7.1 Individual- Level versus Population-level Thinking
Population genetics: relationship of genotype frequency in an offspring population and how it is
related to the genotype frequencies in a parental population.
Qualatative and quantitative predictions:
Biological evolution occurs when genotype frequencies change over time.
Equilibrium: stasis. "steady-state" frequencies. A state where it does not change in the absence of
outside forces or processes acting on it. Genotype frequencies do not change from generation to
Types of Equilibrium:
Stable equilibrium: Marble at the bottom of a rounded cup.
Unstable equilibrium: marble balanced on top of a hill.
Neutral equilibrium: marble at rest on a tabletop.
Mixed equilibrium: marble in a half-pipe
7.2 The Hardy-Weinberg Model: A Null Model for Population Genetics.
Hardy-Weinberg model: provides null model to compare to. It tells what happens to genotype
frequencies when natural selection and other important drivers of evolutionary change re not
The model provides 3 conclusions:
1. The frequencies of the A1 and A2 allies do not change over time in the absence of
evolutionary processes acting on them.
2. Given allele frequencies and random mating, we can predict the equilibrium genotype
frequencies in a population in which evolutionary processes are not acting. (Hardy-
Weinberg equilibrium frequencies.)
3. If no evolutionary processes are operating, a locus that is initially not at Hard-Weinberg
equilibrium will reach Hardy-Weinberg Equilibrium in a single generation.
1. Sexually reproducing diploid organism that reproduces in discrete generations (all parents
reproduce synchronously and then die)
2. Natural selection is not operating on the trait or traits affected by the locus in question.
3. Individuals have no preference for others with similar (or dissimilar) genotypes. Thus mating
in the population is random with respect to the locus n question.
4. No mutation is occurring.
5. There is no migration into or out of the population.
6. The population is effectively infinite in size, so that chance fluctuations in allele frequencies
p= f[A1A1] + f[A1A2]/2
q=f[A2A2] + f[A1A2/2 p+q=1
Hardy- Weinberg Equilibrium Frequency
Example of Hard-Weinberg:
The Myoglobin Protein
7.3 Natural Selection
Selection for Coat Color in Pocket Mice:
Rock pocket mouse:
Environments: light colored or dark rocks
D and d allele: D dark; d light
Location: Mc1R locus
Simple model of Natural Selection:
Allow natural selection to operate on the population.
Using rock pocket mouse: A2 allele is being selected against.
Selection Coefficient: (s) to describe the farness reduction of the light phenotype relative to the
Dark phenotype set to 1. Light phenotype is 1-s.
s=0 means no selection against the allele
s=0.25 indicates a 25% reduction in fitness
s=0.50 indicates 50% reduction in fitness
Fitness’s for a Dominant Locus
A1 dominant to A2
s= the genotype divided by the best fit.
Modes of Frequency-Independent Selection:
Frequency-Independent Selection: the fitness associated with a trait is not directly dependent on
the frequency of the trait in a population.
Directional Selection: one allele is consistently favored over the other alleles. Eventually the
favored allele will become fixed in the population, thus replacing all the other alleles, fixation. Under Hardy-Weinberg, infinite population size assumption will never reach complete
A1 Dominant: Homo-A1 and Heterozygous fit then recessive
A1, A1 Co dominance: Heterozygous between Hetero-A1 and Hetero-A2
A1 Recessive: Hetero and Homo-A2 less fit then A1
Rates of Fixation under Directional Selection:
Why does a rare A1 allele quickly increase in frequency in the dominant and co dominant cases,
but not in the recessive case?
Most copies of the A2 appear in the A1A2 heterozygous. When A2 is dominant to or so
dominant with A2, these heterozygous enjoy a selective advantage, and thus the frequency
of the A2 allele responds with sizable increase.
Once A1 is common, why does it take a long time to go to fixation in the dominant case but not in
the co dominant or recessive case?
When A1 is recessive, the heterozygous have the same fitness as the A2A2 homozygous that
makes up the majority of the population. Selection increases the frequency of the A2 in the
rare events in which A1A1 homozygous is produced.
Over dominance and Under dominance
Over dominance: (heterozygote advantage) the A1A2 heterozygote has a higher fitness than
either the A1A1 or the A2A2. Direction of natural selection depends on the current allele
frequencies in the population. When A2 is rare, it will usually occur in heterozygotes, thus avg
fitness of individuals carrying A1 will be at higher than average fitness. But when it is common it
will occur in A1A1 that have a lower fitness that the avg. Thus A1 increase when rare and
decreases when common.
Balanced Polymorphism: stable equilibrium in which both alleles are present. All frequencies will
return to their equilibrium values after a perturbation away from the equilibrium.
Balancing section: selection that leads to a balanced polymorphism.
Over dominance example:
Under dominance: The A1A2 heterozygote has a lower fitness than the homo. Natural selection
will favor one allele over the other. If A2 is very rare it will appear in A1A2 that have lower than
average fitness. When A1 is common, the A1 will appear in the A1A1 that have higher than
average fitness. Same for A2. Depending on where it starts depends on which will reach fixation
and which will be lost.
Under dominance example:
New Zealand Black/ New Zealand White Mice
Modes of Frequency-Dependent Selection
Frequency-dependent selection: occurs when the costs and benefits associated with a trait
depend on its frequency in the population. Positive or Negative.
Positive: fitness with a trait increases as the frequency of the trait increases. Thus, each
phenotype is favored once it becomes sufficiently common in the population. If the phenotypes
are controlled by two alternative alleles, one of the two will eventually be fixed and the other lost. Negative: the fitness associated with a trait decreases as the frequency of the trait increases. Thus
each phenotype is favored when it is rare. If the phenotypes are controlled by two alternative
alleles, both alleles will be maintained in a balanced polymorphism. Thus, negative frequency-