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
We will observe how genotype and allele frequencies
change through time at different population sizes
Rules
• 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?
Alpine Chipmunk
Image from
Evolution.berkeley.edu
Data from Rubidge et al. 2012
What are the possible consequences
of a decline in genetic variation?
Genetic Drift and Selection
Rules
• 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 Selection
• Make a prediction as to how fluctuating
selection will impact the changes in allele
frequencies in large vs. small populations
Fluctuations in Selection
• Pathogens, Parasites
• Global “weirding”
Monocultures
The Irish Potato Famine
Image from evolution.berkeley.edu
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
Image from evolution.berkeley.edu
Wolves on Isle Royale
http://www.michiganradio.org/post/decisiontime-isle-royale-only-8-wolves-left
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
Questions?
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 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)