Chapter 7 Flashcards
What does population genetics integrate?
It integrates evolution by natural selection with Mendelian genetics.
What is microevolution?
It is the change in genotype frequencies over time within a population.
What is the Hardy-Weinberg principle?
It is a null model that predicts genotype frequencies if no evolution is occurring.
What five main evolutionary processes disrupt Hardy-Weinberg equilibrium?
Selection, mutation, migration, genetic drift (finite population size), and non-random mating.
What happens when a population is in Hardy-Weinberg equilibrium?
Allele frequencies remain constant from generation to generation.
How do you calculate allele frequencies from genotype frequencies?
Freq(A) = Freq(AA) + ½ Freq(Aa)
Freq(a) = Freq(aa) + ½ Freq(Aa)
Given a population where the genotype frequencies are AA = 0.25, Aa = 0.50, and aa = 0.25, what are the allele frequencies?
Freq(A) = 0.25 + (0.5 × 0.50) = 0.50
Freq(a) = 0.25 + (0.5 × 0.50) = 0.50
What is the Hardy-Weinberg equation?
p² + 2pq + q² = 1, where:
p² = Frequency of homozygous dominant (AA)
2pq = Frequency of heterozygotes (Aa)
q² = Frequency of homozygous recessive (aa)
If a population is not in Hardy-Weinberg equilibrium, what does that indicate?
It suggests that one or more evolutionary forces are acting on the population.
In a study of a mosquito population, 40% were homozygous resistant (rr), 50% were heterozygous (rR), and 10% were homozygous susceptible (RR). What is the frequency of the resistant allele (r)?
Freq(r) = (1 × 0.40) + (0.5 × 0.50) = 0.40 + 0.25 = 0.65
If an allele (D) that decreases pesticide penetration has a frequency of 0.8 in a population, what are the expected genotype frequencies under Hardy-Weinberg equilibrium?
Freq(DD) = 0.8² = 0.64
Freq(Dd) = 2(0.8)(0.2) = 0.32
Freq(dd) = 0.2² = 0.04
What can cause deviations from Hardy-Weinberg equilibrium?
Natural selection, genetic drift, mutation, migration, and non-random mating.
In a deer population, the allele for shorter antlers (a) has a frequency of 0.23. The measured genotypic frequencies differ from Hardy-Weinberg predictions. What does this suggest?
Evolution is occurring in the population.
What is an example of a real-world application of Hardy-Weinberg principles?
Studying how malaria resistance alleles are maintained in human populations.
In a study on malaria resistance, observed genotype frequencies did not match Hardy-Weinberg predictions. What does this suggest?
Selection is occurring, possibly due to heterozygote advantage.
What is heterozygote advantage?
It is when heterozygous individuals have higher fitness than either homozygous genotype, maintaining both alleles in the population.
What is the main assumption of Hardy-Weinberg equilibrium that selection violates?
The assumption of no selection.
Why can’t we use Hardy-Weinberg equilibrium to predict genotype frequencies when selection is occurring?
Because one of the key assumptions (no selection) is violated, altering allele frequencies over generations.
How does selection affect allele frequencies?
Beneficial alleles increase in frequency, while harmful alleles decrease, depending on their dominance and selection strength.
What happens when selection acts against a common recessive allele?
Evolution occurs rapidly because many individuals express the recessive phenotype.
What happens when selection acts against a rare recessive allele?
Evolution is slow because the allele is mostly hidden in heterozygotes.
Why do recessive alleles persist in a population despite being selected against?
They can ‘hide’ in heterozygous individuals, where they are not subject to selection.
How is the strength of selection (s) calculated?
s = 1 - (fitness of selected genotype).
If homozygous recessive individuals have 40% mortality, what is the selection coefficient (s)?
s = 0.4 (since 60% survive).
If heterozygotes have 55% survival compared to homozygotes, what is the selection coefficient?
s = 1 - 0.55 = 0.45.
How does coat color in rock pocket mice relate to selection?
Dark mice are favored on lava flows, while light mice are favored on desert sand, leading to selection based on habitat.
In rock pocket mice, if the light-colored genotype (A2A2) has a fitness of 0.9, what is the selection coefficient (s)?
s = 1 - 0.9 = 0.1.
What equation predicts the frequency of a recessive allele after selection?
q’ = q(1 - sq) / (1 - sq^2)
where q’ is the new frequency, q is the current frequency, and s is the selection coefficient.
If a recessive allele starts at q = 0.2 and selection coefficient s = 0.1, what will q’ be in the next generation?
q’ ≈ 0.196 (showing a slight decrease in allele frequency due to selection).
Why are most common genetic diseases recessive?
Because recessive alleles can persist in heterozygous carriers, avoiding selection.
Why are dominant disadvantageous alleles rare?
They are always expressed and selected against unless they appear late in life (e.g., Huntington’s disease).
Give an example of an autosomal recessive disease and its prevalence.
Cystic fibrosis (1 in 2,000 Caucasians).
Give an example of an autosomal dominant disease and its prevalence.
Huntington’s disease (1 in 2,500).
What are the three main types of selection?
Directional selection: Shifts the mean trait value in one direction.
Stabilizing selection: Reduces trait variation but maintains the mean.
Disruptive selection: Increases trait variation without changing the mean.
Which type of selection can completely eliminate a recessive allele?
Strong directional selection.
What is heterozygote advantage (overdominance)?
It occurs when heterozygous individuals have higher fitness than either homozygote, maintaining both alleles in the population.
How does overdominance affect allele frequencies?
It leads to an equilibrium frequency where both alleles are maintained in the population.
What is the equation for equilibrium allele frequency under overdominance?
p(hat) = t / (s+t)
Give an example of a condition maintained by overdominance.
Sickle cell trait, where heterozygotes (carriers) have resistance to malaria.
What is frequency dependent selection?
A type of balancing selection where the fitness of a phenotype depends on its frequency in the population.
How does frequency-dependent selection maintain genetic variation?
When a phenotype becomes too common, its fitness decreases, favoring the rarer phenotype and maintaining both alleles in the population.
Give an example of frequency-dependent selection in nature.
Scale-eating cichlid fish: “left-mouthed” and “right-mouthed” forms alternate in frequency as prey adapt to defend against the more common type.
Why does mutation alone not drive rapid evolution?
Mutation rates are typically very low, and selection usually acts against most new mutations.
What is mutation-selection balance?
The equilibrium where the rate of new mutations equals the rate at which selection removes them from the population.
What is the equation for eq. frequency under mutation-selection balance?
q(hat) = sq. rt. (mu/S)
How does mutation-selection balance explain the persistence of some genetic disorders?
Even though selection removes harmful alleles, new mutations continually introduce them.
Why might cystic fibrosis have a higher allele frequency than predicted by mutation-selection balance?
It may also be influenced by overdominance, as carriers might have resistance to typhoid fever.
