Prelim 2 key

Prelim 2 key - 1...

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Unformatted text preview: 1. Which
of
the
following
statements
is
TRUE:
 
 Microevolutionary
change
always
requires
genetic
variation
to
be
present
in
a
population.
 

 2. Imagine
that
you
create
a
randomly
mating,
infinitely
sized,
sexual,
diploid
population
where
 80%
of
the
founding
individuals
are
AA
and
20%
are
Aa.
After
three
generations
of
random
 mating,
what
proportion
of
individuals
will
have
the
genotype
aa?

 
 0.01
 
 
 
 3. Which
of
the
following
statements
is
TRUE:
 
 Answers
A‐D
are
all
false.
 
 4. Consider
a
population
in
which
individuals
vary
at
one
locus,
where
there
are
two
alleles
(A
 and
a).
Individuals
who
are
homozygous
for
either
allele
produce,
on
average,
10
offspring.
 Individuals
who
are
heterozygous
produce,
on
average,
20
offspring.
All
other
HWE
 assumptions
are
met
in
this
population.
At
this
moment,
the
genotype
frequencies
in
the
 population
are:
 
 AA
 
 Aa
 
 
aa
 
 
 0.01
 
 0.1
 
 0.89
 
 Imagine
that
you
return
and
sample
this
population
after
1000
generations
have
passed.
What
 do
you
predict
the
allele
frequencies
to
be
at
that
point?
 


A
 
 



a
 
 
 
 0.5
 
 0.5
 
 
 
 NOTE:
Questions
5
and
6
both
refer
to
the
data
table
below
 
 Genotype
 
 
 
 AA
 
 Aa
 
 aa
 Phenotype
 
 
 
 large
 
 medium
 small
 Mean
age
at
first
reproduction
 5
yrs
 
 1
yr
 
 5
yrs
 Mean
#
of
lifetime
offspring

 2
 
 0
 
 8
 Mean
survival
(age
at
death)
 8
yrs
 
 2
yrs
 
 5
yrs
 
 
 5. What
are
the
relative
fitnesses
(wi)
of
the
three
genotypes?
 
 WAA
=
0.25,
WAa
=
0.0,
Waa
=
1
 
 6. What
are
the
selection
coefficients
(si)
of
the
genotypes
(where
si
=
wref
–
wi)?
 
 sAA
=
0.75,
sAa
=
1,
saa
=
0
 
 
 
 7. How
many
A
alleles
are
present
in
a
population
of
100
individuals
where
all
individuals
 are
Aa
heterozygotes?
 
 100
 
 
 8. In
which
population
below
is
genetic
drift
likely
to
become
directly
responsible
for
a
 rapid
change
in
allele
frequencies?
 
 


 

Census
Population
 
 Ne
 
intensity
of
selection
 
 
 100000
 
 10

 
 low
 
 9.
Which
of
the
following
is
FALSE:
 
 All
other
things
being
equal,
the
effects
of
selection
are
most
likely
to
be
seen
in
populations
 where
genetic
drift
is
also
a
strong
force.
 
 10.
In
discussion
section,
you
read
a
paper
about
Batesian
mimicry
in
snakes.

According
to
the
 paper,
which
of
the
following
is
the
best
explanation
for
why
the
mimetic
form
of
the
scarlet
 kingsnake
occurs
in
areas
where
there
are
no
venomous
eastern
coral
snakes?
 
 Migration
of
scarlet
kingsnakes
from
the
sympatric
range
into
the
allopatric
range
maintains
 the
mimetic
form
in
allopatry
even
though
it
is
disfavored
there
by
natural
selection
 
 2
 11.

Consider
the
graph
below,
which
is
taken
directly
from
the
lecture
powerpoint
slides.
Recall
 that
the
y
axis
shows
the
proportion
of
the
A1
allele
in
the
population:
 
 A. On
the
axes
below,
use
X
symbols
to
plot
relative
fitnesses
of
the
three
genotypes
that
are
 consistent
with
the
scenarios
shown
above,
in
which
A1
is
the
beneficial
allele

that
 determinesfitness
(3
pts):
 
 
 
 
 
 
 
 
 
 
 A1
recessive
 A1
dominant
 A1
co‐dominant
 B. Find
the
narrow
arrow
on
the
x‐axis
above.
At
this
time,
why
does
the
A1
dominant
 scenario
have
the
highest
proportion
of
A1
alleles
(3
pts)?
 When
the
A1
allele
first
arises
in
a
population,
it
is
extremely
rare
and
therefore
is
initially
found
 only
in
heterozygotes.
If
A1
is
dominant,
A1A2
has
same
fitness
as
A1A1,
so
there
is
strong
 selection
for
the
heterozygote
and
freq(A1)
increases
rapidly.
If
A1
is
recessive,
rare
mutations
are
 maintained
in
heterozygotes
that
have
the
same
fitness
as
the
A2A2
individuals
&
so
there
is
no
 selection.
If
A1
co‐dominant,
the
A1A2
heterozygote
is
less
fit
than
the
A1A1
individual,
so
it
takes
 longer
for
the
A1
allele
to
increase
in
frequency
at
first
 C. Find
the
wide
arrow
on
the
x‐axis
above.
At
this
time,
why
does
the
A1
co‐dominant
 scenario
have
the
highest
proportion
of
A1
alleles
(4
pts)?
 Over
time,
the
allele
A1
increases
in
frequency.
When
allele
A1
has
high
frequency,
the
A2
allele
is
 rare
and
is
therefore
mostly
found
in
heterozygotes.
If
A1
is
recessive,
selection
alone
cannot
 increase
the
freq
of
the
A1
allele
much.
A1
reaches
fixation
faster
in
the
co‐dominant
scenario
than
 in
the
dominant
scenario
because
the
heterozygote
has
lower
fitness.
Since
A1A2
individuals
have
 lower
fitness
in
the
co‐dominant
case,
the
A2
allele
is
not
completely
shielded
from
selection
and
 therefore
can
be
eliminated.
On
the
other
hand,
if
A1
is
dominant,
the
heterozygote
has
the
same
 fitness
as
A1A1
individuals,
so
the
disadvantageous
A2
alleles
maintained
in
A1A2
individuals
are
 masked
from
selection
(and
we
need
to
rely
on
drift
to
reach
fixation).
 
 3
 
 
 
 
 Print
your
name:
________________________________
 
 
 
 
 
 
 
 
 
 75
km
 
 
 
 
 
 
 
 
 150
km
 
 12.
Consider
the
schematic
shown
above
in
which
the
grey
oval
represents
the
entire
geographic
 area
occupied
by
a
species.

The
intensity
of
grey
shading
represents
a
“selection
gradient”—
 different
alleles
are
favored
in
different
parts
of
this
species’
range:
 
 
 
 









lightest
grey
shading
 
 







darkest
grey
shading
 
 
 
 AA
 Aa
 aa
 
 
 AA
 Aa
 aa
 Relative
fitness:
 
0.2
 0.2
 1
 
 
 1
 0.3
 0.1
 
 You
sample
1000
individuals
each
from
the
locations
labeled
A
and
B
and
find
the
following
 genotype
proportions:
 

Sample
area
A
 
 
 

Sample
area
B
 
 
 
 
 AA
 Aa
 aa
 
 
 AA
 Aa
 aa
 Genotype
proportions:
 
0.7
 0.2
 0.1
 
 
 0.9
 0.1
 0.0
 
 In
a
few
sentences,
explain
whether
the
genotype
frequencies
in
these
samples
match
what
you
 would
expect
to
see
given
the
environmental
(=selection)
differences
between
sites
A
and
B
(5
 pts).
[Hint:
this
is
not
a
HWE
math
problem
—
you
don’t
need
to
use
any
equations
at
all
to
answer
this
question.]
 Yes,
we
expect
the
genotype
frequencies
in
sample
area
B.

AA
has
the
both
the
highest
fitness
and
 the
highest
genotype
proportion.

No,
we
do
not
expect
the
genotype
frequencies
in
sample
area
A.

 Since
aa
has
the
highest
relative
fitness,
we
would
expect
to
see
aa
as
the
most
common
genotype.

 However,
genotype
proportions
show
that
AA
is
actually
the
highest
and
that
only
10%
of
the
 population
is
genotype
aa.

This
is
unexpected.
 Outline
how
one
population
genetic
process
could,
in
the
real
world,
explain
this
pattern
of
 genotype
frequencies
versus
differential
selection
in
different
locations?
(5
pts).

 Gene
flow
could
explain
this
pattern.

Since
AA
is
the
fittest
and
most
abundant
genotype
in
nearby
 area
B,
migration
from
area
from
area
B
to
area
A
could
explain
the
unexpected
genotype
 frequencies
in
area
A.

This
is
similar
to
the
biological
scenario
described
in
the
coral
snake
paper
 that
we
read
in
section
 
 4
 A
 B
 
 13.

You
are
a
biologist
interested
in
the
variation
in
the
body
sizes
of
Cayuga
Island
Turtles.

You
 know
that
body
size
is
determined
by
a
single
locus
with
two
alleles,
in
which
the
allele
for
large
 body
size
(A1)
is
dominant
to
the
allele
for
small
body
size
(A2).
You
survey
the
body
size
of
100
 turtles,
including
50
from
one
island
and
50
from
a
different
island.

In
the
sample
of
100
total
 turtles
you
observe
the
following
phenotype
frequencies:
 
 
 Large
bodied
turtles
(A1A1
and
A1A2):


 84
 
 Small
bodied
turtles
(A2A2):

 
 16
 
 If
the
population
were
in
Hardy
Weinberg
Equilibrium,
how
many
turtles
of
each
genotype
would
 you
expect
(3
pts)?
 
 A1A1:
____36_____
 
 A1A2:
___48______
 
 A2A2
:___16______
 
Minus
one
point
for
using
frequencies
(.36,
.48,
.16)
instead
of
#
of
turtles
 You
take
DNA
samples
of
these
turtles
to
your
genetics
lab
and
find
that
the
observed
genotype
 frequencies
are
as
follows:
 
 A1A1:


74
 
 A1A2:


10
 
 A2A2:

16
 
 Clearly,
this
sample
in
not
in
Hardy
Weinberg
equilibrium.

If
you
wanted
to
verify
this
conclusion
 with
a
statistical
test,
which
test
would
be
appropriate?
 
 


_______chi
squared
test______________________________
 What
process(es)
might
be
responsible
for
the
observed
deviation
from
the
null
HWE
hypothesis
 (3
pts)?
 
 
 genetic drift migration natural selection non-random mating mutation (although unlikely) 
 
 
 
 You
then
do
similar
calculations
for
within‐island
genotype
frequencies,
analyzing
the
50
turtles
 from
each
island
in
two
separate
analyses.

You
determine
that
genotype
frequencies
within
each
 individual
island
population
are
in
HWE.

Why
might
the
individual
island
populations
be
in
HWE
 but
the
pooled
sample
be
far
from
HWE
(4
pts)?
 
 The
pooled
population
assumes
gene
flow
among
island
populations
with
different
equilibrium
 allele
frequencies,
but
the
ocean
acts
as
a
barrier
preventing
gene
flow
between
populations.
 
 5
 ...
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