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Variety is the Spice of Life!. Genetic Variation- the raw material for evolution. Evolution. A change in allele frequencies across generations. Evolution. A change in allele frequencies across generations Selection Genetic Drift Mutation Migration. A Game of Cards and Musical Chairs.

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  • A change in allele frequencies across generations
  • A change in allele frequencies across generations
    • Selection
    • Genetic Drift
    • Mutation
    • Migration
a game of cards and musical chairs
A Game of Cards and Musical Chairs

We will observe how genotype and allele frequencies change through time at different population sizes


  • Each person gets two playing cards that represent two alleles of the same gene
  • Black cards and red cards each represent different copies of an allele (BB vs. rr)
  • Colors will start at about average frequency in the “population”
  • While music is playing, wander around chair area
  • Sit down when the music stops and exchange one card with the person you are sitting next to, to “mate”. Your new genotype is the offspring of your old genotype.
    • To introduce some element of stochasticity 1-2 students will exchange cards with the deck (contains 50% of each color)

First- what is the probability of getting a certain genotype?

    • BB
    • Br
    • Rr
  • p^2 + 2pq + q^2 – the expected frequency of each genotype in the population

Population genetics simulation of drift

For each population represented on graph A, would you expect the population to be in Hardy-Weinberg equilibrium? For graph B?

If one allele were under selection in graph B, how would this graph differ?


Population genetics simulation of drift

How would each of these graphs differ if starting allele frequencies in the simulations were not 50/50—for example, if the populations began with 30% A alleles and 70% a alleles? Would drift occur?

how does population size impact genetic variation
How does population size impact genetic variation?

Alpine Chipmunk

Image from

Data from Rubidge et al. 2012

genetic drift and selection
Genetic Drift and Selection


  • Game will proceed similarly as before, however before each round of random mating, the dice will be rolled to determine which genotype is selected against
    • 1-2: BB selected against
    • 3-4: Br selected against
    • 5-6: rr selected against
  • If your genotype is selected against, you will discard your hand. You didn’t survive to reproduce. You will hand a green button to an individual that did survive to reproduce. This individual has made 2 offspring (of which you will now be one), therefore you obtain an identical copy of the genotype that they have after mating (from the deck).
    • If there are more individuals that die before reproducing than individuals that survive, some will have to sit out
  • Allele and genotype frequencies will be recorded after mating has occurred
genetic drift and selection1
Genetic Drift and Selection
  • Make a prediction as to how fluctuating selection will impact the changes in allele frequencies in large vs. small populations
fluctuations in selection
Fluctuations in Selection
  • Pathogens, Parasites
  • Global “weirding”
the irish potato famine
The Irish Potato Famine

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the southern corn leaf blight e pidemic
The Southern Corn Leaf Blight Epidemic
  • A particular genotype became prominent in the 1960s (that later turned out to be susceptible to a particular fungus)
  • In 1970 a fungal pathogen destroyed ~15% of corn production resulting in losses of one billion dollars
  • Farmers began using a different genotype to avoid this disease
inbreeding a short term disadvantage to small population sizes
Inbreeding- a short-term disadvantage to small population sizes

Image from

wolves on isle royale
Wolves on Isle Royale

why is genetic variation important
Why is genetic variation important?
  • Natural selection acts on genetic variation
  • Populations with low genetic variation are more vulnerable to changing environmental conditions than diverse populations
  • Variation in immunity can help populations survive diseases
  • Inbreeding can result in less fit populations
  • Implications
    • The world’s food supply
    • Climate change and species persistence
    • Conservation strategies
wolves on isle royale1
Wolves on Isle Royale
  • Population went from a high of 50 individuals in 1980 down to 9 individuals in 2012 (Mlot 2013)
  • Canine parvovirus was brought to island
  • Wolves use to periodically come to island through ice bridge from Canada but due to climate change, winters with ice bridges are becoming rarer
  • Inbreeding prominent
  • If you were a conservationist, what would you recommend doing to help save the wolves?
the impact of inbreeding
The impact of inbreeding
  • The class will break up into 2 groups of different sizes
  • Two jokers (mutations) will be introduced into each group
  • Individuals will mate randomly as before, and will record the number of times the two jokers come up together in the same genotype (out of 10 generations)