trees with soot. On this dark background, the
pale typical moths were no longer well camouflaged and were easily caught by birds, but
the carbonaria moths were harder for birds to see, so more carbonaria survived to breed
and pass on their genes, and this dark type of peppered moth became more common. By
1895, 98% of moths in heavily polluted Manchester were carbonaria. Armed with Darwin’s
new theory of natural selection, J.W. Tutt, an English entomologist, hypothesized in 1896
that the change in color was due to selection pressure based on how often birds were able
to spot the moths.
After the Clean Air Act was introduced in the 1950s, trees regained their former
appearance reversing the selection pressure, and with the mottled typical form now falling
victim to birds less often their numbers began to increase. Sure enough, between 1959 and
1984 the population of the typicall form rose from 6% to 30%, and is continuing to increase
its numbers with the forecast that the carbonaria form will be pushed to extinction by
2020. This shows that natural selection can work in both directions, and the decline
in carbonaria has been monitored ever since.
NEW YORK TIMES
August 8, 2011
A Colorful Way to Watch Evolution in Nebraska’s Sand Dunes
By HILLARY ROSNER
VALENTINE, Neb. — The nearest ocean is a thousand miles away. Yet across central
Nebraska, peeking out from rolling hills of prairie grass, is sand — beige and reminiscent of
a Caribbean beach. Nebraska’s sandhills are the largest sand dunes in North America,
spreading for more than 20,000 square miles, over more than a quarter of the state. The
sand hails from the Rocky Mountains: tiny specks of eroded quartz that blew east and
landed here, 8,000 years ago.
For a team of evolutionary biologists and geneticists from Harvard University who spent
part of their summer in Valentine, on the sandhills’ northern edge, the draw is not the
quartz itself, but a particular mouse whose coat color matches it. They’re using
pigmentation to study a central question of biology: How do organisms adapt to their
environments? Color has preoccupied naturalists for centuries, and was a popular subject
for early genetics research because it’s so visually obvious. Today, the Harvard group, run
by Hopi Hoekstra, professor and curator of mammals for the university’s Museum of
Comparative Zoology, is studying color to piece together evidence of how genetic diversity
occurs, and how natural selection acts on that diversity.
They want to understand the specific genetic mechanisms that lead to changes in physical
appearance, and then how those physical changes affect an organism’s fitness — how likely
it is to survive and reproduce. “Fitness is the most important concept in biology,” said Dr.
Hoekstra. “But no one ever measures it.” The deer mouse, Peromyscus maniculatus, is the
most populous mammal on the continent, a versatile little rodent that can thrive in virtually
any environment. Before the sandhills formed, Nebraska’s soil was dark — and so were its
deer mice. Even today, most of the region’s mice have medium- to dark-brown coats, except
for the ones that scurry around on the sand. Their fur tends toward tan or orangey-blond.
The reason seems obvious enough: Mice that have dark fur and live on light sand are easy
pickings for hungry hawks and owls. A genetic mutation led to lighter fur, and the sandhills
mice lucky enough to be born mutants were more likely to survive and breed. With each
generation, the population of light mice would grow, while that of the dark mice would
shrink, until eventually, the bulk of the sandhills mice sported lovely beige fur. It’s a
textbook tale that any high school student could explain.
The only problem is, it might not be true. To Dr. Hoekstra and Rowan Barrett, a
postdoctoral researcher in her lab, the fact that it makes sense says nothing about its
scientific credibility. “People love to tell stories about how species adapt and evolve,” said
Dr. Barrett, “but no one ever sees it happen.”
Thanks to an accumulation of knowledge about both the ecology and the genetics of the
Nebraska deer mice, the animals offer a rare opportunity to directly test the story, and to
measure the effect of environmental conditions on evolution. Several years ago, Dr.
Hoekstra and another postdoctoral researcher, Catherine Linnen — now at the University
of Kentucky — discovered that a specific gene controls much of the fur coloring in P.
maniculatus by telling pigment-producing cells how long to spend making certain shades.
More recently, they identified four different parts of the gene where mutations affect coat
color, and showed that natural selection is involved in the color changes.
“It’s kind of fun to find the mutations,” said Dr. Hoekstra, “because that’s the basic material
of change. But at a bigger level, there are two general things we can learn. One is about how
changes in gene expression evolve. And the other is, does evolution occur through big leaps
or the accumulation of small, gradual changes?”
To answer this for the specific case of the deer mice, the scientists are examining the
mutations that arise in a group of wild mice, how they affect physical appearance, and how
that gives certain individuals a better shot at reproducing. To watch evolution in action, Dr.
Barrett, who has a reputation for devising grand, slightly crazy experiments, dreamed up a
project: construct eight enclosures, four on the light sand and four on the darker soil, stock
them with an equal number of dark and light mice and watch what happens over time.
At a reservoir 20 miles south of Valentine, in the heart of sandhill country, Dr. Barrett and a
rotating crew from Cambridge built four pens out of galvanized steel sheets. Each enclosure
is a 150-foot square with three-foot-high metal walls dug two feet into the ground,
anchored by steel rods hammered deep into the sand. It’s designed to keep mice in, and
other rodents and ground-dwelling predators — like rattlesnakes — out. East of town, on a
ridge of alfalfa fields, Dr. Barrett built four additional pens, identical to the reservoir
enclosures except for the soil, which is dark.
“Mice are these tiny little animals,” Dr. Barrett mused one evening, “and yet we used tens of
thousands of pounds of materials just to do this. Imagine if it was squirrels.”
Each evening for nearly a month, the crew caught mice, setting live traps — long metal
boxes that snap shut with the pressure of little rodent feet — at light and dark locations.
Fanning out in a corn field, on sandy ridges along the highway, or near the enclosures
themselves, the team would proceed methodically: open the trap, set it down, throw in a
handful of sunflower seeds. Each morning at 6:30, before the heat of the day would cook
any mice caught in the metal boxes, they would return and collect the traps.
The garage of their rented house functioned as a makeshift lab. The scientists would sedate
each mouse with a few whiffs of isoflurane, and then weigh it, measure it, punch a tiny hole
in its ear for visual identification, and measure its coat color using a spectrophotometer, an
instrument that tracks light waves. They photographed each mouse beside a color chart,
and inserted a tiny microchip between its shoulder blades. Finally — usually after a bit
more isoflurane — they cut a snip off the end of each mouse’s tail, to collect its DNA. These
they stored in vials in the kitchen, in a refrigerator vegetable bin.
In the evening, before the next round of trapping, they released the mice into the
enclosures. Each enclosure will eventually house 100 mice. Dr. Barrett will return roughly
every six weeks, setting traps inside the pens to check the frequencies of genetic variants
and monitor changes. This will help the scientists understand how genes are linked to
physical appearance, and how both are linked to fitness. “We may be able to detect that a
gene is being favored not because it controls coat color but because it’s doing something
else to the organism that’s having some correlated effect,” said Dr. Barrett.
“People typically have this view of evolution as being an extremely gradual process,” said
Dr. Barrett. “But you have situations like antibiotic resistance, and systems that have been
exploited by humans, like overfishing, where you get incredibly rapid changes. So doing
these experiments can give you an idea, if you push wild animals, how quickly could they
potentially evolve? It allows you to make predictions about how things are going to change
in the future.”
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