Evolution Lecture 9 : Population Genetics: Selection

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Evolution Lecture 9 Population Genetics
Selection
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What happens to Hardy-Weinberg Equations When
Assumptions Violated

• If we violate assumptions, then we can have
  predictable effects on the outcomes of allele or
  genotype frequencies
• Therefore, we can determine which factors (or
  lack of assumptions) are effecting our populations

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Selection

• Selection favoring certain genotypes should cause
  alleles to increase in frequency and vice versa
• If flies with two types of ADH (f and s) exist,
  and ADH f is better at digesting alcohol, then
  what should we predict about the frequency of
  that allele in the presence of alcohol relative
  to a control?

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How does selection affect H-W equations

• Imagine that we have the following genotypes from
  100 zygotes B1B125, B1B250, and B2B225.
• Selection for w1w10.6, w1w21.0, and w2w20.6.
  Then we get B1B115, B1B250, and B2B215.
• If each genotype makes 10 gametes, then we still
  get 50B1 and 50B2 alleles...has the population
  not evolved?
• Yes, because this would predict that our adult
  population would beB1B10.5x0.50.25
  B1B22(0.5x0.5)0.50 and B2B20.25.
• However, we see that our adult population
  genotype is already B1B115/800.1875
  B1B250/800.625 and B2B215/800.1875
• Selection for heterozygotes!!

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Empirical Research on Selection

• Jaeken Syndromeabnormal distribution of fat,
  skeletal deformities, and inadequate liver
  function
• PMM Enzyme disrupted. Found 24 different loss of
  function(LOF) mutations
• All Jaeken subjects have some or both alleles
  disrupted. They can be disrupted by the R141H
  LOF (R) allele or a set of other LOFs(O).
• In 54 patients, we find that R141H0.4 alleles
  and other 0.6 alleles.
• This should yield the following
  genotypesO/O0.36O/R0.48 and R/R0.16
• However, in reality we have O/O0.2, O/R0.8 and
  R/R0.0
• R/R is lethal prior to birth. Too much LOF of
  PMM
• Examining 1,370 LOF PMM patients has never
  yielded an R/R genotype

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Predicted Changes in Allele Frequencies
If population of D 32 genes where high, and the
number of aids infections were also high, then
we might predict this graph. This has never
happened!
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With incidence of infection below 1 in parts of
Europe, this a more true prediction
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Sad Reality in Africa, with 25 infection and low
numbers of D32 allele
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Testing Predictions of Pop Gen Theory on Various
Patterns of Selection

• Selection on recessives in Dawsons Tribolium
  beetle experiment.
• Two alleles /, /l, and l/l. l/l Does not
  survive.
• Therefore, if we begin with a population that is
  100 /l, then we predict, without selection that
  that we would obtain 25,50, and 25 for each
  respective genotype.
• With selection, we get very different results
  over time.

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Predictions about rate of evolution

• Dominance and allele frequency can determine the
  rate of evolution
• If recessive allele is common, evolution by
  natural selection is quick
• If a recessive allele is rare, then evolution by
  natural selection is slow.

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Selection favoring heterozygotes

• Mukai and Burdick experimented with flies that
  have two alleles L and V
• VValive, VLalive, LLdead
• We might predict the same result as the Tribolium
  experiment
• Comparatively, the V allele should reach 94 by
  generation 15. It did not. It only reached 79
• We have overdominance or heterozygote
  superiority.
• The L allele is being maintained in the
  heterozygote condition through overdominance and
  thus we cannot reach a V greater than 94

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To demonstrate this theory, they must also
start at a high homozygous V population
(0.95) and see what happens
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Selection favoring homozygotes

• This is favoring not just one of the homozygote
  conditions, but both
• Another way to say this is, heterozygotes are not
  favored.
• Foster examined fruit flies with compound
  chromosome types. Populations can have C2 or C3
  type. C2 x C3 matings produces no offspring
• What should we predict about the frequency of
  genotypes?
• Based on how many C2C2s there are in the
  beginning, we should either go to 1.0 or zero.
  Why is this?

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Underdominance
When underdominance occurs, one allele tends to
go to fixation, while the other is lost
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Frequency-dependent selection

• Gigord et al. (2001) examined Elderflower orchids
• These plants come in purple and yellow.
• Neither give nectar, but still attract bees, due
  to their bright colors. The bees visit one
  flower and then leave to another flower. They
  seek out the rarer color, expecting nectar in the
  different type
• Selection by bees favor yellow, until it becomes
  too common. The bees realize that the yellow
  yields no nectar at higher frequencies.

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Elderflower orchids
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