
Homework:
Get started on PS 17.


Do Now:
 Take out last night’s HW so you have it handy
 What do you think is the difference between macroevolution
and microevolution?
 What does evolution have to do with genetics?
Today’s Goals:
 Explain the difference between macro and microevolution
 Apply the Hardy-Weinberg equations to analyzing gene pools in
populations
 Describe microevolutionary forces that cause changes in gene
pools over time
EVOLUTION
Macroevolution –
• One species can branch into two or more
species
• All life on earth is descended from
common ancestors
Microevolution –
• Changes in the heritable characteristics in
a population over time (population
genetics)
Populations Genetics
(Microevolution)
Are populations evolving? How? Why?
Some definitions…
• Species –
– a group of organisms with similar
characteristics and genetics that can
reproduce and make fertile offspring
• Population –
– a group of organisms of the same species
that live in a given area
• Gene pool –
– the collection of all the genes (and alleles) in
a population
• Allele frequency –
– the proportion of one allele in the gene pool
Example:
In cats, black hair is dominant over white
hair. (B = black, b = white)
Imagine a population of 500 cats:
320 are homozygous dominant
160 are heterozygous
20 are homozygous recessive
• How many alleles are in the gene pool for
this gene?
• What is the frequency of the B allele?
Hardy-Weinberg Theorem
of Genetic Equilibrium
In a non-evolving population, frequencies of
alleles and genotypes should remain
constant across generations.
Hardy-Weinberg equations
Allele frequencies:
p = freq. A allele
q = freq. a allele
By definition:
p+q=1
Genotype frequencies:
Freq. of AA genotype = p2
Freq. of Aa genotype = pq + qp = 2pq
Freq. of aa genotype = q2
Prediction under HW equilibrium:
p2 + 2pq + q2 = 1
figure 21-07.jpg
Use HW to do two
things:
Calculate allele
frequencies
(given genotype freq’s)
Predict genotype
frequencies
(given allele freq’s)
Populations Genetics
Are populations evolving? How? Why?
H-W equations let us answer a specific
aspect of this question:
Are gene (allele) and/or genotype
frequencies changing over time?
So what?
If ACTUAL genotype frequencies match
predictions, then…
• the population is in HW equilibrium
• allele and genotype freq. constant across
generations (no evolution)
If ACTUAL genotype frequencies differ from
predictions, then…
• population is not in HW equilibrium
• some evolutionary force is acting!
Requirements for HardyWeinberg Equilibrium
• No natural selection (due to limited
resources, etc.)
• No mutation
• Random mating
• No immigration or emigration (no gene
flow)
• Large population size
Evolutionary forces that violate
Hardy-Weinberg:
•
•
•
•
•
mutation
migration
non-random mating
genetic drift
selection
How does each affect genetic variation
within populations?
Mutation – transformation of one
allele into another
• generates genetic variation
• alone, not a strong evolutionary force
• provides the “raw material” of genetic
variation on which selection and drift
can act
Migration – movement of individuals
between populations
• “gene flow” between gene pools
• maintains genetic variation within
populations by bringing in new alleles
• prevents populations from genetically
diverging (and eventually becoming
separate species)
Non-random mating
• does not change allele freq’s
• DOES change genotype freq’s
Assortative mating – individuals prefer mates
with same genotype
increases homozygosity at a
particular locus
Disassortative – individuals prefer mates with
different genotypes
increases heterozygosity at a
particular locus
Inbreeding – mating between
close relatives
• increases homozygosity across the
genome
• danger: “inbreeding depression”
– inbred populations often show
decreased fitness (due to greater risk of
homozygous recessive disorders)
(Freeman & Herron 2001)

Homework:
Lab 8 Analysis Questions

Do Now: Do some math!
 Imagine a population of 1,000 fruit flies:
 510 have gray bodies (wild-type, dominant)
 490 have black bodies (recessive)
 What is the frequency of the recessive genotype?
 What is the frequency of the recessive allele?
 What is the frequency of the dominant allele?
 How many flies would you predict to be…
 Homozygous dominant?
 Heterozygous?
Evolutionary forces that violate
Hardy-Weinberg:
•
•
•
•
•
mutation
migration
non-random mating
genetic drift
selection
Genetic drift – random changes
in allele frequencies between
generations
• due to sampling error
• greatest effect in small populations
– population bottlenecks
– founder effect
Genetic drift via
population bottleneck or
founder effect
Simulation:
Genetic drift at one locus
http://darwin.eeb.uconn.edu/simulations/drift
.html
Selection – differential survival and reproduction
of individuals with different genotypes
• Non-random process (unlike genetic drift)
• Natural selection involves…
– More offspring are born than can survive
– Competition/struggle for survival for limited resources
– Variation between individuals that makes some
better able to survive and reproduce
– This variation is heritable/genetic (can be passed on)
Result: Over many generations, the genotypes that are
better able to survive and reproduce become more
common in the population.
Simulation:
Selection at one locus
http://darwin.eeb.uconn.edu/simulations/sele
ction.html

Homework:
Lab 8 Analysis Questions

Today’s Goals:
 Use H-W equations to calculate allele and genotype frequencies
 Explain how genetic drift and natural selection affect
microevolution
 Simulate population genetics and calculate allele and
genotype frequencies in “The Mating Game”


Homework:
Due Friday: PS 17 and Lab 8
We will also have a short practice quiz on Friday – based on the
problem set
Do Now:
 Take out Lab 8 and turn to Part B
 Get 2 sample papers and lick them. See if one tastes bad.

Today’s Goals:
 Calculate allele and genotype frequencies for a REAL trait in our
class (Part B)
 Describe various effects of natural selection and sexual
selection on the range of traits in a population.
Selection – differential survival and reproduction
of individuals with different genotypes
• Non-random process (unlike genetic drift)
• Natural selection involves…
– More offspring are born than can survive
– Competition/struggle for survival for limited resources
– Variation between individuals that makes some
better able to survive and reproduce
– This variation is heritable/genetic (can be passed on)
Result: Over many generations, the genotypes that are
better able to survive and reproduce become more
common in the population.
Classify the following examples as Directional,
Stabilizing, or Disruptive Selection:
• Human birth weight tends to stay around 7
lbs. (too big = trouble for mom; too small = trouble for newborn)
• Many seeds have seed coats of medium
thickness (if too thin, no protection; if too thick,
germinating seed can’t break through)
• Finches need either small beaks (to eat tiny
seeds) or large beaks (to crack large
seeds).
Galapagos finches
Balanced Polymorphism
(aka Heterozygote Advantage)
Ex: sickle cell anemia
Frequency-dependent selection
Sexual Selection
Peacock Experiments
Sexual Selection
• Intrasexual – Members of one sex (usually
male) compete for access to the other sex
– Ex: Sea lions, rams
• Intersexual – Members of one sex (usually
female) choose certain members of the
opposite sex over others
– Ex: Pea hens choosing pea cocks