Download Lecture 13: Speciation Continued

Lecture 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)
Clines
•  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??
If 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
Parapatric Speciation
Adjacent Populations
Mechanism
1) CLINE evolves in hybrid zone
2) REINFORCEMENT:
• Repro. isol’n 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
Parapatric cont’d
Most hybrid zones = no  fitness of hybrid
Most researchers think: hybrid zones are 2
contact
Sympatric Speciation
• No sep’n of ancestral pop’ns’ geog. range
• Need: stable polymorphism & assortative
mating
A) Instantaneous Sympatric
Speciation
Polyploidy
>2 sets genes
• Immediate repro isol’n
• Fertile
• Restores chromosomal segregation
• Need > 1 ind. for repro
• Sometimes called:
STATISPATRIC SPECIATION
• e.g. Grasshoppers
Examples
2N
Backcross
3N
2N

4N (close inbreeding)
• Plants
• Some parasitic Hymenoptera ( sib mating)
•  diversity of spp.
Inversions
• DNA segment reversed
Inversion Loop:
b/c: homologous areas align
Inversion results
• Inviable gametes:
- dicentric bridges & acentric fragments
(paracentric inversions)
- duplications & deletions (pericentric inversions)
Result:
Non-viable gametes:
• Duplicate some info
• Lose other info
B) Gradual Models
Disruptive Selection: NS favours
forms that deviate from pop’n mean
If random mating generates
phenotypes matched to resource
dist’n:
• NO select’n for assortative mating
(e.g. seed & beak sizes)
• No speciation b/c equal fitness
But…
Nonnormal resource dist’n:
• random mating = unequal fitness
• assortative mating matches dist’n better 
speciation
Heterogeneous Env’ts: Selection maintains
Diversity
Multiple Niche Polymorphisms:
• Coarse vs. Fine – Grained
• Spatial vs. Temporal
Resource distribution
Fitness
AA
Aa
aa
eg. Papilio (Butterflies)
AA
(Host 1)
aa
(Host 2)
Aa
LOW FITNESS
- selection for assortative mating
Locus B: BB, Bb – mate on host 1
bb – mate on host 2
RIM (premating isolation)
Conditions for Sympatric Speciation
• Strong linkage b/w A (resource) &
B (host choice)
• Strong selection against Aa (hybrid)
•  gene flow b/c var’n in host preference
• Few loci involved in mate preference
Why 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!!!
e.g. Lacewings
• colour & niche & seasonal diff’ns
(multiple niche polymorphisms)
• currently sympatric
• assortative mating b/c poor camouflage of
heterozygote
• NOT proof of
sympatric speciation
Host 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?
• Envt’l segreg’n, diff’t dev’t times
•  maybe don’t need more selection for
isolation
Evidence
• 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 > ALLOPATRIC b/c threat of
hybridization lowers fitness
e.g. Damselflies
• Wing Colour
(Courtship – diff’n in colour with sympatry)
• Interpopulation comparisons convincing
• Interspecific comparisons ….not convincing
– Sympatric spp. with low repro isol’n already fused
 artificially inflates repro isol’n
Damselflies Cont’d
Allopatric w Recontact
1b
1a
Past
(no interbreeding)
Present
1a 1b
Sympatric w High Isol’n
2a
Allopatric w Low Isol’n
(interbreeding)
2b
2b
2a
Past
Fused
2b
Present
Sympatric sp. only ever show spp. with high isolation
But, doesn’t explain…
1a
1a1b
1b
Hybrid Zone
If mate then allopatric w low isolation
If won’t mate – sympatric w high isol’n
Genetic Models of Speciation
Freq of x
Fitness
b
1) Divergence model
• isolated popn
• Select’n for lower x
• divergence to equilibria a & b
a
2) Peak Shift
a
P2
•
•
•
•
drift
P1
small population (drift more likely)
character moves past “saddle” by drift
NS won’t push into area of lower fitness
moved to peak z by selection
b
selection
Recontact…
• Differentiation in populations by adapting
to different niches
• May incidentally confer repro isolation
when later meet
How do R.I.M. arise?
Sexual Selection – F pref. arise through drift
Runaway Selection – rapid divergence
Coevolution
• drift in flower phenotype in local popn’s
• selec’n on pollinator, isol’n of flower, drives
divergence
Do R.I.M. arise to prevent
hybridization?
• Evidence: repro. isol’n arises allopatrically by
sex. selection, drift, ecol. selection
• e.g. Sticklebacks (predation vs. sexual selection)
– Intermediate b/w red/black (hybrid)
=  fitness
Rapid Speciation
Can occur through:
• strong sexual selection
• high trophic specialization
• few competitors
Lake Malawi Cichlids
- Highest speciat’n rate of any vertebrate
group living or extinct (450 spp. in 2 MY)
- Hypothesis: rapid divergence due to sexual
selection
Summary
• Reproductive isolation can evolve by
selection & drift whether “threatened” by
hybridization or not
• Speciation need not be adaptive in itself
• Byproduct of selection & drift