Exam 2 Flashcards
Explain why natural selection is most rapid when it acts on common recessive alleles (and rare dominant alleles).
- When a recessive allele is rare, most copies are hidden in heterozygotes and protected from selection.
- So as A2 becomes less common, natural selection acts more slowly.
define overdominance
- heterozygote advantage
- heterozygotes have higher fitness than homozygotes
overdominance - predict how it should affect allele and genotype frequencies
natural selection will produce equilibrium allele frequencies and fixation will not occur
overdominance example
- Sickle cell and Mukai and Burdick fruit fly experiment
- both alleles meet at a certain number (no fixation)
define underdominance
- homozygote advantage
- homozygotes have higher fitness than heterozygotes
underdominance - predict how it should affect allele and genotype frequencies
natural selection will fix one allele or the other depending on which is more common
underdominance example
- Foster et al. fruit fly experiment
- 2 alleles goes to fixation
- starting frequency affects which one becomes fixed
what happens in negative frequency dependent selection (NFDS)
Rare genotype more fit than common genotype
NFDS - predict how it should affect allele and genotype frequencies
natural selection will produce equilibrium allele frequencies and fixation will not occur
NFDS example
- Gigord et al. orchid experiment
- when yellow trait is rare, it has a higher fitness
- when it is common, it has lower fitness
- frequency of yellow morph and reproductive success has a neg. correlation
Explain why mutation on its own is a weak evolutionary mechanisms.
- mutation rates are too small
- needs to act on selection for it to become rapid
how is mutation-selection balance is a potential explanation for the high-frequency of a deleterious allele in a population
- mutation-selection balance
- The frequency of a deleterious allele can remain at equilibrium if the opposing mechanisms of selection and mutation are equal
Explain how genetic drift affects heterozygosity, genotypic diversity, and allelic diversity in a focal population
decreases all three
For a focal population that is evolving in response to genetic drift, you should be able to predict that likelihood that a particular allele will drift to fixation.
it will be the starting/initial frequency
Understand what neutral theory is, three predictions it makes (that are supported)
- Beneficial mutations are rare, start at low frequency, and are often lost—uncommon—can be fixed positive selection
- Deleterious mutations are removed by purifying selection (so don’t contribute to evolution)
- Neutral mutations rise and fall b/c drift
why is neutral theory an important tool (detect selection).
- comparing ratio of nonsynonymous vs.
synonymous substitutions - N/S<1 = deleterious
- N/S=1 = neutral
- N/S>1 = positive
how does inbreeding affects allele and genotype frequencies using a selfing example.
- no change in allele frequencies but change in genotypic frequencies
- Homozygotes increase in frequency
- Heterozygotes decrease in frequency
explain why is nonrandom mating should not be considered an “Evolutionary Mechanism”
- allele frequencies do not change (conclusion 1 is not violated) but it violates conclusion 2 of HW bc it cannot be predicted using the equation
explain inbreeding depression
effect deleterious recessive alleles have on the average fitness of offspring in the population
why does inbreeding depression occur as well as examples of fitness consequences
By increasing the proportion of homozygotes, inbreeding increases the frequency with which deleterious recessive (loss-of-function) alleles affect phenotypes –there are more “aa” phenotypes in the population
define conservation genetics
application of population genetics to understand and reduce the risk of population and species extinctions
recognize how/why conservation genetics fits in with chapters 6 & 7 (drift and gene flow)
- populations that need conservation are small due to extinction
- makes them more vulnerable to drift and gene flow
list and explain 3 processes that cause LD
- selection
- drift
- migration
be able to explain how sex restores LE.
- Genetic recombination randomizes genotypes at one locus with respect to genotypes at another
- reduces LD
Understand how sex increases genetic variation.
- Genetic Recombination (crossing over)
- Creates chromosomes with new combination of alleles for all genes one a chromosome
describe the Red Queen Hypothesis
individuals must continually evolve new adaptations in response to other organisms adaptations to avoid extinction
red queen hypothesis - how does it explains the value of sex
sex allows organisms to ‘outrun’ extinction through the creation of new alleles via crossing over
describe Mueller’s ratchet in detail.
- Asexual populations occasionally sustains deleterious mutations
- If the zero mutation group is small, chance events may cause its extinction
- Then the one mutation group will now have highest fitness
- Loss of a group by drift is easier than is replacement by backmutation
generally describe what QTL mapping is and what it is used for
- a statistical analysis
- used to identify which molecular markers lead to a quantitative change of a particular trait
distinguish “broad-sense” from “narrow-sense” heritability
- broad-sense = the proportion of the total variance that is due to genetic variance so h^2 = var(P)/var(G)
- narrow-sense = proportion of total variance in a trait that is due to additive genetic variance so h^2 = var(A)/var(G)
define the terms in this equation: Var (P) = Var (A) + Var (I) + Var (D) + Var (E).
- var(A), var(I), and var(D) = genetic variance
- var(P) = variation in phenotype
- var(A) = additive genetic variance
- var(I) = variance in Interactions or epistasis
- var(D) = variation in dominance
- var(E) = environmental variance
explain all of the terms of the breeder’s equation
- equation: R = h^2*S
- R = response to selection
- h^2 = heritability
S = selection differential
to define each of the 3 ways of selection, give a hypothetical example of each, and be able to graph trait distributions and selection curves (case study slides).
- disruptive: extreme traits are selected for
- stabilizing: intermediate trait is selected for
- directional: one extreme is selected over the other extreme
explain what an adaptive hypothesis is and list the components of an adaptive explanation
- an explanation for the evolution of a trait from an ancestral condition that describes the specific environmental condition that selected for the trait (the selective pressure)
- should explain function and fitness
why might a trait evolved from an ancestor that did not possess that trait.
- drift
- mutation
- vestigial
- pleiotropy (genes interacting w other genes, one is adaptive and another is not)
- constraints and trade-offs
explain how observational studies can be used to evaluate aspects of an adaptive hypothesis.
- it can address function only if there is a significant relationship between traits and fitness
- if that is the case, drift can be rejected
- cannot address phylogenetic history
observational studies - giraffe example.
- researchers tested theory that giraffe evolved long necks to eat off leaves
- observational studies show that giraffes do not eat at their maximum height
observational studies: giraffe - what is another explanation for their necks
adaptation for male-male combat
explain how experimental studies can be used to evaluate aspects of an adaptive hypothesis
- can address function AND causation if designed well
- can reject drift if there is a significant relationship between traits and fitness
experimental studies - flower marker example
hypothesis: If pollinators find well-marked flowers more attractive (function), the plants that display them should enjoy higher reproductive success (fitness)
experimental studies: flower marker example - how did they conduct the study
- used black ink to fill in the white arrowheads on experimental flowers
- results showed that pollinators visited the control more (matches hypothesis)
explain how the comparative method can be used to evaluate aspects of an adaptive hypothesis
- cannot address function
- can reject drift if there is a significant relationship between the traits that species have and the environment those species inhabit
- phylogenetic history can be addressed instead of current environmental conditions as a good explanation for trait variation among lineages
comparative studies - Phylogenetically Independent contrasts
the name of the main statistical tool people use to take phylogenetic relationships into account when applying the comparative method
comparative studies - explain why phylogenetic studies are a necessary complement to the comparative method
allows for more accurate hypotheses on how traits evolved due to selection pressures rather than inheritance alone
describe some of the assumptions that are inherent to parent/offspring regressions
- assumes that the trait variance is primarily due to additive genetic effects
- environmental factors shared between parents and offspring don’t significantly impact the trait.
- parents do not selectively invest in offspring based on their trait values
- a consistent environment between the parental and offspring generations.
discuss the limitations of estimating heritability using parent/offspring regression
- Shared environmental factors can be difficult to completely rule out, making it challenging to disentangle genetic contributions from environmental ones
- Limited to Linear Additive Effects
- over- or Underestimation of Heritability
- it doesn’t directly account for how selection pressures might act on the trait across generation
true or false: a heritability of 0.6 indicates that 60% of the trait in an animal is determined by its genes
- false
- 60% of the variation in the trait within a population can be attributed to genetic differences among individuals
- the remaining 40% of the variation is due to environmental factors or other influences.
true or false: if a trait has no heritability (h^2 = 0), that means the trait is not determined by genes
- false
- there is no genetic variation in the population for that trait
- all individuals in the population have the same genetic contribution to the trait
- any differences we observe are due to environmental factors.
is it possible for heritability to be very low if var(P) is not explained by environmental variation
- false
- h^2 can only be low if a large portion of the phenotypic variance is due to environmental effects or other non-genetic factors
if a trait has a heritability as one does that mean the environment is constant
- no
- for the population in question, environmental differences are not influencing tat particular trait’s variation.
