Download Mc1r - National Center for Case Study Teaching in Science

Selection and the
Blond Beach Mouse
by
Joan Sharp
Department of Biological Sciences
Simon Fraser University, BC, Canada
Part I: The Adaptive Value of Cryptic Coloration
Introducing
Dr. Hopi Hoekstra
Dr. Hoekstra is
Professor of Zoology
and Curator of
Mammals at Harvard
University. She
studies the oldfield
deer mouse,
Peromyscus
polionotus, to
understand the
evolution of its cryptic
coloration and
behavior.
2
The Oldfield Deer Mouse
Peromyscus polionotus
3
The Oldfield Deer Mouse
• Oldfield deer mice (Peromyscus
polionotus) live in abandoned agricultural
fields throughout the SE United States.
• They live in loosely packed sandy or clay
soils, where they can dig their burrows.
• Mainland oldfield deer mice have a dark
brown dorsal coat, a light grey belly, and a
striped tail.
4
The Beach Mouse
The Alabama beach mouse
Peromyscus polionotus subspecies ammobates
5
The Beach Mouse
• Beach mouse populations of Peromyscus
polionotus have colonized sand dune and
barrier island habitats, where they live on
brilliant white sand with sparse vegetation.
• Beach mice look very different from their
mainland cousins, with a lightly colored
dorsal coat and no pigment on their face,
belly, and tail.
6
Eight Subspecies of Beach Mice
Eight subspecies of beach mice live on sand dunes on
beaches and barrier islands that formed ~6000 years
ago along the Gulf Coast of Alabama and Northern
Florida and the Atlantic Coast of Florida.
7

Discuss in your small groups:
1. What random process was necessary for
beach mice to evolve from dark mainland
mice?
2. What non-random process was necessary
for beach mice to evolve from dark
mainland mice?
8
Evolution of Blond Beach Mice
Mutation:
A random mutation in one or more of the
genes affecting coat coloration caused light
coloration in the coat of the mutant mouse.
Natural selection:
Beach mice with mutations that produced
light coat color had higher survival and
reproductive success than dark beach mice.
9
Understanding Natural Selection
• Biologists ask ultimate questions about
why natural selection has favored a
particular trait.
• Ultimate explanations address the
adaptive value of the trait, exploring how
the trait increases evolutionary fitness in a
specific environment.
10
Understanding Natural Selection
• Biologists also ask proximate questions
about how a particular trait is expressed.
• Proximate explanations address the
mechanisms that produce the trait,
including its genetic, developmental, or
physiological basis.
11
Definitions
• Evolutionary fitness measures the
relative ability of an individual to produce
viable, fertile offspring, relative to other
individuals in the population.
• An adaptation increases an individual’s
evolutionary fitness relative to other
individuals in the population.
12
CQ#1: Japanese cranes breed in
spring and early summer. Choose the
ultimate explanation:
A. Breeding is most likely
to be successful in
spring and early
summer.
B. Hormonal changes
bring about breeding
behaviors.
C. Breeding is triggered
by the effect of
increased day length
on the birds’
photoreceptors.
13
CQ#1: Japanese cranes breed in
spring and early summer. Choose the
ultimate explanation:
A. Breeding is most
likely to be
successful in spring
and early summer.
B. Hormonal changes
bring about breeding
behaviors.
C. Breeding is triggered
by the effect of
increased day length
on the birds’
photoreceptors.
14
CQ#2: Japanese cranes breed in
spring and early summer. Choose the
proximate explanation:
A. Breeding is most likely
to be successful in
spring and early
summer.
B. Increasing day length
triggers the release of
breeding hormones.
C. Ample food is available
for chicks at this time.
15
CQ#2: Japanese cranes breed in
spring and early summer. Choose the
proximate explanation:
A. Breeding is most likely
to be successful in
spring and early
summer.
B. Increasing day length
triggers the release
of breeding
hormones.
C. Ample food is available
for chicks at this time.
16
Ultimate Questions
Hoekstra’s team asked two ultimate
questions about natural selection for
coloration in oldfield deer mice:
1. Why is blond fur adaptive in beach mice?
2. Why is dark fur adaptive in mainland
mice?
Oldfield deer mice have many visual
predators, including hawks, owls, and
coyotes.
17
Two Complementary Hypotheses
Hypothesis 1:
Blond beach mice are camouflaged and less
vulnerable to predation in a beach habitat.
Hypothesis 2:
Dark mainland mice are camouflaged and
less vulnerable to predation in a mainland
grassy field habitat.
18
Work in your small groups to plan an
experiment to test these hypotheses, marking
sure that you include the following:
• Describe your experimental setup.
• Independent variable: What will you vary in your
experiment?
• Dependent variable: What will you measure in
your experiment?
• What variables will you control or standardize?
• How will you provide replication?
19
Experimental Design Can Be Tricky!
If you plan an experiment to test predation
risk on blond and dark mice in natural beach
and field habitats, consider:
• The color of mice in a particular habitat
may not vary much.
• It can be hard (or impossible) to follow an
individual mouse and determine its fate.
20
Experimental Design Can Be Tricky!
If you plan an experiment to test predation
risk on blond and dark mice in enclosures
that mimic natural beach and field habitats,
consider:
• It’s hard to provide a full set of natural
predators in an enclosure.
• Blond and dark mice may vary in other
ways, such as odor, activity level, and
behavior. (Do mice try to hide? Escape?)
21
The Experiment
• Dr. Hoekstra and her colleagues made
models of oldfield deer mice with beach
and mainland coloration to assess the
adaptive value of camouflaged coloration
in nature.
• They made 250 soft Plasticine models of
crouching Peromyscus polionotus.
– Half were painted to look like beach mice.
– Half were painted to look like mainland mice.
22
Plasticine Mice
23
The Experiment
• The models were set out in 8 linear
transects where Peromyscus polionotus
lives.
– 4 transects were in beach habitats
– 4 transects were in mainland habitats
• 14 light and 14 dark model mice were
randomly set out on open patches 10 m
apart along each transect.
24
The Experiment
• Model mice were checked every 24 hours and
attacked models were replaced with identical
models.
• Predatory attacks were scored by the presence
of bite marks, bill marks, dragging of models, or
other “injuries.”
• The proportion of attacked model mice that
matched or mismatched their habitat were
calculated (relative to the total number of mice
attacked).
25
Work in your small groups to identify
the following features of Dr. Hoekstra’s
experiment:
1. What is the independent variable?
2. What is the dependent variable?
3. What variable(s) are controlled or
standardized?
4. How is replication provided?
26
What results would you expect if the
data support Hypothesis 1?
Hypothesis 1: Blond beach mice are
camouflaged and less vulnerable to
predation in a beach habitat.
Make a prediction!
27
Proportion of attacks
Predict the proportion of attacks
on dark and light models in a beach habitat
Dark models
Light models
28
What results would you expect if the
data support Hypothesis 2?
Hypothesis 2: Dark mainland mice are
camouflaged and less vulnerable to
predation in a mainland grassy field
habitat.
Make a prediction!
29
Proportion of attacks
Predict the proportion of attacks
on dark and light models in a field habitat
Dark models
Light models
30
The Results
Predation on dark and light models in beach or field habitats
31
CQ#3: What do these results show?
Predation on dark and light models in beach or mainland habitats.
A. Predation on dark and light models is similar in both
habitats.
B. Camouflaged models suffer no predation in both habitats.
C. Predation on camouflaged models is reduced relative to
non-camouflaged models in both habitats.
32
CQ#3: What do these results show?
Predation on dark and light models in beach or mainland habitats.
A. Predation on dark and light models is similar in both
habitats.
B. Camouflaged models suffer no predation in both habitats.
C. Predation on camouflaged models is reduced relative
to non-camouflaged models in both habitats.
33
Work in your small groups to discuss
whether these results allow you to
support or reject the two hypotheses.
Hypothesis 1:
Blond beach mice are camouflaged and less
vulnerable to predation in a beach habitat.
Hypothesis 2:
Dark mainland mice are camouflaged and
less vulnerable to predation in a mainland
grassy field habitat.
34
Dr. Hoekstra and
her colleagues
have answered
the ultimate
question about
the evolution of
blond coloration
in beach mice.
35
Why is blond color adaptive?
• Blond coloration is adaptive in beach
populations of Peromyscus polionotus
because blond mice suffer reduced
predation by visual predators.
• With increased survival, blond beach mice
are more likely to reproduce successfully
and have higher evolutionary fitness than
dark beach mice.
36
Proximate Questions
Hoekstra’s team asked two proximate
questions about blond coloration in beach
mice:
1.What are the genes that affect coat color in
mice?
2.What mutations in these genes produce
blond coat color in beach mice?
37
Part II (Detailed Version)
The Genes and Alleles Affecting
Coat Color in Mice
38
How is fur coloration determined?
• Mammals are unique in having hair or fur.
• Each hair grows from a hair follicle, and
melanocytes at the base of each follicle
produce pigments that are deposited in the
growing hair.
• In mammals, variation in pigmentation is
determined by the distribution and relative
amounts of two pigments: pheomelanin and
eumelanin.
39
Mammalian Pigments
Eumelanin pigments produce dark colored
hairs that vary from brown to black,
depending on the distribution and number of
pigment molecules in each hair.
Pheomelanin pigments produce light
colored hairs that vary from blond to red.
A hair can have a banded pattern if it
switches between these pigments as it
grows.
40
Genes Affecting Hair Color
Many genes affect hair color in mammals,
but we’ll consider two primary genes that
affect coat color in oldfield deer mice:
1. Melanocortin-1 receptor (Mc1r)
2. Agouti
41
Genes Affecting Coat Color in Mice
Hopi Hoekstra and
her colleagues
have identified
mutant alleles of
these pigment
genes that produce
blond coloration in
beach mice.
Dr. Hopi Hoekstra
42
Melanocortin-1 receptor (Mc1r) Gene
The product of Mc1r is a transmembrane receptor protein
that inserts into the plasma membrane of a melanocyte and
signals the cell to produce black/brown eumelanin pigment.
43
Why are Santa Rosa Island
beach mice blond?
Sand dunes of Santa
Rosa Island, Florida
Hopi Hoekstra and
her colleagues
captured blond beach
mice from the sand
dunes of Santa Rosa
Island, on Florida’s
Gulf Coast.
They found a point
mutation in the Mc1r
gene, changing one
of the 954 base pairs
that make up the
gene.
44
Why are Santa Rosa Island beach
mice blond?
Due to a mutation altering
a single base pair (CT) in
the Mc1r gene, Santa Rosa
beach mice have cysteine
(a small, uncharged amino
acid) at position 65 in the
Mc1r transmembrane
protein.
Field mice have arginine
(a large, positively
charged amino acid) in
position 65.
45
Why are Santa Rosa Island beach
mice blond?
The Arg65Cys mutation in
Santa Rosa Island beach
mice reduces the ability of
Mc1r to signal the
melanocyte to produce
black/brown eumelanin.
Instead, melanocytes of
mutant mice produce
pheomelanin, a blond/red
pigment.
46
CQ#4: How did the Arg65Cys
mutation arise?
A. The Arg65Cys mutation arose by chance, due to
random substitution of one DNA nucleotide for
another.
B. The Arg65Cys mutation arose because beach
mice needed blond coloration in order to be
camouflaged in their new habitat.
C. The Arg65Cys mutation arose due to natural
selection for blond coloration.
47
CQ#4: How did the Arg65Cys
mutation arise?
A. The Arg65Cys mutation arose by chance, due
to random substitution of one DNA nucleotide
for another.
B. The Arg65Cys mutation arose because beach
mice needed blond coloration in order to be
camouflaged in their new habitat.
C. The Arg65Cys mutation arose due to natural
selection for blond coloration.
48
Gulf Coast Beach Mice
Dr. Hoekstra’s team examined the five Gulf
Coast subspecies of beach mice, looking for the
Arg65Cys mutation in the Mc1r gene.
49
Gulf Coast Beach Mice
The Arg65Cys mutation is found in 4 of the 5 beach
mouse subspecies found on the Gulf Coast, with
varying frequency.
50
Gulf Coast Beach Mice
Frequency of the Arg65Cys mutation correlates with
coloration of the subspecies.
• The darkest population (Alabama) lacks the
mutation.
• 100% of the mice in the lightest population
(Santa Rosa Island) have the mutation.
51
CQ #5: Four Gulf Coast subspecies
share the Arg65Cys mutation. Select a
possible explanation for this finding.
A. The Arg65Cys mutation arose independently in
each of the four subspecies.
B. A single founding population with the Arg65Cys
mutation colonized the beach habitat and
differentiated into the four subspecies.
52
It’s most likely that a single founding population
with the Arg65Cys mutation colonized the beach
habitat and differentiated into the four subspecies.
However, it’s also possible that the mutation arose
independently in each subspecies.
Work in your small groups to consider what
additional information would help Dr. Hoekstra’s
research team to distinguish between these two
possible explanations.
53
The Woolly Mammoth
Examination of the DNA of a 43,000 year old woolly
mammoth from Siberia found that one of the two
alleles for the Mc1r gene showed the same
Arg65Cys mutation as the beach mice! There was
likely variation in fur color in mammoths.
Red-haired woolly mammoth
54
CQ #6: Woolly mammoths and Santa
Rosa Island beach mice share the
Arg65Cys mutation. Select a likely
explanation for this finding.
A. The Arg65Cys mutation arose independently in
beach mice and mammoths.
B. The Arg65Cys mutation arose in a common
ancestor to beach mice and woolly mammoths.
55
CQ #6: Woolly mammoths and Santa
Rosa Island beach mice share the
Arg65Cys mutation. Select a likely
explanation for this finding.
A. The Arg65Cys mutation arose independently in
beach mice and mammoths.
B. The Arg65Cys mutation arose in a common
ancestor to beach mice and woolly mammoths.
56
Agouti Gene
• The Agouti gene is expressed in skin cells
near the hair follicle and releases a small
protein called Agouti.
• Agouti protein binds to Mc1r protein in the
melanocyte membrane, preventing Mc1r
from signaling the melanocyte to produce
black/brown eumelanin.
• With inhibition of Mc1r, the melanocyte
produces red/blond pheomelanin.
57
Does Agouti contribute to blond
coloration?
• Beach and mainland mice have Agouti proteins
with identical amino acid sequences.
• Beach mice have mutations in Agouti that affect
gene expression, not protein structure or
function. The skin cells of Santa Rosa Island
beach mice express Agouti at a higher rate than
the skin cells of mainland mice.
How would increased expression of Agouti alter
the coat color of these mice? Why?
58
Atlantic Coast Beach Mice
Three subspecies of
beach mice live along
Florida’s Atlantic Coast.
59
Why are Atlantic Coast beach
mice blond?
• Is the light coloration of
the Atlantic Coast
beach mice due to the
same Arg65Cys
mutation found in the
Gulf Coast beach mice?
Anastasia Island
beach mouse
60
Why are Atlantic Coast beach
mice blond?
• Is the light coloration of the Atlantic Coast
beach mice due to the same Arg65Cys
mutation found in the Gulf Coast beach
mice?
• No! None of the Atlantic Coast subspecies
have the Arg65Cys mutation that
contributes to blond coloration.
61
Why are Atlantic Coast beach
mice blond?
• Mc1r does not appear to be responsible
for the blond coloration of Atlantic Coast
beach mice.
• Are mutations in the coding or regulatory
regions of Agouti responsible?
• The Hoekstra lab continues to work on this
question…
62
Atlantic and Gulf Coast Beach Mice
Gulf Coast beach mouse
Atlantic coast beach mouse
It seems that different
mutations in different
genes lead to blond
color in Atlantic Coast
and Gulf Coast
subspecies of beach
mice.
63
CQ #7: What is the best explanation for
these genetic differences?
A. Gulf Coast beach mice
evolved from Atlantic
Coast beach mice.
B. Atlantic Coast beach mice
evolved from Gulf Coast
beach mice.
C. Gulf Coast and Atlantic
Coast beach mice evolved
independently from
populations of mainland
field mice.
64
CQ #7: What is the best explanation for
these genetic differences?
A. Gulf Coast beach mice
evolved from Atlantic
Coast beach mice.
B. Atlantic Coast beach mice
evolved from Gulf Coast
beach mice.
C. Gulf Coast and Atlantic
Coast beach mice
evolved independently
from populations of
mainland field mice.
65
Why did blond fur evolve in
beach mouse populations?
Ultimate explanation:
Blond mice suffer reduced predation
by visual predators. With increased
survival, blond beach mice are more
likely to reproduce successfully and
have higher evolutionary fitness
than dark beach mice.
66
How did blond fur evolve in
beach mouse populations?
Proximate explanation #1:
A point mutation in the Mc1r gene
altered the protein coded by that gene
by one amino acid (Arg65Cys). Mc1r
could no longer signal melanocytes to
make eumelanin.
67
How did blond fur evolve in
beach mouse populations?
Proximate explanation #2:
A mutation in the regulatory region of
the Agouti gene increased gene
expression, producing more Agouti
protein. Agouti binds to Mc1r on the
melanocyte membrane and prevents
signaling to trigger eumelanin
production.
68
Part II (Simplified Version)
The Genes and Alleles Affecting
Coat Color in Mice
69
How is fur coloration determined?
• Mammals are unique in having hair or fur.
• As each hair grows, cells at the base of
the hair follicle produce pigments that are
deposited in the growing hair.
• Mammals can deposit a light pigment
(red/blond) or a dark pigment
(black/brown).
• Pigment molecules may also vary in
distribution and number.
70
Genes Affecting Hair Color
Many genes affect hair color in mammals,
but we’ll consider two primary genes that
affect coat color in oldfield deer mice:
1. Melanocortin-1 receptor (Mc1r)
2. Agouti
71
Genes Affecting Coat Color in Mice
Hopi Hoekstra and
her colleagues
have identified
mutant alleles of
these pigment
genes that produce
blond coloration in
beach mice.
Dr. Hopi Hoekstra
72
Melanocortin-1 receptor (Mc1r) gene
The Mc1r gene determines whether light
(blond/red) or dark (black/brown)
pigments are deposited in the growing
hair.
73
Why are Santa Rosa Island
beach mice blond?
Sand dunes of Santa
Rosa Island, Florida
Hopi Hoekstra and
her colleagues
captured blond beach
mice from the sand
dunes of Santa Rosa
Island, on Florida’s
Gulf Coast.
They found a point
mutation in the Mc1r
gene, changing one
of the 954 base pairs
that make up the
gene.
74
Why are Santa Rosa Island
beach mice blond?
All of the Santa Rosa Island beach mice had a
mutation in the Mc1r gene, causing them to
produce the light pigment and deposit it in their
fur.
75
CQ #4: How did the Mc1r mutation causing
light fur arise?
A. The Mc1r mutation arose by chance, due to a
random error during DNA replication.
B. The Mc1r mutation arose because beach mice
needed blond coloration in order to be
camouflaged in their new habitat.
C. The Mc1r mutation arose due to natural selection
for blond coloration.
76
CQ #4: How did the Mc1r mutation causing
light fur arise?
A. The Mc1r mutation arose by chance, due to a
random error during DNA replication.
B. The Mc1r mutation arose because beach mice
needed blond coloration in order to be
camouflaged in their new habitat.
C. The Mc1r mutation arose due to natural selection
for blond coloration.
77
Gulf Coast Beach Mice
Dr. Hoekstra’s team examined the five Gulf Coast
subspecies of beach mice, looking for the Arg65Cys
mutation in the Mc1r gene.
78
Gulf Coast Beach Mice
The Mc1r mutation is found in 4 of the 5 beach
mouse subspecies, with varying frequency.
79
Gulf Coast Beach Mice
Frequency of the Mc1r mutation correlates with
coloration of the subspecies.
• The darkest population (Alabama) lacks the
mutation.
• 100% of the mice in the lightest population
(Santa Rosa Island) have the mutation.
80
CQ #5: Four Gulf Coast subspecies share
the Mc1r mutation for light fur. Select a
possible explanation for this finding.
A. The Mc1r mutation arose independently in each
of the four subspecies.
B. A single founding population with the Mc1r
mutation colonized the beach habitat and
differentiated into the four subspecies.
81
It’s most likely that a single founding population
with the Mc1r mutation colonized the beach habitat
and differentiated into the four subspecies.
However, it’s also possible that the Mc1r mutation
arose independently in each subspecies.
Work in your small groups to consider what
additional information would help Dr. Hoekstra’s
research team to distinguish between these two
possible explanations.
82
The Woolly Mammoth
Examination of the DNA of a 43,000 year old woolly
mammoth from Siberia found that one of the two
alleles for the Mc1r gene showed the same
mutation as the beach mice! There was likely
variation in fur color in mammoths.
Red-haired woolly mammoth
83
CQ #6: Woolly mammoths and Santa
Rosa Island beach mice share the
Mc1r mutation for light fur. Select a
likely explanation for this finding.
A. The Mc1r mutation arose independently in beach
mice and mammoths.
B. The Mc1r mutation arose in a common ancestor
to beach mice and woolly mammoths.
84
CQ #6: Woolly mammoths and Santa
Rosa Island beach mice share the
Mc1r mutation for light fur. Select a
likely explanation for this finding.
A. The Mc1r mutation arose independently in
beach mice and mammoths.
B. The Mc1r mutation arose in a common ancestor
to beach mice and woolly mammoths.
85
Agouti Gene
• The Agouti gene is expressed in skin cells
near the hair follicle, producing a small
protein.
• Agouti protein binds to the pigmentproducing cells, causing them to produce
the light blond/red pigment.
86
Does Agouti contribute to blond
coloration?
• Beach and mainland mice have Agouti proteins
with identical amino acid sequences.
• Beach mice have mutations in Agouti that affect
gene expression, not protein structure or
function: The skin cells of Santa Rosa Island
beach mice express Agouti at a higher rate than
the skin cells of mainland mice.
How would increased expression of Agouti alter
the coat color of these mice? Why?
87
Atlantic Coast Beach Mice
Three subspecies of
beach mice live along
Florida’s Atlantic Coast.
88
Why are Atlantic
Coast beach mice
blond?
Anastasia Island
beach mouse
• Is the light coloration of the Atlantic Coast
beach mice due to the same Mc1r
mutation found in the Gulf Coast beach
mice?
89
Why are Atlantic Coast beach
mice blond?
• Is the light coloration of the Atlantic Coast
beach mice due to the same Mc1r
mutation found in the Gulf Coast beach
mice?
• No! None of the Atlantic Coast subspecies
have the Mc1r mutation that contributes to
blond coloration.
90
Why are Atlantic coast beach
mice blond?
• Mc1r does not appear to be responsible
for the blond coloration of Atlantic Coast
beach mice.
• Are mutations in the coding or regulatory
regions of Agouti responsible?
• The Hoekstra lab continues to work on this
question…
91
Atlantic and Gulf Coast Beach Mice
Gulf Coast beach mouse
Atlantic coast beach mouse
It seems that different
mutations in different
genes lead to blond
color in Atlantic Coast
and Gulf Coast
populations of beach
mice.
92
CQ #7: What is the best explanation for
these genetic differences?
A. Gulf Coast beach mice
evolved from Atlantic
Coast beach mice.
B. Atlantic Coast beach mice
evolved from Gulf Coast
beach mice.
C. Gulf Coast and Atlantic
Coast beach mice evolved
independently from
populations of mainland
field mice.
93
CQ #7: What is the best explanation for
these genetic differences?
A. Gulf Coast beach mice
evolved from Atlantic
Coast beach mice.
B. Atlantic Coast beach mice
evolved from Gulf Coast
beach mice.
C. Gulf Coast and Atlantic
Coast beach mice
evolved independently
from populations of
mainland field mice.
94
Why did blond fur evolve in
beach mouse populations?
Ultimate explanation:
Blond mice suffer reduced predation by
visual predators. With increased
survival, blond beach mice are more
likely to reproduce successfully and
have higher evolutionary fitness than
dark beach mice.
95
How did blond fur evolve in
beach mouse populations?
Proximate explanation:
Mutations in the Mc1r and Agouti genes
cause beach mice to produce the light
pigment and deposit it in their fur.
96
Part III: The Origin of the Blond Mc1r Allele
The Blond Mc1r Allele
Four subspecies of beach mice on
the Gulf Coast of Florida share the
mutant Mc1r allele that contributes
to blond fur color.
97
Gulf Coast Beach Mice
• 100% of the mice in
the lightest population
(Santa Rosa Island)
have the mutation.
• 3 other populations
share the mutation
with frequencies
ranging from 10% to
75%.
98
Where did the blond allele come from?
Hopi Hoekstra and her colleagues addressed this
question by studying the Santa Rosa Island
population, in which the blond Mc1r allele is fixed.
99
Two Opposing Hypotheses
Hypothesis 1:
The mutant allele was present in the ancestral
oldfield mouse population that invaded the new
beach habitat.
Hypothesis 2:
The mutant allele arose in the beach mouse
population after it colonized the beach habitat
and quickly became fixed.
100
Mc1r is a small gene, made up of 954 base
pairs that are expressed as a single
transcript.
• Hopi Hoekstra’s team examined a DNA region of
4000 base pairs that includes the Mc1r gene.
• They noted all the variant forms of this region
that occurred in two populations: an Alabama
oldfield mouse population and the Santa Rosa
Island beach population.
101
H2
H1
Origin of blond Mc1r mutant allele
102
Work in your groups to decide what
evidence would support each of
these two contrasting hypotheses.
Hypothesis 1:
The mutant allele was present in the ancestral oldfield
mouse population that invaded the new beach habitat.
Hypothesis 2:
The mutant allele arose in the beach mouse population
after it colonized the beach habitat and quickly became
fixed.
103
The results:
Most of the variant forms of the 4000 base
pair region that included the Mc1r gene
found in the Santa Rosa Island beach
mouse population were also found in the
Alabama field mouse population.
104
CQ #8: Which hypothesis do these results
support?
A. H1: The mutant allele was
present in the ancestral
oldfield mouse population
that invaded the new
beach habitat.
B. H2: The mutant allele
arose in the beach mouse
population after it
colonized the beach
habitat and quickly
became fixed.
105
CQ #8: Which hypothesis do these results
support?
A. H1: The mutant allele
was present in the
ancestral oldfield mouse
population that invaded
the new beach habitat.
B. H2: The mutant allele
arose in the beach mouse
population after it
colonized the beach
habitat and quickly
became fixed.
106
Mutation fixed in Santa
Rosa Island population
Mutation arose in
ancestral population
107
In your groups, discuss why these
results support the first hypothesis.
H1: The mutant allele was present in the
ancestral oldfield mouse population that
invaded the new beach habitat.
108
The Evolution of Blond
Coloration in Beach Mice
Dr. Hoekstra’s research tells a
remarkably complete story of evolution by
natural selection, explaining why, how,
and where selection has favored the
evolution of blond coloration in beach
populations of oldfield deer mice.
109
Selection in the Valley of Fire
You will now watch a brief film about another
story of evolution by natural selection: The
Making of the Fittest: Natural Selection and
Adaptation.
http://www.hhmi.org/biointeractive/makingfittest-natural-selection-and-adaptation
Predation has selected for dark coloration in
rock pocket mice living on lava fields in New
Mexico’s Valley of Fire.
110
Image Credits
Slides 1, 5, 6, 8, 63, 92, 99:
Photo of Alabama Beach Mouse (Peromyscus polionotus ammobates) in the Bon Secour National Wildlife Refuge, Alabama. USFWS, p.d.
http://digitalmedia.fws.gov/cdm/singleitem/collection/natdiglib/id/10113/rec/1
Slides 2:, 23, 35, 42, 72:
Various photos of Hopi Hoekstra and lab, courtesy of Hopi Hoekstra.
Slides 3,4, and 8:
Photo of Peromyscus polionotus, the oldfield mouse a.k.a. beach mouse. USFWS, p.d.,
https://commons.wikimedia.org/wiki/File:Peromyscus_polionotus_oldfield_mouse.jpg.
Slide 7, 49, 50, 51, 59, 78, 79, 80, 88, 98:
Figure 1 in Steiner, C.C., H. Römpler, L.M. Boettger, T. Schöneberg, and H.E. Hoekstra. 2009. The Genetic Basis of Phenotypic Convergence in Beach
Mice: Similar Pigment Patterns but Different Genes. Mol Biol Evol 26 (1): 35-45. doi: 10.1093/molbev/msn218. Used with permission of Oxford
University Press.
Slides 13–16:
Photo of courting Japanese cranes by Francesco Veronesi, CC BY-SA 2.0,
https://www.flickr.com/photos/francesco_veronesi/15525814846/.
Slides 31–33:
By NCCSTS, based on Figure 1 from Vignieri SN, Larson J, Hoekstra HE. 2010. The selective advantage of cryptic coloration in mice. Evolution.
64:2153-2158.
Slide 44:
Photo of Dunes of Santa Rosa Island, Florida, by User:Danielos, CC-BY-SA-2.5, https://commons.wikimedia.org/wiki/File:Santarosaislanddunes.jpg.
Slides 43, 45, 46:
By NCCSTS.
Slide 54:
Photo of woolly mammoth restoration at the Royal British Columbia Museum, Victoria, British Columbia, by User:WolfmanSF, CC BY-SA 3.0,
https://commons.wikimedia.org/wiki/File:Woolly_Mammoth-RBC.jpg.
Slides 60, 63-65, 89, 92, 93, 94, 105, 106:
Photo of Anastasia Island Beach Mouse, US NPS, p.d., https://commons.wikimedia.org/wiki/File:Peromyscus_polionotus_phasma.jpg.
111