exam 2

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.

linkage disequilibrium

probability that alleles are inherited together (LINKED)

"Common garden" experiments, which expose different genotypes to a common environment, are useful for

determining the extent to which genotype influences variation in phenotype.

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 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.

quantitative traits

traits that show continuous variation

The rate of evolution at a single locus that goes to fixation for an allele Py is

zero because genetic diversity is zero

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 statement about genetic drift is true?

Mildly disadvantageous alleles can sometimes increase in frequency, due to genetic drift.

Which mode of selection preserves genetic variation at the locus under selection?

Negative-frequency dependent

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.

What are traits influenced by

genetic factors and 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.

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

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.

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.

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

environmental variance

large effect on phenotype variance

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...

maintains genetic variation within subpopulations

heterozygosity

measure of genetic diversity

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.

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

what occurs when a beneficial mutation spreads to fixtation

selective sweep eliminates polymorphism

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 all of the following EXCEPT

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

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

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

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

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

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 Fst 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

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

Which mode of selection preserves genetic variation at the locus under selection?

Overdominance

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.

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 results 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 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 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.

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.

A population that is polymorphic for the Za and Zi 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, 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

Total phenotypic variance of a quantitative trait is comprised of which of the following?

additive genetic variance environmental variance dominance variance epistatic variance

frequency-dependent selection

allele fitness depends on frequency

linkage equilibrium

alleles are fully shuffled before the end meiosis (UNLINKED)

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.

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

standing genetic vaiation

degree of heterozygosity in population

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.

three modes of selection

directional, stabilizing, disruptive

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.

phenotypic plasticity

environment-dependent phenotype expression