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
development 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 be pre-mating or
post-mating 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 be pre-mating
or post-mating 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
isolation of populations allopatry genetic
divergence of populations allopatry developmen
t of reproductive isolation when
populations come back into contact sympatry
(secondary contact and reinforcement) Not
unique to allopatric speciation first two
stages frequently happen in sympatry differenti
ation despite gene flow due to selection
reproductive isolation is a consequence of
selection on ecological attributes
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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
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Sexual selection drives speciation in African
great lakes cichlids
Couldridge and Alexander 2002 Behavioral Ecology
1359 Pseudotropheus zebra complex of at least
four species (Lake Malawi) differ in color
pattern similar in other morphology, ecology,
behavior Test for assortative mating female
preference with choice (conspecifics and
heterospecifics) female preference without
choice (heterospecifics only)
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C P. callainos pale blue, no bars R P.
zebra red dorsal pale blue dark
blue bars orange dorsal fin G P. zebra
gold pale orange, faint brown bars L
P. lombardoi yellow, 5 vertical black
bars
Predict 1) in choice tests, females will
prefer conspecific males 2) in no choice tests,
females will prefer the heterospecific male
whose color pattern is most similar
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Disruptive selection and speciation in red
crossbills, Loxia curvirostra
highly specialized -- seeds of coniferous trees
(seeds retained in cones) use the crossed bill
to extract seeds from within the cone
extraction efficiency depends on bill shape and
palate structure bill structure and cone
structure affect feeding performance ---gt no
generalist crossbill nine different call types
seven are specialized on different species of
conifers
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Benkman 2003 Evolution 571176 detailed study
of one population in southern Idaho feeds on
Rocky Mountain lodgepole pine (Pinus contorta
latifolia) estimated form of selection on bill
shape (bill depth) relationship between bill
depth and fitness (survival) fitness surface
based on feeding performance (bill depth and
palate structure) for five species
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Selection on bill depth is stabilizing
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Selection and speciation Natural Selection for
different feeding methods or habitat
preferences Sexual Selection for mate preferences
Hybrids do poorly because they do not work well
in the environment (physical or social). They
may function properly as organisms i.e. viable
and fertile
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What are the requirements for this to
work? Strong selection Little genetic
exchange/assortative mating Predictions
Prerequisite of close linkage/pleiotropy
between the traits of mating and
performance or, Recombination will erode
weaker associations leading to no speciation
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Genetics of speciation -- performance and
habitat preference in pea aphids Via
2002 two host races/incipient species --
alfalfa and clover known trade-off between
performance on clover and alfalfa
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reaction norms mean fitness of
clones tested on both hosts
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Genetics of speciation -- performance and
habitat preference in pea aphids Via
2002 two host races/incipient species --
alfalfa and clover known trade-off between
performance on clover and alfalfa what is the
genetic architecture of this trade-off?
are genes for performance independent of genes
for habitat choice/assortative
mating? negative genetic correlation pleiotro
py or LDE of closely linked loci vs. LDE of
unlinked loci QTL analysis
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Performance
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Acceptance and Performance on alfalfa
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Acceptance and per- formance on clover
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Intrinsic postmating incompatibility
Drosophila frogs Lepidoptera
birds
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Post
Lepidoptera
Pre- plus post
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Hybrid sterility and inviability evolve
gradually positively correlated with the time
since divergence -- gradual accumulation
of deleterious epistatic interactions between
species Hybrid sterility evolves faster than
inviability Haldanes Rule if hybrid
sterility or inviability is present in only one
sex of F1 offspring, it will be the
heterogametic sex
For Drosophila it is males--for Lepidoptera it is
females
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Haldanes Rule if hybrid sterility or
inviability is present in only one sex of F1
offspring, it will be the heterogametic sex
inviability due to lethal incompatibilities
between autosomal and X-linked loci dominance
theory hybrid incompatibilities are partially
recessive XX? hybrids are heterozygous
and protected X Y hybrids express all
X-linked recessives faster-male evolution
- sexual selection drives rapid divergence of
male fertility factors ---gt hybrid male
sterility - spermatogenesis is inherently more
sensitive to perturbation
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Species are evolutionarily independent groups. A
species is often defined as a group of
populations that are capable of interbreeding,
but this definition applies best to
diploid, sexually reproducing taxa. Populations
which are phenotypically or ecologically
distinct, yet regularly interbreed, or species
that reproduce largely asexually may still be
valid species. The process of adaptive
divergence that leads to speciation may occur
either in allopatry or sympatry. Speciation
is a result of divergent natural (ecological) or
sexual selection. Genetic drift may
accelerate the process of divergence during
founder events. Reproductive isolation between
species may result from assortative mating
(prezygotic) or from hybrid sterility/inviability
(postzygotic). Haldanes rule is a general
description of the early stages of
speciation. Genetic correlations for performance
in different environments and for habitat
choice may facilitate the rate at which
divergence occurs.