GENES ON SAME CHROMOSOME ARE LINKED Sweet Peas: flower colour

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SUMMARY OF LAST TIME
POPULATION GENETICS Evolution change in
allele frequencies Allele frequencies add up
to 1 Four evolutionary processes
Mutation gene flow add new alleles
Selection genetic drift alter allele

frequencies HARDY-WEINBERG RATIO of p2 2pq
q2 predicted if random mating no evolution

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OUR EXAMPLE HOMOZYGOTE ADVANTAGE with incomplete
dominance
AA AB BB
1- s 1- t 1
with s 2t 0.5
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BETTER ADAPTED VARIANT SPREADS
s 2t 0.5
0 Generations
50
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HETEROZYGOTEADVANTAGE
Heterozygote has highest fitness
AA AB BB
Relative Fitness
1- s 1 1- t
s and t selection coefficients
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HETEROZYGOTE ADVANTAGELEADS TO A MIXTURE OF
ALLELES
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HETEROZYGOTE ADVANTAGELEADS TO A MIXTURE OF
ALLELES
s 2t 0.5
0 Generations 50
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HETEROZYGOTE ADVANTAGELEADS TO A MIXTURE OF
ALLELES
AA AB BB
Relative Fitness
1 - s 1 1 - t
Allele frequencies evolve to
p t q s s t s t
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LECTURE SIXADDING ALLELES
Mutation Gene Flow
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MUTATION - any heritable genetic change-
original source of all genetic variation- far
too rare to distort Hardy-Weinberg GENE FLOW-
movement of alleles by movement of organisms,
gametes, seeds, etc. - varies from too rare to
distort Hardy- Weinberg to being very powerful
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MENDEL DISCOVERED DOMINANCE
smooth rough
Smooth dried pea seeds are dominant
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DNA
starch
TRANSCRIPTION
glucose
mRNA
enzyme
ribosome
TRANSLATION
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SS

glucose starch
starch is insoluble, attracts little
water by osmosis
water
pea rigid as dries out
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rr

one amino acid different
little glucose converted to starch
glucose is soluble, attracts much
water by osmosis
water
swollen pea
pea wrinkles as dries out
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Sr
glucose starch
pea rigid as dries out
Smooth is dominant because heterozygote
converts most of its glucose
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SO IS r ALLELE BROKEN?
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NO rr is the normal genotype in many species of
pea
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MUTATIONS VARY IN SCALEPOINT MUTATIONGENE
DUPLICATIONPOLYPLOIDY
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MUTATIONS VARY IN RESULTPOINT MUTATION
NEW ALLELE GENE DUPLICATION
NEW GENE POLYPLOIDY
NEW SPECIES
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MUTANTS VARY IN FITNESS 0
1
Worse adapted Better adapted
Selected against Selected for
Lethal Neutral mutation
mutation
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GENE FLOW
LOW
HIGH
MAINLAND
ISLAND
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BEST ON
BEST ON MAINLAND
ISLANDS

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TUESDAYS EXAMPLE HOMOZYGOTE ADVANTAGE with
incomplete dominance
1- s 1- t 1
with s 2t 0.5
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BETTER ADAPTED VARIANT SPREADS
s 2t 0.5
0 Generations
50
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SELECTION ALONE
MAINLAND
ISLAND
ISLAND
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GENE FLOW
HIGH GENE FLOW CAN SWAMP SELECTION
MAINLAND
ISLAND
ISLAND
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HOW MUCH GENE FLOWTO PREVENT ADAPTATION?
m proportion of island population
that are migrants
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GENE FLOW WINS IF
m gt 2st
st
In our example
s 2t 0.5 m gt 1 3
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BLUE TITS, OAK TREES CATERPILLARS
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FRENCH OAKS
Downy
Holm
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FRENCH MAINLAND
Downy
Holm Common
Rare Deciduous
Evergreen Caterpillar peak May
Caterpillar peak June
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FRENCH MAINLAND

Holm Oaks
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BLUE TITS IN FRANCE HATCH IN MAY
Jacques Blondel
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FRENCH BLUE TITS MALADAPTED TOHOLM OAK MANY
CHICKS STARVE
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CORSICA HOLM OAK ONLY
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BLUE TITS IN CORSICA HATCH IN JUNE
Jacques Blondel
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DIFFERENCE HERITABLE