Ch. 7 The Genetics of Populations

underdominance

-there is a stable equilibrium in which one allele or the other is fixed -Heterozygotes have the lowest fitness among the three genotypes. -There is an unstable intermediate (nonzero) equilibrium frequency of the A1 allele.

Imagine a population in which mutation is the only driver of allele frequency change. Which of the following statements is true for a locus with two alleles and the mutation rates shown in the figure to the right?

The A1 allele will reach an equilibrium frequency of v/(μ+v)

Which phenomenon occurs when genotype frequencies in a population change from one generation to the next?

biological evolution

what factor decreases variation?

directional selection

Migration _______ variation within a population and _______ variation between populations.

increases; decreases

If a genotype frequency is at a stable equilibrium in a population, a small perturbation that increases the genotype frequency will be followed by a ________ in frequency. If a genotype frequency is at an unstable equilibrium in a population, a small perturbation that increases the genotype frequency will be followed by an ________ in frequency. If a genotype frequency is at a neutral equilibrium in a population, a small perturbation that increases the genotype frequency will be followed by _________ in frequency.

decrease; increase; no further change

Directional selection ________ variation within a population. If environmental conditions are such that directional selection favors one allele in one population and a different allele in another, then directional selection ________ variation between those populations.

decreases; increases

The graph below depicts trajectories of allele frequency change predicted by a simple model of natural selection on a dominant advantageous allele. Which of the following conclusions can you draw from the figure?

the greater the selective advantage of the allele, the faster the allele approaches fixation

When testing for Hardy-Weinberg equilibrium in a population, what does a significant chi-square test imply?

the observed genotype frequencies deviate significantly from Hardy-Weinberg expected proportions.

population genetics answers which questions?

-How are genotype frequencies in an offspring population related to genotype frequencies in the parental population? -How can we mathematically model the evolutionary dynamics of a population in which a genetic locus is under natural selection? -Is the genotype frequency observed in a population at a stable equilibrium?

indicate whether each process increases, decreases, or does not change allelic variation within a population.

-Increases: migration and mutation -decreases: directional selection favoring a dominant advantageous allele, directional selection favoring a recessive advantageious allele, and underdominance -does not change: Mendelian inheritance and nonrandom mating

Hardy-Weinberg assumptions:

-There is no mutation or migration. -Population size is effectively infinite. -There is no natural selection.

overdominance

-there is a stable intermediate allele frequency -this is a form of balancing selection -There is a stable intermediate (nonzero) equilibrium frequency of the A1 allele. -Heterozygotes have the highest fitness among the three genotypes.

negative frequency dependent selection

-this is a form of balancing selection -there is a stable intermediate allele frequency

Given the genotype frequencies below, what is the frequency of the V1 allele in this population? Give your answer to one decimal place. f[V1V1] = 0.6 f[V1V2] = 0.2 f[V2V2] = 0.2

0.6 + 0.5(0.2) = 0.7

An equilibrium is at a stable point if which of the following is true of the system?

1. When at this point, the system does not change. 2. If perturbed or displaced by some small amount, the system will return to its original position at rest. (it returns to that point after a small local perturbation)

fecundity

A measure of the ability to produce offspring.

identify whether each statement is correct or incorrect of inheritance and evolution in a sexually reproducing diploid population.

Correct: We can model population-level change in genotype frequencies without knowing the genotypic compositions of specific families. If two parents are heterozygous at a locus, we know the odds that an individual offspring will have a particular genotype. Incorrect: When two specified gametes come together, there is uncertainty about what genotype will develop. If two parents are heterozygous at a locus, we can say with certainty which parental alleles an individual will inherit.

Which of the following statements is true of the locus associated with familial adenomatous polyposis (FAP), a genetic disease that predisposes individuals to colon cancer?

The frequency of disease in Denmark is consistent with the hypothesis that mutation-selection balance maintains the disease-causing allele at an equilibrium frequency

positive frequency-dependent selection

There is a stable equilibrium in which one allele or the other is fixed.

Assume a locus with two alleles and complete dominance. Mutation from the advantageous allele to the deleterious allele = μ. Back mutation is negligible. Drag each statement to the correct box to indicate whether it is true or false of mutation-selection balance at this locus.

True: The deleterious allele will reach a frequency at which the rate of removal by natural selection equals the rate of introduction by mutation. The two alleles will persist in the population at stable equilibrium frequencies. False: Natural selection will fix the advantageous allele when it is dominant but not when it is recessive. The deleterious allele will persist at a lower equilibrium frequency if it is recessive than if it is dominant.

Given the following fitnesses and starting genotype frequencies, what is the frequency of the A1A1 genotype after selection? A1A1 relative fitness = 1 genotype frequency = 0.36 A1A2 relative fitness = 1 genotype frequency = 0.48 A2A2 relative fitness = 0.6 genotype frequency = 0.16

[(0.36)(1.0)]/[0.36(1.0) + 0.48(0.1) + 0.16(0.6)]= 0.38

What factors increase variation?

migration mutation balancing selection

When the fitness associated with a trait decreases as the trait becomes more common, there is _________

negative frequency-dependent selection

When the fitness associated with a trait increases as the trait becomes more common in the population, there is __________

positive frequency-dependent selection

Violation of which Hardy-Weinberg assumption does not, on its own, change allele frequencies in a population from one generation to the next?

random mating

Not a Prediction of Mendel's Laws:

-Any Mendelian trait should appear at an approximate 3:1 ratio of dominant to recessive among the members of a whole population. -By virtue of their dominant effect on phenotype, dominant alleles should eventually replace recessive alleles in populations.

Drag the steps below into the correct order to show how you would test whether the population is in Hardy-Weinberg equilibrium at this locus.

1.) collect a sample of butterflies and count the number in each phenotypic class 2.) divide the number of individuals in each phenotypic class by the total number of individuals in the sample to arrive at relative genotype frequencies. use these values to calculate observed allele frequencides 3.) calculate Hardy-Weinberg expected genotype frequencies 4.) for each genotypic class, subtract the expected number from the observed number, square the value, and divide the squared value by the expected number for that class. sum these values over all three genotypic classes. 5.) compare the chi-square test statistic to the critical value with one degree of freedom

You are given the following data and asked to determine whether the population is in Hardy-Weinberg equilibrium at the locus in question. What is the value of the chi-square statistic for this test? GG: 500 individuals Gg: 200 individuals gg: 300 individuals

340.3 (This value is larger than 3.84, the critical value at a 5% significance level with one degree of freedom. This population thus deviates significantly from Hardy-Weinberg expected genotype frequencies and is therefore evolving.)

Prediction of Mendel's Laws:

An individual with two copies of a recessive allele at a locus will exhibit the recessive trait. Any Mendelian trait should appear at an approximate 3:1 ratio of dominant to recessive among the offspring of a cross between two heterozygotes.

Which statements are true of a population that is in Hardy-Weinberg equilibrium for a particular locus?

For a locus with two alleles of frequencies p and q, heterozygote genotypes are expected to occur at a frequency of 2pq. Genotype frequencies at the locus stay the same from one generation to the next.

The figure to the right shows that, regardless of how common each foraging behavior is in the population of Drosophila larvae (as shown along the x axis), rovers always have higher fitness than sitters when there is no resource competition. There is thus _________ on foraging behavior under these conditions. However, when there is resource competition, the fitness associated with each behavioral mode varies with trait frequency. Under resource competition, there is ________ on larval foraging behavior.

frequency-independent selection; negative frequency-dependent selection

According to the data, when wild and hybrid sunflowers are grown together, the alleles found in wild sunflowers should do which of the following?

increase in frequency due to fecundity selection

Mutation is the origin of new alleles. This process_______ variation within populations and __________variation between populations.

increases; increases

What do we mean when we say that the Hardy-Weinberg model provides a "null model" for population genetics?

it models a population in which drivers of evolutionary change are not acting.