SUMMARY OF

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SUMMARY OF LAST TIME
RIMs are anything - except geography -preventing
gene flow between populations Pre-zygotic RIMs
prevent fertilisation e.g. ecological,
seasonal, behavioural, mechanical, chemical
Post-zygotic RIMs select against hybrids e.g.
hybrid inviability, hybrid sterility,
hybrid breakdown
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14 GEOGRAPHY OF SPECIATION
How sexual species split into two
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Three models of speciation
Allopatry Parapatry Sympatry
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ALLOPATRICSPECIATION
Geography prevents gene flow Separated
populations diverge Most widely accepted idea
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Origin of allopatry
time
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Allopatric divergence
time
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Allopatric divergence is inevitable over time
MUTATIONS rare, so will differ GENE FLOW from
3rd population GENETIC DRIFT especially if
either population
small SELECTION especially if habitats differ
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Return to sympatry
?
time
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Outcome 1
NO RIMS NO SPECIATION
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Outcome 2
Perfect Pre-Zygotic RIMS SPECIATION
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Outcome 3
Imperfect RIMS Hybrids less fit
Selection for Pre-Zygotic RIMS
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HOW QUICK?
How quickly do allopatric populations diverge
enough to speciate?
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HOW QUICK?
Sometimes very fast e.g. Hawaii Wallabies c.
100 years
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HOW QUICK?
Sometimes very slow e.g. Tulip Trees
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TULIP TREES
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SEEDS KILLED BY SALT WATER
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FOSSIL TULIP TREE
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FOSSILS
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GENETICS OF HYBRID INVIABILITY
Two gene loci collaborate to produce trait
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TWO POPULATIONS HAVE DIVERGED
AA bb aaBB
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HYBRID
Aa bB
A phenotype B phenotype
LESS FIT even lethal
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THAT WAS AN EXTREME CASE
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CHANGE DOMINANCE IN EXAMPLE
AA BB aabb
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HYBRID F1
Aa Bb
A phenotype B phenotype
AS FIT AS ONE PARENT
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HYBRID F2
AA Aa aa
BB Bb bb
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THAT WAS MILD HYBRID BREAKDOWN
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GENETICS OF HYBRID STERILITY
During MEIOSIS homologous chromosomes have to
pair up so that one set goes to each gamete
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HYBRID STERILITY
Homologous chromosomes fail to recognise each
other Spare chromosomes often left over Gametes
almost never have perfect set
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HYBRID STERILITY
Horse Donkey 64 chromosomes 62

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HYBRID STERILITY
Mum Dad Horse
Donkey 32 chromosomes 31
Mule 63

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HYBRID STERILITY
Mum Dad Donkey
Horse 31 chromosomes 32
Hinny

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HYBRID STERILITY
Chromosome re-arrangement in Hawaii
population Cannot interbreed
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PARAPATRICSPECIATION
Occurs despite gene flow Requires limited
dispersal or strong selection Best evidence
Ring species
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RING SPECIES
space
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RING SPECIES 1
song
wing bar(s)
Greenish Warblers
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SIBERIA
HIMALAYAS
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1 wing bar Short trilling song
SIBERIA 2 wing bars Long soft song
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RING SPECIES 2
Herring Gull complex
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Herring
Herring
Herring
LBB
Herring
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Textbook claim
Lesser Black-backed Gull
Herring Gull
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Reality
American Herring Gull
British Herring Gull
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SYMPATRICSPECIATION
Requires an intrinsic barrier to gene flow
strong selection