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 butterfliesPapilio memnon

Parapatric Speciation

Adjacent Populations

 

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

Mechanism

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 2N

4N (close inbreeding)

• Plants

• Some parasitic Hymenoptera ( sib mating) diversity of spp.

Backcross

3N

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

speciationHeterogeneous Env’ts: Selection maintains

DiversityMultiple Niche Polymorphisms:• Coarse vs. Fine – Grained• Spatial vs. Temporal  

AA Aa aa

Resource distribution

Fitness

eg. Papilio (Butterflies) AA aa(Host 1) (Host 2)

A aLOW FITNESS

- selection for assortative mating  Locus B: BB, Bb – mate on host 1

bb – mate on host 2RIM (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

1a 1b

1a 1b

2a

2b

2b

2b2a

Past

Present

Past

Present

Allopatric w Recontact

(no interbreeding)

Sympatric w High Isol’n

Allopatric w Low Isol’n

(interbreeding) Fused

Sympatric sp. only ever show spp. with high isolation

Damselflies Cont’d

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

 1) Divergence model

• isolated popn

• Select’n for lower x

• divergence to equilibria a & b

a

b

Freq of x

Fitness

2) Peak Shift

• 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

drift

P2 P1

aselectionb

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