Transcript Genes in Populations II: Deviations from Hardy

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?
HW equations
By definition: p + q = 1
p = freq. A allele
q = freq. a allele
Prediction under HW equilibrium:
p2 + 2pq + q2 = 1
p2 = freq. AA genotype
2pq = freq. Aa genotype
q2 = freq. aa genotype
figure 21-07.jpg
Use HW to do two
things:
Calculate allele
frequencies
(given genotype freq’s)
Predict genotype
frequencies
(given allele freq’s)
So what?
If ACTUAL genotype frequencies match
predictions, then…
• the population is in HW equilibrium
• allele and genotype freq. constant across
generations
If ACTUAL genotype frequencies differ from
predictions, then…
• population is not in HW equilibrium
• some evolutionary force is acting!
Evolutionary forces that violate
Hardy-Weinberg:
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•
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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)
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
Galapagos finches
Classify the following examples:
• 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).
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