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Speciation
most important problem in evolutionary biology- change over time- diversification
Species concepts
Mechanisms of speciation
Species Concepts
Mayden (1997) 22 different species concepts
species as categories -- taxa
species as evolutionary groups -- “smallestindependent evolutionary unit”
importance -- conservation of biological diversity understanding the process of diversification
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, doesn’t apply to many species:
asexual species -- bacteria, fungi, many invertebrates and plants
hybridizing species -- wolves and coyotes; cottonwood and balsam poplar
fossil species
Phylogenetic Species Concept -- Joel Cracraft (1989)
“An irreducible (basal) cluster of organisms diagnosably differentfrom other such clusters, and within which there is a parentalpattern 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
Morphospecies Concept
fossil species
problems of cryptic species because of limited characters
Geographical classification (pass)
allopatricvicariancefounder event
parapatricsympatric
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)
geographic isolation due to range splitting -- vicariance
populations may diverge due to:natural selectionsexual selection(genetic drift)
type of divergence may be pre-mating or post-mating or both
Vicariant speciation in snapping shrimp
Isthmus of Panama closes ~3 mya
Caribbean, Pacific populations
both pre-mating and post-mating divergence
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 environmentsexual selection
type of divergence may be pre-mating or post-mating or both
Patterns of speciation in Hawaiian Drosophila are consistent with founder events
outcomes of divergence in allopatry
insufficent divergence ---> fusion
reproductive isolation due to assortative mating withinpopulations ---> speciation
mating between populations leading to a hybrid zone
hybrid fitness hybrid zone eventual outcome
lower than narrow, short- reinforcementparental forms lived
equal to wide, parental populationsparental forms persistent coalesce
greater than ecotone stable hybrid zone,parental forms new habitat new species
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)
Not unique to allopatric speciation
*first two stages frequently happen in ‘sympatry’differentiation despite gene flow due to selection
reproductive isolation is a consequence of selection onecological attributes
Ecological speciation in three-spined sticklebacks
Rundle et al. 2000 Science 287:306
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 reproductivelyisolated from each other
limnetic forms from one lake should be isolated from benthicforms from a different lake
species pairs from three different lakes
different lakessame lake
Natural selection, sexual selection selection and speciation in the cichlids of Lake Victoria
Verheyen et al. 2003 Science 300:325
Lake Victoria -- more than 500 species of haplochromine cichlids
morphological, ecological, and behavioral diversity
Lake Victoria cichlids are a monophyletic group most closely related to taxa from Lake Kivu
algal grazer insect eater piscivore algal grazer
Sexual selection drives speciation in African great lakes cichlids
Couldridge and Alexander 2002 Behavioral Ecology 13:59
Pseudotropheus zebracomplex of at least four species (Lake Malawi)differ in color pattern; similar in other morphology, ecology, behavior
Test for assortative matingfemale preference with choice (conspecifics and heterospecifics)female preference without choice (heterospecifics only)
C P. callainos pale blue, no bars
R P. zebra “red dorsal” pale blue; dark blue
bars; orangedorsal 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
C P. callainos pale blue, no bars
R P. zebra “red dorsal” pale blue; dark blue
bars; orangedorsal fin
G P. zebra “gold” pale orange, faint
brown bars
L P. lombardoi yellow, 5 vertical
black bars
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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 structurebill structure and cone structure affect feeding performance
---> no generalist crossbill
nine different call types; seven are specialized on different species of conifers
Benkman 2003 Evolution 57:1176
detailed study of one population in southern Idahofeeds 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 depthand palate structure) for five species
Selection on bill depth is stabilizing
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 inthe environment (physical or social).
They may function properly as organismsi.e. viable and fertile
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
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
reaction norms mean fitness of clones tested on both hosts
Genetics of speciation -- performance and habitat preference in pea aphids
Via 2002
two host races/incipient species -- alfalfa and cloverknown trade-off between performance on clover and alfalfa
what is the genetic architecture of this trade-off?
are genes for performance independent of genes forhabitat choice/assortative mating?
negative genetic correlation:pleiotropy or LDE of closely linked loci
vs. LDE of unlinked loci
QTL analysis
Performance
Acceptance andPerformance on alfalfa
Acceptance and per-formance on clover
Intrinsic postmating incompatibility
Drosophila frogs Lepidoptera birds
Lepidoptera
Post
Pre- plus post
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
Haldane’s Rule: if hybrid sterility or inviability is present in only one
sex of F1 offspring, it will be the heterogametic sexFor Drosophila it is males--for Lepidoptera it is females
Haldane’s 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 ---> hybrid male sterility- spermatogenesis is inherently more sensitive to perturbation
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).
Haldane’s 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.