Chapter 7 Flashcards
What are three criticisms of Mendel’s conclusions?
- seemed to be opposed to most biological observations
- unclear as to whether his observations were consistent with Darwin’s theory
- trait frequencies as observed in nature were not consistent with frequencies expected under Mendelian inheritance
Why won’t alleles disappear?
Heterozygotes can “hide” the recessiveness
What did GH Hardy do? (4)
- developed model to predict the population level consequences of Mendelian inheritance
- Showed Punnett was correct
- demonstrated that dominant alleles would not replace recessive alleles over time
- The frequency of an allele neither decreases or increases simply because its recessive or dominant
Population genetics
=> provides math description & modilizes evolution process
-Investigates how the genotype frequencies in an offspring population are related to the genotype frequencies in a parental population.
Individual versus Population level thinking
Individual thinking: what gametes and offspring are produced, in what frequencies, from a given pair of parents?
Population thinking: how do the characteristics of the population change over time as the result of evolutionary processes?
Qualitative prediction
If a trait is beneficial we would expect to see its frequency in the population increase
Quantitative prediction
Numerical predictions about evolutionary dynamics
Stasis
When genotype frequencies (AA/Aa/aa) or allele frequencies (A or a) stay the same
Steady state frequencies
equilibria of our models
When is a system in equilibrium?
When the system has reached a state where it does not change in the absence of outside forces or processes acting on it
Stable equilibrium conditions (2)
- at this point the system DOES NOT CHANGE
- If perturbed by small amount the system will return to this point
*upside down cup with ball on the bottom
Unstable equilibrium conditions (2)
At this point the system DOES NOT change
If perturbed or displaced by some small amount, the
system will move further away from its initial position at rest.
Neutral equilibrium conditions (2)
At this point, system does not change
If perturbed by small amount the system will stay in its displaced position, rather than returning to the original position as it would in a stable equilibrium, or moving further away as it would in an unstable equilibrium.
Mixed equilibrium conditions (2)
At this point the system does not change
If perturbed to the left or the right, the system will return to its starting point. If perturbed forward or backward, the system will stay in the new displaced position
What does the Hardy Weinberg model serve as & what does it tell us about genotype frequencies?
A null hypothesis for population genetics [opposite]
Tells us what happens to genotype frequencies when natural selection and other important drivers of evolutionary change are NOT happening
What are the three conclusions if there is an absence of evolutionary processes acting on alleles?
- Frequencies of A1 and A2 do not change over time
- With allele frequencies and random mating it is possible to predict the equilibrium genotype frequencies
- All alleles no matter their initial frequencies will reach Hardy-Weinberg equilibrium in a single generation
First and Second Hardy Weinberg assumptions?
- Natural selection is not operating on the trait or traits affected by the locus in question => no natural selection
- Individuals have no preference for others with similar (or dissimilar) genotypes => random mating
Third, fourth, and fifth HW assumptions:
- No mutation is occurring
- There is no migration into or out of the population => no new alleles
- Population is infinite in size => no genetic drift
What are the 3 possible genotypes of A1 and A2? Sum of genotype frequencies?
A1A1, A1A2, A2,A2 = 1
What equilibrium does p2, 2pq, and q2 represent, given allele frequencies p and q?
Stable equilibrium
What type of equilibrium are the allele frequencies p and q?
Neutral
What frequency should be reached by heterozygous frequency at HW equilibrium?
f[A1A2] = 0.5
What is the basic procedure of the HW Model?
- Experimentally collect frequency of alleles in population
- Determine genotype frequencies
- Use genotype frequencies to calculate the actual allele frequencies in population
- Calculate the expected HW
- Use chi square test to compare expected to observed for significance
What is the whole idea of HW model, and an example?
To be able to tweak an assumption and see their affect.
If mutation is introduced and it fluctuates your data, then you know that’s driving your population
Selection coeffiecient
Describes the fitness reduction of the light phenotype relative to dark phenotype.
How is fitness set up in light and dark phenotypes? What does it mean is s=0 or 0.25?
Fitness of dark type is set to 1
Fitness of light type set to 1-s
If s=0; no selection against alllele
If s=0.25; 25% reduction in fitness
Frequency-independent selection & example
Fitness associated with a trait is not directly dependent on the frequency of the trait in a population
Example: pocket mice
Directional selection
One allele is consistently favored over the other allele; takes long because it’s being masked
What if A1 is codominant or dominant to A2?
heterozygotes have selection advantage
What if A1 is recessive to A2?
Heterozygotes have same fitness as A2A2 homozygotes
Codominant
equal expression
Overdominance
heterozygotes has highest fitness
Balanced polymorphism
-A stable equilibrium that’s polymorphic; allele frequencies will return to their equilibrium values after a perturbation away from equilibrium
Balancing selection, and example
selection that leads to balanced polymorphism
- generally rare
- classic example is sickle cell anemia
Underdominance
Heterozygote has lower fitness than homozygotes
Overdominance and underdominance are very rare, why?
If the allele goes above critical threshold frequency, it goes to fixation
If the allele goes below this frequency, it’s lost from the population
Why are overdominance and underdominance classified as frequency independent selection?
Fitness of each genotype, and its corresponding phenotype, is constant and independent of the frequencies of genotypes in the population
Frequency-Dependent selection
Occurs when the costs and benefits associated with a trait depend on its frequency in the population
What is POSITIVE frequency dependent selection? What happens when phenotype is favored and controlled by two alleles at a single locus?
Fitness associated with trait that INCREASES as the frequency of the trait increases in population
- Each phenotype favored once becomes sufficiently common in the population.
- If phenotype controlled at a single locus, one of the two alleles will become fixed and the other will be lost
What is NEGATIVE frequency dependent selection? When are phenotypes favored and controlled by two alleles at a single locus?
Fitness associated with trait DECREASED as the frequency of the trait increases in population.
- Each phenotype favored when rare
- If phenotype controlled by two alleles at a single locus, both will be maintained in a balanced polymorphism (form of balancing selection)
Example of positive frequency-dependent selection
Flat nail species; The higher the frequency of either coil direction in the population the higher their fitness
Example of Negative frequency-dependent selection
Scale-eating cichlid fish (two morphs: right and left handed mouth openings); As one mouth opening became dominant, its prey became aware of protecting that side decreasing the fitness of the most common mouth morphology = maintaining it ~50%
Viability selection
Fitness differences that arise because of differences in rates of survival and mortality
Fecundity selection
Natural selection operating on the number of offspring produced
Semelparous species
Organisms that reproduce once at the end of their lives
Mutation; two facts?
Ultimate source of variation
Can change allele frequencies in a population
Which acts slower, mutation or selection?
Mutation operate acts slower than selection.
How does mutation affect genotype frequencies?
If other HW assumptions are met, the genotype frequencies will always be in standard HW proportions
Even if A1 gets favored, why won’t it ever get fixed? What will be reached?
A1 although favored will never be fixed because A2 alleles are constantly being generated by mutation. => equilibrium will eventually be reached.
Mutation selection balance
Equilibrium state where the action of natural selection to DECREASE the frequency of A2 is exactly balanced by the action of mutation to produce new A2 alleles by mutation from A1
Since several human diseases negatively affect fitness, why haven’t they been eliminated from population?
Mutation-selection balance
