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
