NAME: ____________________________
Bio 350 – Evolution
________
Midterm
Spring 2015
80 pts
Circle the best choice out of the options. Read all choices carefully before answering. (1 point each)
1. Mutations may or may not affect the protein that a gene codes for. Which of the following will always
alter the protein product of a gene?
a. a transition mutation
b. substitutions at the 3rd position
c. frameshift mutations
d. silent substitutions
e. point mutations
2. A transposable element, or “jumping gene,” inserts into the coding region of a gene. This will cause a:
a. loss-of-function mutation
b. silent substitution
c. transversion
d. synonymous change
e. neutral change
3. Natural selection acts:
a. against all mutations
b. on all traits, whether they are heritable or not
c. directly on genotypes
d. on pre-existing variation
e. by creating new, beneficial mutations
4. A gene under strong positive selection is likely to:
a. show many non-synonymous changes between alleles
b. show only silent substitutions between alleles
c. promote asexual reproduction
d. have no significant effect on the phenotype of the organism
e. both b and d
5. A key difference between underdominance and inbreeding is:
a. underdominance results from genetic drift
b. inbreeding tends to fix one allele at any given locus
c. their effect on heterozygosity
d. underdominance is a form of natural selection
e. none of the above
6. Which of the following does not change allele frequencies?
a. migration of new individuals into a population
b. inbreeding
c. mutation
d. random mating
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6 pts
e. both b and d
1
7. Natural selection acts more strongly than genetic drift:
a. in small populations
b. on silent substitutions
c. in large populations
d. when individuals colonize a new habitat
e. in all populations, regardless of size
8. Inversions can promote adaptation to the environment if they:
a. prevent recombination from breaking up beneficial allele combinations
b. occur in asexual lineages
c. decrease the rate at which point mutations occur
d. benefit one sex at the expense of the other sex
e. both b and c
9. A population may be prevented from adapting to its environment by:
a. the results of strong natural selection
b. continuous migration of new individuals into the population
c. the effects of random mating
d. conflict between the sexes leading to antagonistic co-evolution
e. both b and d
10. Which of the following was not one of Darwin’s postulates of evolution by natural selection?
a. within a species, individuals are variable
b. new variation is constantly introduced into populations by mutation
c. most offspring die before reproducing
d. some variation in traits is passed on from parents to their offspring
e. individuals with the most favorable variations survive and reproduce the most
11. A mutation that swaps a DNA base that is a purine (A or G) for one that is a pyrimidine (T or C) is:
a. a transition mutation
b. a tranversion mutation
c. usually noticed and corrected by DNA proofreading enzymes
d. always going to alter the protein product of a gene
e. both b and c
12. A small island has several species of birds, a resident population of about 500 bats, and no native
snakes. In which scenario would evolution rapidly result from genetic drift:
a. female birds of one species are attracted to males with red tails, while another species likes blue tails
b. a storm washes five snakes onto the island, which give rise to a population of several thousand
c. beak size of one bird species changes after a drought, because only large-beaked birds can get food
d. every year, a hundred bats fly from the mainland onto the island
e. both c and d
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6 pts
2
13. Define the following terms in 1-2 sentences each. (2 points each)
evolution – A change in the allele frequencies of a population over time.
genetic drift – Random sampling of alleles that occurs when offspring are
produced in every generation, causing alleles to randomly drift to
fixation in proportion to their initial frequency in the population;
works more strongly in small populations.
Neutral Theory of molecular evolution – Theory states that most mutations are harmful
and quickly removed by selection; all surviving alleles present in a
population are neutral, or make no difference to fitness.
List the terms that best answer the question. Be brief; complete sentences aren’t necessary.
14. List the 5 things that must be true for a population to stay in Hardy-Weinberg equilibrium. (5 points)
1) No natural selection; all individuals survive and reproduce equally
2) No mutation; alleles don’t change to other alleles
3) No migration; no individuals moved into or out of the population
4) No chance events caused some individuals to pass on more alleles to
the next generation (genetic drift)
5) Individuals mate at random; are not more likely to mate with others
of their own genotype (= no non-random mating)
15. List 4 factors that maintain genetic variation in a population. (4 pts)
sexual reproduction / crossing over/ independent assortment / meiosis
mutation / mutation-selection balance
over-dominance / heterozygote advantage / selection against homozygotes
frequency-dependent selection
migration
[slash = same basic answer]
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15 pts
3
Short answer. Answer each numbered question in a brief paragraph. (6 points each)
16. Explain the controversy over directed mutation - what did some people think bacteria were capable of?
Why did this conflict with the basic premise of Darwinian evolution by natural selection?
Observations suggested that bacteria could somehow (a) sense what gene
had to mutate for them to survive when the only energy source in their
environment was one they could not metabolize, and (b) direct or focus
mutation on that one crucial gene. Darwinian evolution holds that
“variation precedes adaptation”, meaning mutations are random with
respect to the adaptive needs of individuals; mutations happen first, by
chance, and the lucky possessor of a suddenly-advantageous allele
survives and passes that allele to its offspring. The observation (later
proved wrong) suggested organisms could mutate a lac- frameshift
mutation to a working lac+ allele in response to the presence of
lactose; later experiments confirmed the stress (only lactose available
as food) selected for a pre-existing mutation to lac+.
17. What are two advantages of asexual reproduction? Why then do most organisms reproduce sexually at
least some of the time?
(1) asexual populations have a 2x faster rate of population growth,
because all individuals are females and produce offspring; no wasting of
resources on non-offspring-producing males; (2) well-adapted individuals
pass on their allele combinations to all offspring; (3) no need to find
a mate.
Sex creates new alleles combinations not present in the parents, that
may be adaptive if the environment changes. Recombination allows new
beneficial mutations to escape the effects of background trapping, and
spares a population from Muller’s Ratchet (the continuous accumulation
of harmful mutations over time).
18. Why can’t selection efficiently remove rare recessive alleles from a population? When might it
be beneficial for a population to maintain recessive alleles that are lethal in homozygotes?
When recessive alleles are rare, they are mostly masked in
heterozygotes, so they do not affect the individual’s phenotype and
cannot be selected against. In cases of overdominance, the heterozygote
has superior fitness to the homozygote. The fitness advantage of the
recessive allele in heterozygotes balances the fitness cost of the
lethal allele in homozygotes, maintaining that allele in the population;
e.g., the recessive-lethal allele for sickle cell anemia confers malaria
resistance to the heterozygote.
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18 pts
4
Compute the answers to questions 19-20; you must show your work to get credit.
19. You are in charge of managing the last five surviving specimens of the Loud Laughing Llama, an
annoying but endangered creature that you want to preserve.
(a) Despite your efforts to arrange random matings among the surviving plants, how much heterozygosity do
you expect to lose after one generation? (5 points)
Hg+1 = Hg (1 - 1/2N)
heterozygosity in
the next generation
For N =
5, 1 - __1__
2 x 5
=
current
heterozygosity
=
Thus, heterozygosity in
the next generation
1 - 1/10
=
1 -
1__
2N
= 9/10 = 0.90
current
heterozygosity
0.90
= 90% of current heterozygosity
Thus you will lose 10% of the heterozygosity in one generation
(b) What process is responsible for the steady loss of heterozygosity over time? (3 pts)
This is
alleles
allele.
overall
a manifestation of genetic drift - the random sampling of
from a population resulting in the eventual fixation of one
Every time an allele fixes at some locus, the population loses
heterozygosity.
(c) Why is the loss of heterozygosity likely to pose a problem for the long-term survival of this species?
Under what conditions will this be a particular problem? (2 pts)
Genetic diversity is the raw material on which natural selection works,
to increase adaptation to an ever-changing environment. An excess of
homozygotes exposes harmful recessive alleles normally masked in
heterozygotes, lowering fitness. This is most apparent in stressful
environmental conditions, when there may be insufficient diversity for
any individuals to survive.
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10 pts
5
20. You collect 10 slugs under a rock in your garden. You run a sample from each slug on a protein gel,
and stain the gel so that one particular enzyme shows up. Your gel looks like this:
A
a
(a) What is the frequency of each allele in this population? (4 pts)
# of AA individuals = 1, and 2 x 1 = 2 A alleles from AA specimens
# of Aa individuals = 8, so 8 A alleles from AB specimens
total # of A alleles = 8 + 2 = 10 copies of A
total # of individuals = 10
total # of alleles in population = 20
Frequency of A allele =
# of copies of A
total # of alleles
=
10
20
= 0.5
Frequency of a allele = 1-(0.5) = 0.5
(b) What are the genotype frequencies in this population? What genotype frequencies would you expect if
the population were in Hardy-Weinberg equilibrium? (6 pts)
Actual genotype frequencies:
Hardy-Weinberg prediction:
AA = 1 = 0.1
10 individuals
p2 + 2pq + q2
= (0.5) 2 + 2(0.5)(0.5) + (0.5) 2
Aa = 8 = 0.8
10
aa = 1 = 0.1
10
= 0.25 + 0.50 + 0.25
AA = p2 = 0.25
Aa = 2pq = 0.5
aa = q2 = 0.25
(c) Name one force that may be operating on this population, and explain your answer. (2 pt)
There are more heterozygotes than you would predict from HW equation.
Some form of overdominance, or heterozygote advantage, is likely at
work, since otherwise you should see about half homozygotes.
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12 pts
6
21. You find 2 streams with populations of guppies. In a stream with no large predators, male guppies have
big red tails but females are dull brown. In a stream with predators, both males and females are brown.
(a) How did males evolve red tails if tail color in an honest indicator of male fitness? (3 pt)
Sexual selection by female choice, in which females with highest
fitness are those who are attracted to a male display trait that is an
honest indicator of his fitness. Selection favors females that prefer
such traits, as they will pass on favorable alleles to their offspring.
If males with red tails father offspring that grow faster and lived
longer, then tail color may be a reliable cue for females.
(b) If red tails are unrelated to male fitness, how might they have evolved? (3 pt)
Such traits may evolve by run-away sexual selection, where a male
trait exploits a pre-existing receiver bias in females. For instance,
the eyes of females may perceive red better than other colors, or
females may feed on red food and so are more attuned to red items in
their surroundings. Females are therefore initially attracted to red
males, but selection will eventually favor females that ignore traits
unrelated to male fitness. This sets up a cycle where the male trait is
exaggerated and female resistance increases until all males are stuck
with red tails needed to get any female attention.
(c) Why do no females have red tails, even though alleles for red tails are present in the population? (4 pt)
Since red tails are caused by male-advantage alleles that would be
harmful if expressed by a male’s daughters, the highest fitness male is
one who carries these alleles on his Y chromosome, which is never part
of the female genome. This ensures that a male’s good alleles for sexual
display traits are inherited by his sons, but not by his daughters for
whom they would just attract predators. The highest fitness strategy is
usually the one that goes to fixation in a population, so all males end
up with the Y chromosome carrying male-advantage alleles.
(d) Propose an explanation for why males and females are both dull brown when predators are present. (3 pt)
Whereas sexual selection favors red-tailed males, natural selection
imposed by predators favors dull fish that are camouflaged in their
environment (because they don’t get eaten). In this case, natural
selection overpowers sexual selection, since there are no red-tailed
fish.
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13 pts
7