Evolution unit 2
In this population of 16 dots, calculate the estimated change in the A2 allele frequency in one generation if s = 0.01.
0.246 % increase
The product N e m is equal to the
number of reproductive migrants that arrive in a population in a generation.
When the environment affects the phenotypes of all the genotypes in a population and the reaction norms of the genotypes are parallel,
only genetic variance and environmental variation contribute to phenotypic variance
Phenotypic variance that is due to genetic variance can be attributed to
pleiotropy. linkage disequilibrium. dominance. epistasis.
What will happen to DNA sequence variation in the regions of the genome immediately adjacent to an allele undergoing a selective sweep, or strong positive selection?
Decreased variation and high levels of linkage disequilibrium at nearby sites
"Common garden" experiments, which expose different genotypes to a common environment, are useful for
determining the extent to which genotype influences variation in phenotype
The change in the average trait value of a population under selection is equal to the heritability multiplied by the selection differential. The selection differential is the
difference in population trait mean before and after selection
If a population initially shows a symmetric phenotypic distribution (left graph), then shows a left skewed distribution after selection (right graph) what is the most-likely cause?
directional selection
The amount of change expected in one generation following positive selection depends on ______ and _______
heritability of the trait in question and the difference in the phenotype between adults who survived versus died after selection
What is meant by the statement "Evolution may proceed along 'genetic lines of least resistance'"?
Characters with little genetic variation will constrain the rate of natural selection; correlated characters may increase in fitness less rapidly, because they can evolve only along the greatest axis of variation
Which observation would be the best evidence for a recent founder event or population bottleneck?
Low genetic diversity in a single population of a widely distributed species
Which statement about genetic drift is true?
Mildly disadvantageous alleles can sometimes increase in frequency, due to genetic drift
Natural selection and genetic drift are the two most important causes of evolutionary change. How do they differ?
Genetic drift is nonadaptive; it changes allele frequency without regard to fitness
Sewall Wright's metaphor of an "adaptive landscape" that includes "peaks" and "valleys" is used widely in evolutionary biology. Natural selection will always move populations' mean fitness up the peaks. Which influence is most likely to cause a population to move away from a fitness peak towards a valley?
genetic drift
Given the relationship between h2 and total phenotypic variance (P) where P = additive genetic variance + environmental variance + dominance variance + epistatic variance. What can be concluded about the h2 heritability measure?
h2 heritability is a characteristic of a population in a given environment
Consider a hypothetical locus with several alleles in a population that is small. Mutation is absent, and none of the alleles has a selective advantage. Which of the following is likely to occur after a long period of time (many generations)?
Genetic variation will decline as alleles randomly go extinct.
What happens to the heritability of a trait as the most fit allele in the population approaches fixation?
Heritability drops towards zero because phenotypic variation is only contributed by environmental variance.
In a population of 16 dots with the genotypes and phenotypes shown below, calculate the difference in allele frequency for A2 after selection
0.104
In a scenario where a population is evolving only in response to natural selection, allele frequencies will change at a rate proportional to the selection coefficient and the
level of genetic variation at the locus.
Genetic correlations can have two causes: _______ and _______.
linkage disequilibrium; pleiotropy
Migration-selection balance pt 2
maintains genetic variation within subpopulations
Fruit fly larvae in the wild are polymorphic for a gene that controls foraging behavior. Larvae that roam around looking for food (roamers, forR) and larvae move much less (sitters, forS). Either allele confers a fitness benefit when it is rare compared to the other allele. The maintenance of this polymorphism is an example of
negative frequency-dependent selection.
A large population of Prothonotary warblers (birds) breed in the flooded woodlands of central to coastal Virginia. Consider a neutral locus in the genome has two alleles present in the population. The allele frequency of the first allele is 0.68. What is the most likely frequency of that allele in the next generation?
0.67
If a trait is encoded by many loci in the organisms genome and the population shows continuous variation in the trait, we refer to it as a
quantitative trait
The distribution of a quantitative phenotype in a population that experiences directional selection should
show a change in the mean value of the phenotype and maintain the amount of variation in the population.
Tracking the frequency of an allele under selection over geographic distance shows clines of various widths. The width of a cline depends on ____ and ___
the amount of gene flow and the strength of selection
The rate of evolution of an allele under positive selection is a function of
the strength of selection the amount of genetic variation in the population the mode of inheritance of the alleles NOT the size of the population
Stabilizing selection typically occurs when
there is a good match between most of the individuals' phenotype and the environment and the environment is stable
An allele that is not directly under positive selection can increase in frequency in the population over generations if
there is strong linkage disequilibrium between this allele and the allele that is under positive selection
The distribution of phenotypes in natural populations are not always aligned with the theoretical adaptive peak. What is one mechanism that could constrain phenotypes from evolving to adaptive peaks of the highest fitness?
trade-offs.
The rate of evolution at a single locus that goes to fixation for an allele Py is
zero because genetic diversity is zero
Given these absolute fitnesses BaBa, W =15 BaBi, W = 10 BiBi, W = 19 What is the relative fitness of BiBi if BaBa is the reference genotype?
1.267
The top part of this figure shows the fitness curves of different bill thickness of the Red Crossbill bird. Specific bill depths correspond to higher fitness when they are adapted to different sizes of pine cone. The bottom graph shows the distribution of bill phenotypes for 3 populations of Crossbills. Which population is most likely to experience stabilizing selection?
3
Which mutation is most likely to become fixed
A beneficial mutation in an extremely large population
Which mutation is most likely to become fixed?
A beneficial mutation in an extremely large population
The Neutral Theory of Molecular Evolution suggests that
A large amount of genetic variation observed among species is due to genetic drift acting on loci that don't affect fitness
The total number of offspring an individual produces over its lifetime is one way to define
Absolute fitness
Which assumption does not underlie Kimura's neutral theory of molecular evolution?
Advantageous mutations occur often, and many fixation events are due to positive selection.
Which bit of genetic material would be least likely to evolve via drift.
An exon that codes for the active site of a protein enzyme
Given these absolute fitnesses BaBa, W =15 BaBi, W = 10 BiBi, W = 19 Which of the following statements is correct if BiBi is the reference genotype?
BaBa has about 80% of the relative fitness of BiBi
A population that is fixed for the Za allele at a locus that encodes a key enzyme in the pathway that enables cellular respiration. A glaciation event rapidly cools the climate over 1000 generations. The Za allele was an adaptation to warmer climates. A new mutation Zi appears and produces an enzyme that can operate better in cold temperatures. If the relative fitness is as follows, what is predicted to happen? ZaZa w = 0.4 ZaZi w = 0.4 ZiZi w = 1
Despite the fitness advantage, Zi will barely increase in frequency for many generations, then suddenly rapidly increase.
Imagine a population of Galápagos finches that vary for bill size. If the population mean is near the optimum size for eating the seeds found on the island, what would we expect to occur if their main seed resource goes extinct, and another plant with much larger seeds replaces it?
Directional selection increasing bill size towards a new optimum
The top part of this figure shows the fitness curves of different bill thickness of the Red Crossbill bird. Specific bill depths correspond to higher fitness when they are adapted to different sizes of pine cone . The bottom graph shows the distribution of bill phenotypes for 3 populations of Crossbills. What mode of selection would lead to adaptive evolution in bill depth for population 2?
Disruptive selection
Underdominance will eventually result in which pattern of selection
Disruptive selection
You observe a very narrow cline in the middle of a broadly distributed species' range. From this you could conclude that
Either strong selection or low migration
Consider a population undergoing positive selection for one allele at a diploid locus. The fitness advantage of this allele is moderate (s = 0.07). After many many generations, what outcome is most likely?
Genetic diversity at the locus under selection will decrease.
What is most likely to increase rates of gene flow for a benthic (bottom dwelling) marine invertebrate?
Increased lifespan of planktonic larvae
Which condition would result in the least evolution as a result of gene flow?
Individuals that are very successful at migrating to new populations, but cannot mate in the new locations
A population of songbirds consists of individuals with variable amounts of brown and red coloration. The figure above shows the relationship between the two phenotypes where the red dots are individual measurements and the blue star is the average value of the population. After selection, the population shows a change in the mean of brown coloration. Which of the following statements is consistent with these results
Individuals with more brown coloration were favored by selection, so the population mean increased. Red coloration was not under selection and is not correlated with brown coloration so there was no change in the population mean for that trait.
The Congo River formed about 1.5 million years ago and has acted as a barrier, preventing interbreeding by chimpanzees ( Pan troglodytes) and bonobos ( Pan paniscus). What effect does the river have?
It acts as a barrier to gene flow and could cause large F ST values across the genome.
Refer to the graph. Ten variable loci were found in an island population of an organism. F ST was calculated for each to compare with the mainland population.What can we conclude from the data?
Locus D shows signs of local adaptation because of its relatively high Fst.
Which observation would be the best evidence for a recent founder event or population bottleneck
Low genetic diversity in a single population of a widely distributed species
This figure shows a pattern consistent with:
Overdominance
Which mode of selection preserves genetic variation at the locus under selection?
Overdominance and Negative-frequency dependent
Consider the very real situation in which purple cone flower plants show continuous variation in flower color from light lilac to deep purple and adaptive fitness peaks occur when flower colors are at either extreme. Then, the late-great musical artist formerly known as Prince's song "Purple Rain" becomes a reality, and seasonal purple rain pours down and sticks to plants like paint before the flowers emerged every year. After the rain and the flowers emerge, pollinators can no longer easily find the deep purple flowers. How will the distribution of flower colors change in response to this new selective environment brought on by Purple Rain?
Phenotypic variance should decrease as the selective environment changes and disfavors the previously bimodal distribution.
The average value of a particular phenotype in a population can change despite selection not acting directly on that trait. Which scenario describes a mechanism that can produce this outcome?
Purifying selection on alleles that encode low respiratory capacity are in linkage disequilibrium with another alleles that encode traits associated with longer lifespan. Over generations, the population shows shorter longevity, but higher respiratory capacity.
Selective sweep on standing genetic variation differs from a sweep on a newly mutated allele in which way?
Selective sweeps on standing variation results in more polymorphic loci nearby the locus under selection, than sweeps on a new allele.
Consider an island population of birds at equilibrium for a particular neutral locus. Birds from the mainland are blown to the islands during very rare storm events and a few survive and reproduce with island birds of the same species. In the absence of selection of this locus in the island population, what is expected to happen?
The rare mainland migrant creates gene flow between the populations strong enough to prevent drift from causing high divergence at this locus.
Populations of small crustaceans called Tigriopus copepods have experienced positive selection for alleles at multiple loci that encode high temperature tolerance over tens of 1000's of generations since it diverged from it's cooler-water ancestor. Researcher's want to test for the evolvability of high temperature tolerance in Tigriopus copepods, but find that experiments fail to result in increased temperature tolerance beyond their current capacity. What scenario best explains this constraint on evolution?
The alleles that confer high temp tolerance went to fixation. Despite selection imposed by researchers, there was no variation for selection to act on.
Populations of small crustaceans called Tigriopus copepods have experienced positive selection for alleles at multiple loci that encode high temperature tolerance over tens of 1000's of generations since it diverged from it's cooler-water ancestor.Researcher's want to test for the evolvability of high temperature tolerance in Tigriopus copepods, but find that experiments fail to result in increased temperature tolerance beyond their current capacity.What scenario best explains this constraint on evolution?
The alleles that confer high temp tolerance went to fixation. Despite selection imposed by researchers, there was no variation for selection to act on.
Consider a population undergoing positive selection for one allele at a diploid locus. The fitness advantage of this allele is small (s = 0.001). After many many generations, what outcome is most likely?
The beneficial allele will increase in frequency and eventually will be fixed by natural selection.
Which is the best explanation for what F ST represents?
The fraction of genetic variance in a group of populations that result from differences between them
Florida panthers have undergone a severe bottleneck and habitat fragmentation caused by the construction of a major interstate highway. The effective population size on either side of the highway is about 75 and genome-wide scans show an Fst value of 0.19.How has the highway impacted this species?
The highway severely blocks the southern population, allowing only a tiny amount of gene flow.
Which condition is most likely to account for a difference between the total population size and the effective population size ( N e) in a rapidly growing population of humans?
The population is recovering from a bottleneck, which drastically reduced the amount of genetic variation in the population.
A population is fixed for the Za allele at a locus that encodes a key enzyme in the pathway that enables cellular respiration. A glaciation event rapidly changes the climate over 1000 generations. The Za allele is an adaptation to warmer climates. A new mutation Zi appears and produces an enzyme that can operate better in cold temperatures. If the relative fitness is as follows, what is predicted to happen? ZaZa w = 0 ZaZi w = 1 ZiZi w = .75
The population will reach a polymorphic equilibrium because ZaZi shows overdominance.
A population is polymorphic for the Za and Zi alleles at a locus that encodes a key enzyme in the pathway that enables cellular respiration. A glaciation event rapidly changes the climate over 1000 generations. The Za allele is an adaptation to warmer climates, Zi encodes an enzyme that can operate better in cold temperatures. If the relative fitness is as follows, what is predicted to happen? ZaZa w = 0.25 ZaZi w = 0 ZiZi w = 1
Za will be fixed if the initial frequency is above some threshold because ZaZi shows underdominance.
Genetic recombination of alleles at different loci results in
a decrease in the likelihood that those alleles will be inherited together
If a trait is encoded by many loci in the organisms genome and the population shows continuous variation in the trait, we refer to it as
a quantitative trait
Total phenotypic variance of a quantitative trait is comprised of which of the following?
additive genetic varianceenvironmental variancedominance varianceepistatic variance
Consider the following relative fitnesses A1A1, w = 0.5 A1A2, w = 1.0 A2A2, w = 0.75 Which of the following scenarios best describes how the mean fitness of the population will evolve?
an adaptive peak will evolve around the A2 frequency of 0.625
Phenotypic variance that is not due to genetic variance can be attributed to
environmental variance.
This histogram shows a
bimodal distribution consistent with disruptive selection.
The colors of individuals in a snail population are determined by a single autosomal locus. A 1 A 1 homozygotes are red, A 1 A 2 heterozygotes are pink, and A 2 A 2 homozygotes are white. Genotypic fitnesses are as follows: w A 1 A 1 = 0.5; w A 1 A 2 = 1.0; w A 2 A 2 = 0.75. At equilibrium,
both alleles are maintained in the population, and allele A 2 is more common than allele A 1.
In the late eighteenth century, a typhoon swept through the Pacific atoll of Pingelap, leaving approximately 20 human survivors. A large percentage of the present-day inhabitants of Pingelap are color blind. One can conclude, therefore, that the population experienced a
bottleneck
Migration-selection balance...
can result in an allele with a low relative fitness to persist in populations despite being selected against
Genetic drift results in
changes in allele frequencies in a population in the absence of selection
As _____ increases, the ______ decreases
effective population size, strength of genetic drift
Two alleles segregate at a locus. Assuming that no stabilizing forces exist, the first allele will eventually be
either lost or fixed.