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Lecture 21: Speciation II
• Speciation processes– Isolation: Dispersal, Vicariance
– Divergence: Drift, NaturalSelection, Sexual Selection
– Secondary contact: post-zygotic and pre-zygoticisolation
– How to avoid hybridization:sympatric speciation, sexualselection
Cichlid fish diversity:
1000’s of speciesAllopatric Speciation
1. initial isolation
2. divergence
3. secondary contact & “reinforcement”
Allopatric Speciation
IsolationColonization
Vicariance
DivergenceDrift
Natural selection
Sexual selection
Secondary contactHybrid offspring
unfit (postzygotic
isolation)
Selection against
hybridization(prezygotic
isolation)
Reinforcement
(?)
Allopatric Speciation
1. initial isolation
2. divergence
1. Genetic drift
2. Natural selection (Ecology)
3. Sexual selection (courtship, mating)
What Causes Divergence?
What Causes Divergence?
1. Genetic drift
• Dispersal and colonization: Small
populations
• Allele frequencies diverge
What Causes Divergence?
2. Natural selection (Ecology)
• diverge in traits, exploitation of
resources in new environment
Trait value
Fre
quenc
yF
itness
Divergence: Natural selection
• Three-spined stickleback
• Lakes in B.C.
Divergence: Natural selection
• Open water,planktonfeeders
• Bottomfeeders benthic
limnetic
Divergence: Drift and N.S.
Does divergence lead to evolutionary
independence (reproductive isolation)?
What Causes Divergence?
3. Sexual selection
• divergence in secondary sexual
traits affecting mate choice
Isolation
Mating
preferences
change
Genetic
divergence
Knight et al. (2004) Proc Roy Soc B 271: 675-680
Cichlid fishes: Lake Malawi• Geographic
populations
• Differ in malecolor morphs:secondarysexual traits
• Speciation bysexualselection?
Knight et al. (2004) Proc Roy Soc B 271: 675-680
Mate choice experiment
• Males hold territories
• Females spawn, then brood larvae in mouth
• Experiment: Partial partition mate choice– Females, Males from 5 geographic populations
– Number of spawnings
Females choose male with local color
morph
81100RW
426101EM
12908NB
22291MR
5111119CH
RWEMNBMRCHMale/
Female
Hawaiian crickets
• Divergence in secondary
sexual traits: male
courtship song– No ecological differences
• Rapid: 4 species/million
years
Allopatric Speciation
1. initial isolation
2. divergence
3. secondary contact & “reinforcement”
Secondary contact
• Fitness of hybrids?
Hybrids
Secondary contact
Hybrids fit:
Introgression
Hybrids unfit:
Speciation
1. Genetic incompatibility
2. Sexual selection against hybrids
3. Ecological inferiority of hybrids (Nat. Sel.)
Low hybrid fitness? Sticklebacks
Growth slower inboth environments
Low fitnessHybrids
Post-zygoticisolation
• Reinforcement: Selection for reducedhybrid matings, e.g.– Assortative mating– Timing of breeding
• Produces prezygotic reproductive isolation
Hybrids
• Rhagoletis pomonella
• Eggs, larva develop onHawthorne fruit
• Adults return to larval fruitfor mating
How to avoid hybrid mating 1
Hawthorne
winter
larvae pupae adultsadults
Rhagoletis pomonella
• Colonized Apple in 1800s
• Mating, habitat choice arelinked
• Hybrid matings (Hawthornevs. apple flies) rare
How to avoid hybrid mating 1
Apples
winter
larvae pupae adultsadults
Apples
winter
larvae pupae adultsadults
Hawthorne
winter
larvae pupae adultsadults
Linn et al. (2004) PNAS 101: 17753-17758
Sympatric Speciation How to avoid hybrid mating 2?
Divergence by N.S., drift
Isolation Divergence
Mating
preferences
change
reinforcement
How to avoid hybrid mating 2?
Divergence by Sexual selection: Prezygotic
isolation is a consequence of divergence
IsolationMating
preferences
change
Divergence
No reinfocement needed