Title: Lecture 13: Speciation Continued
1Lecture 13 Speciation Continued
- Hybrid zone
- area where differentiated populations interbreed
(incomplete speciation) - Stepped cline in allele freq.
- Introgressive hybridization
- cline widths differ among loci (selection varies)
-
2Clines
- ? in NS ? cline width more abrupt ?s
- Cline width ? (SD of dispersal dist)
- ? s (selection coefficient against
Aa) - Hybrid Zone 2? contact or start of parapatric
speciation??
3If clines are concordant
- 2? contact
- But
- Linkage Disequilibrium genes combine nonrandomly
- Epistasis fitness of 1 allele depends on
occurrence of a 2nd allele - e.g. Mimetic butterflies
Papilio memnon
4Parapatric Speciation
5Mechanism
- 1) CLINE evolves in hybrid zone
- 2) REINFORCEMENT
- Repro. isoln b/w incipient spp. by NS
- (assortative mating)
- if no selection against hybrid - zone is
STABLE - counteracted by gene flow elimination of rarer
allele - ? need fast strong reinforcement
6Parapatric contd
- Most hybrid zones no ? fitness of hybrid
- Most researchers think hybrid zones are 2?
contact
7Sympatric Speciation
- No sepn of ancestral popns geog. range
- Need stable polymorphism assortative mating
8A) Instantaneous Sympatric Speciation
- Polyploidy
- gt2 sets genes
- Immediate repro isoln
- Fertile
- Restores chromosomal segregation
- Need gt 1 ind. for repro
- Sometimes called
- STATISPATRIC SPECIATION
- e.g. Grasshoppers
9Examples
- 2N 2N
- ?
- 4N (close inbreeding)
- Plants
- Some parasitic Hymenoptera ( sib mating)
- ? diversity of spp.
Backcross 3N
10Inversions
Inversion Loop b/c homologous areas align
11Inversion results
- Inviable gametes
- - dicentric bridges acentric fragments
(paracentric inversions) - - duplications deletions (pericentric
inversions) - Result
- Non-viable gametes
- Duplicate some info
- Lose other info
12B) Gradual Models
- Disruptive Selection NS favours
- forms that deviate from popn mean
- If random mating generates phenotypes matched to
resource distn - NO selectn for assortative mating
- (e.g. seed beak sizes)
- No speciation b/c equal fitness
13- But
- Nonnormal resource distn
- random mating unequal fitness
- assortative mating matches distn better ?
speciation - Heterogeneous Envts Selection maintains
Diversity - Multiple Niche Polymorphisms
- Coarse vs. Fine Grained
- Spatial vs. Temporal
14eg. Papilio (Butterflies)
- AA aa
- (Host 1) (Host 2)
- A a
- LOW FITNESS
-
- - selection for assortative mating
- Locus B BB, Bb mate on host 1
- bb mate on host 2
- RIM (premating isolation)
15Conditions for Sympatric Speciation
- Strong linkage b/w A (resource)
- B (host choice)
- Strong selection against Aa (hybrid)
- ? gene flow b/c varn in host preference
- Few loci involved in mate preference
16Why few mate preference loci?
- Recombination causes ? linkage disequilibrium
- ? right alleles for mate preference no longer
linked with right alleles for host selection. - These conditions are Exceptional Circumstances!!!
17e.g. Lacewings
- colour niche seasonal diffns
- (multiple niche polymorphisms)
- currently sympatric
- assortative mating b/c poor camouflage of
heterozygote - NOT proof of
- sympatric speciation
-
18Host shifts
- e.g. Apple pest from Hawthorn
- breed on hatching fruit type
- different development times for 2 fruits
- Assortative mating but hybridize in lab
- What maintains Diversity?
- Envtl segregn, difft devt times
- ? maybe dont need more selection for isolation
19Evidence
- Little for Sympatric Speciation
- Parapatric Sympatric models require
Reinforcement - Character Displacement (increased difference in
traits between related spp. in sympatry) - ? suggests Reinforcement
- Isolating characters
- SYMPATRIC gt ALLOPATRIC b/c threat of
hybridization lowers fitness
20e.g. Damselflies
- Wing Colour
- (Courtship ? diffn in colour with sympatry)
- Interpopulation comparisons convincing
- Interspecific comparisons .not convincing
- Sympatric spp. with low repro isoln already
fused ? artificially inflates repro isoln -
21Damselflies Contd
Past Present Past Present
1b
Allopatric w Recontact (no interbreeding) Sympatri
c w High Isoln
1a
1a 1b
2a
2b
Allopatric w Low Isoln (interbreeding)
Fused
2b
2b
2a
Sympatric sp. only ever show spp. with high
isolation
22But, doesnt explain
Hybrid Zone
If mate then allopatric w low isolation If wont
mate sympatric w high isoln
23Genetic Models of Speciation
a
Freq of x Fitness
b
- 1) Divergence model
- isolated popn
- Selectn for lower x
- divergence to equilibria a b
242) Peak Shift
b
selection
drift
a
P2 P1
- small population (drift more likely)
- character moves past saddle by drift
- NS wont push into area of lower fitness
- moved to peak z by selection
25Recontact
- Differentiation in populations by adapting to
different niches - May incidentally confer repro isolation when
later meet
26How do R.I.M. arise?
- Sexual Selection F pref. arise through drift
- Runaway Selection rapid divergence
- Coevolution
- drift in flower phenotype in local popns
- selecn on pollinator, isoln of flower, drives
divergence
27Do R.I.M. arise to prevent hybridization?
- Evidence repro. isoln arises allopatrically by
sex. selection, drift, ecol. selection - e.g. Sticklebacks (predation vs. sexual
selection) - Intermediate b/w red/black (hybrid)
- ? fitness
28Rapid Speciation
- Can occur through
- strong sexual selection
- high trophic specialization
- few competitors
29Lake Malawi Cichlids
- Highest speciatn rate of any vertebrate
- group living or extinct (450 spp. in 2 MY)
- Hypothesis rapid divergence due to sexual
selection
30Summary
- Reproductive isolation can evolve by selection
drift whether threatened by hybridization or
not - Speciation need not be adaptive in itself
- Byproduct of selection drift