Deck 16: The Evolution of Sex

Question 1

In a population that is polymorphic for sexual/asexual reproduction,a sexually transmitted disease has just eliminated most of the sexually reproducing individuals.When you expose samples of asexuals and sexuals from this population to harsh environmental conditions in the lab,you discover that the sexual individuals are worse at adapting to the new environment than the asexual individuals.(Note that your lab population is free from the sexually transmitted disease.)Which of the following statements is a likely explanation for your results?

A)All asexually reproducing strains independently acquired the same beneficial mutation that offset the advantage of the sexuals.
B)The bottleneck caused by the disease resulted in the fixation of many alleles in the sexually reproducing population.
C)Asexuals are favored under harsh environmental conditions.
D)The results support standard evolutionary theory.
E)B and C

Answer 1

B

Question 2

The figure below illustrates the proportion of a particular asexual snail clone in a population and the degree to which it was infected with a particular parasite.Which prediction of the Red Queen hypothesis do these results appear to reflect?

A)A correlation between parasite load and sexual reproduction
B)Oscillation in the relative frequency of asexual strains in the presence of parasites
C)Emergent host defenses are synchronously tracked by parasite responses
D)A general decline of asexual lineages in the presence of parasites

Answer 2

B

Question 3

Water fleas,Daphnia magna,can switch between asexual and sexual reproduction cycles.Which of the following observations support the hypothesis that sex is favored in unpredictable environments?

A)The introduction of predators triggers sexual reproduction.
B)D. magna reproduces asexually every other generation in a variety of habitats.
C)D. magna reproduces sexually when its aquatic habitat dries up.
D)Asexual D. magna strains outcompete introduced competitors.
E)A and C

Answer 3

E

Question 4

What evolutionary model for the maintenance of sexual reproduction do the following data appear to support?

A)Muller's ratchet
B)The Red Queen hypothesis
C)The Fisher-Muller hypothesis
D)The environmental unpredictability hypothesis

Question 5

Are there any costs when asexual organisms reproduce automictically as opposed to apomictically?

A)Yes, recombination may disrupt coadapted gene complexes.
B)Yes, recombination is the cause of the twofold cost of sex.
C)No, both are forms of asexual reproduction.
D)No, recombination will produce better adapted individuals.
E)C and D

Question 6

The Amazon molly,Poecillia formosa,is a freshwater fish that is gynogenetic.In order to reproduce,individuals of this all-female species must mate with a male from a closely related species to initiate the development of their offspring.The male's genetic material does not,however,enter the Amazon molly's egg.What costs of sexual reproduction does the Amazon molly potentially experience?

A)None. It reproduces asexually.
B)The costs associated with finding a mate.
C)The fish may spend energy competing with other females for mates.
D)It has no means of turning back Muller's ratchet.
E)B and C

Question 7

During bacterial conjugation,small circular chromosomes of DNA,called plasmids,are transferred from one bacterial cell to another.Why does this mechanism not fit the common definition of sexual reproduction?

A)No new offspring is produced.
B)No new allelic combinations are formed.
C)The cell receiving the plasmid does not contain genetic material from both parents.
D)The plasmid can recombine with the recipient cell's chromosome.

Question 8

Why is the cost of sex sometimes referred to as the "cost of males"?

A)Males do not contribute genes to the next generation.
B)Males do not produce new offspring.
C)With finite resources, males take the place of offspring-producing females.
D)Sperm cells do not contribute any substantial energy resources to the eggs they fertilize.
E)B, C, and D

Question 9

What is the consequence of a deleterious mutation becoming fixed in a population?

A)Muller's ratchet will automatically move an additional turn.
B)Recombination cannot remove the mutation.
C)Mating between unrelated individuals can produce offspring that do not carry the mutation.
D)Muller's ratchet may move an additional turn.
E)B and D

Question 10

Which of the following statements illustrates a potential cost of sexual reproduction?

A)Proteins in the seminal fluid of Drosophila males shorten the life expectancy of inseminated females.
B)Female birds of paradise inspect several males before deciding who to mate with.
C)Male and female damselflies form characteristic mating wheels, which allow the partners to fly, albeit at slower speeds than single damselflies.
D)All of the above

Question 11

Some mammals have demonstrated preferential attraction to individuals that carry immune system alleles that are complementary to their own.Which hypothesis does this observation support?

A)The Red Queen hypothesis
C)Muller's ratchet
B)The Fisher-Muller hypothesis
D)The Attribution Bias hypothesis

Question 12

Why is sexual reproduction believed to be the ancestral state in eukaryotes?

A)Asexual reproduction is completely absent from basal unicellular eukaryotes.
B)Under the parsimony principle, the most common form of a trait's state is always the ancestral state.
C)The "twiggy" distribution of obligate asexual reproduction in phylogenetic trees suggests that asexuality is a derived trait.
D)Evolutionary models suggest that obligate asexual taxa are more prone to extinction than sexual ones.

Question 13

According to the reasoning of Maynard Smith,the rate at which the proportion of asexual females increase declines as the overall proportion of asexual to sexual individuals rises because

A)as asexuals become more common, their proportional increase slows.
B)the cost of sex per individual becomes smaller when sexuals become more rare in the population.
C)under Maynard Smith's model, populations are finite.
D)This scenario does not represent an accurate representation of real populations.

Question 14

The two figures below represent an asexual and a sexual population,respectively.The letters indicate beneficial mutations.The shaded areas represent the frequency of particular combinations of mutations in the population over time.Why does it take longer for the beneficial allele combination of ABC to go to fixation in the asexual population than in the sexual population?

A)In the asexual population, new allelic combinations have to arise via de novo mutations.
B)There is no principal difference between asexual and sexual populations. In the sexual population shown, the favorable mutations just happen to be on different chromosomes.
C)The effective population size of both populations is the same.
D)Sexual populations tend to have higher mutation rates.

Question 15

In an experiment,Jokela and colleagues reared 14 replicate populations of a snail species that is polymorphic for sexual/asexual reproduction under controlled,identical conditions in the lab.Each population started with 35% asexual and 65% sexual individuals.The figure below shows the percent of asexual individuals after a year-long experimental run.Which of the following statements is a possible explanation for these results?

A)Asexual females survived better under the experimental conditions.
B)Asexual females produced more offspring per individual than sexual females.
C)Avoiding the cost of males allows faster population growth for the asexual subpopulation.
D)A novel mutation after the start of the experiment led to the increased survival of the asexual line with the mutations.
E)A, B, and C

Question 16

What are coadapted gene complexes?

A)Genes that are located on the same chromosome
B)Codominant alleles
C)Favorable combinations of alleles at different loci
D)Any epistatic interaction between different loci
E)A and C

Question 17

Which of the following statements describes an element of the Fisher-Muller hypothesis for adaptive advantage of sexual reproduction?

A)Sex builds up linkage disequilibrium.
B)Sex slows down adaptive evolution by disrupting coadapted gene complexes.
C)Sex facilitates the combination of favorable mutations that originated in different individuals into advantageous genotypes.
D)Sex accelerates adaptive evolution.
E)C and D

Question 18

Would there be any disadvantage of sex if sexual reproduction was isogamous?

A)No, females would not pay the twofold cost of sex.
B)Yes, sex may break up coadapted gene complexes.
C)No, females would still transmit the same proportion of their genes to the next generation.
D)Yes, despite contributing the same proportion of offspring as asexual females, isogamous sexual females would only transmit half the proportion of their genes to the next generation.

Question 19

Which of the following systems lends itself best to the study of the environmental factors that lead to the maintenance of sexual reproduction?

A)Cyclical parthenogenetic populations that follow highly predictable seasonal patterns independent of environmental changes
B)Obligate sexual species taxa, such as most birds
C)Species that are polymorphic for sexual/asexual reproduction, and show the same proportion of sexual reproduction in all environments
D)Populations that respond to environmental changes with a change from asexual to sexual reproduction

Question 20

Why do some scientists consider automixis sexual reproduction?

A)It results in only heterozygous gametes.
B)It allows for crossing-over.
C)It can turn back Muller's ratchet.
D)It reshuffles variation that originated in a bisexually reproducing population.
E)B and C

Question 21

Is it possible for a recombination event to turn Muller's ratchet "forward" in the population of chromosomes below? The dots on the chromosomes indicate deleterious mutations.Indicate the crossing-over event by placing an X on the figure.

Question 22

Why do pathogens tend to evolve faster than their hosts?

Question 23

Counter the following argument: "Sexual reproduction is just an accident of evolution,once established it is irreversible.One does not need to postulate specific benefits to explain its maintenance."

Question 24

Does Muller's ratchet apply to both apomictic and automictic asexual populations?

Question 25

Under what conditions are asexual and sexual reproduction favored by the three evolutionary models,involving environmental unpredictability,number of niches,and parasite load?
A)Low environmental unpredictability
B)High environmental unpredictability:
C)Low number of niches
D)High number of niches
E)Low parasite load
F)High parasite load

Question 26

Which of the following statements is an element of the DNA repair hypothesis?

A)Double-stranded mutations can be repaired in haploid organisms.
B)Homologous chromosomes serve equally well as templates for repair in both asexual and sexual organisms.
C)Functional alleles on one chromosome can mask the deleterious allele on its homologue.
D)A and B
E)B and C

Question 27

What are the two immediate benefits to sexual reproduction in unpredictable environments,according to Kondrashov?

Question 28

Could HIV infection be considered a cost of sexual reproduction? Keep your answer to one sentence.

Question 29

You catalog all obligate asexual species within a family of insects and notice that asexual species are rare,and they are all members of genera that consist predominantly of sexual species.You conclude that the asexual species tend to be short lived.Why?

Question 30

Do each of the following forms of reproduction represent asexual or sexual reproduction?
A.Amphimixis
B.Automixis
C.Apomixis

Question 31

What are two examples of asexual-like reproduction in the genomes of obligate sexually reproducing organisms?

Question 32

Develop a short hypothetical explanation (one sentence)for the observed distribution of obligate asexual reproduction under the environmental unpredictability model.