Speciation

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Speciation most important problem in
evolutionary biology - change over time -
diversification Species concepts Mechanisms of
speciation
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Species Concepts Mayden (1997) 22
different species concepts species as
categories -- taxa species as evolutionary
groups -- smallest independent evolutionary
unit importance -- conservation of
biological diversity understanding the
process of diversification
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Biological Species Concept -- Ernst Mayr
(1947) Species are actual or potentially
interbreeding natural populations which
are reproductively isolated from other
populations emphasizes genetic
exchange within species reproductive
isolation between species can be difficult
to apply and, doesnt apply to many
species asexual species -- bacteria, fungi,
many invertebrates and plants hybridizing
species -- wolves and coyotes cottonwood
and balsam poplar fossil species
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Phylogenetic Species Concept -- Joel Cracraft
(1989) An irreducible (basal) cluster of
organisms diagnosably different from other such
clusters, and within which there is a
parental pattern of ancestry and descent
species are monphyletic groups can be applied
to any type of organism can be difficult to
implement difficulties with matching gene
trees to speciation events
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Morphospecies Concept fossil species
problems of cryptic species because of limited
characters
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Geographical classification (passJ) allopatric
vicariance founder event parapatric sympatri
c Classical model of allopatric
speciation isolation of populations allopatry
genetic divergence of populations allopatry dev
elopment of reproductive isolation when
populations come back into contact sympatry
(secondary contact and reinforcement)
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geographic isolation due to range splitting --
vicariance
populations may diverge due to natural
selection sexual selection (genetic
drift) type of divergence may result in
pre-mating isolation or post-mating isolation
or both
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Vicariant speciation in snapping
shrimp Isthmus of Panama closes 3
mya Caribbean, Pacific populations both
pre-mating and post-mating divergence
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geographic isolation due to a founder event
founder population is genetically different from
source population genetic drift is important at
the start of the process divergence via natural
selection in a novel environment sexual
selection type of divergence may result in
pre-mating isolation or post-mating isolation or
both
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Patterns of speciation in Hawaiian Drosophila are
consistent with founder events
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outcomes of divergence in allopatry
insufficent divergence ---gt fusion
reproductive isolation due to assortative mating
within populations ---gt speciation
mating between populations leading to a hybrid
zone hybrid fitness hybrid zone eventual
outcome lower than narrow, short- reinforcemen
t parental forms lived equal to
wide, parental populations parental forms
persistent coalesce greater
than ecotone stable hybrid zone, parental
forms new habitat new species
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Classical model of allopatric speciation (also
passJ) isolation of populations allopatry gen
etic divergence of populations allopatry develop
ment of reproductive isolation when
populations come back into contact sympatry
(secondary contact and reinforcement)
first two stages frequently happen in
sympatry differentiation despite gene flow due
to selection reproductive isolation is a
consequence of selection on ecological
attributes or divergent sexual selection
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Ecological speciation in three-spined
sticklebacks Rundle et al. 2000 Science
287306 pairs of species in freshwater lakes
independently derived from a marine
stickleback species benthic form -- large,
heavy-bodied feeds on invertebrates in the
near-shore habitats limnetic form
slender-bodied feeds on plankton in open water
Predict limnetic forms from different lakes
should not be reproductively isolated from each
other limnetic forms from one lake should be
isolated from benthic forms from a different
lake species pairs from three different
lakes
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different lakes same lake
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Natural selection, sexual selection selection
and speciation in the cichlids of Lake
Victoria Verheyen et al. 2003 Science
300325
Lake Victoria -- more than 500 species of
haplochromine cichlids morphological,
ecological, and behavioral diversity
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Lake Victoria cichlids are a monophyletic
group most closely related to taxa from
Lake Kivu
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algal grazer insect eater piscivore
algal grazer