Lecture 12: SPECIATION CONTINUED

HYBRID ZONE = geographic area where differentiated populations interbreed

Causes a STEPPED CLINE in allele freq.

INTROGRESSIVE HYBRIDIZATION:

cline widths differ among loci (selectn varies)

in NS = cline width = more abrupt Ds

SD of dispersal distance

Cline Width

selectn coefficient

against AA’

Hybrid Zone = 2° contact or start of parapatric speciation??

CONCORDANT CLINES:

- same geographic variation in character 1

& character 2 = 2° contact

But:

LINKAGE DISEQUILIBRIUM:

- when linked, selectn on 1 = selectn on 2nd

EPISTASIS:

- fitness of 1 allele depends on occurrence of a 2nd allele

2) PARAPATRIC SPECIATION

Adjacent Populations

1) CLINE evolves in hybrid zone

2) REINFORCEMENT:

repro isoln b/w incipient spp. by NS

- *** b/c no selection against hybrid - zone is STABLE

3) counteracted by gene flow

4) elimination of rarer allele

5) therefore need fast and strong reinforcement

Most hybrid zones = no fitness of hybrid

Most researchers: hybrid zones are 2° contact

3) SYMPATRIC SPECIATION

No sep’n of ancestral popns’ geog range

A) Instantaneous:

POLYPLOIDY/ INVERSIONS

- Plants

- Some parasitic Hymenoptera

( sib mating: diversity of spp. )

B) Gradual Models :

DISRUPTIVE SELECTION:

- NS favours forms that deviate from popn mean

If random mating generates phenotypes matched to resource distn then NO selectn for assortative mating (e.g. seed & beak sizes).

Nonnormal resource distn: random mating = unequal fitness, assortative mating matches distn better \ speciation

Multiple Niche Polymorphisms:

- Coarse vs. Fine – Grained

- Spatial vs. Temporal

eg. Papilio (Butterflies)

AA A’A’

(Host 1) (Host 2)

A A’

LOW FITNESS

- selectn for assortative mating

Locus B: BB, Bb – mate on host 1

bb – mate on host 2

Condns of Model: (for sympatric speciatn)

- Strong linkage b/w A & B

- Strong selection against A A’

- gene flow b/c varn in host preference

- Few loci involved in mate preference

Why ? Recombination causes linkage disequilibrium \ right alleles for mate preference no longer linked with right alleles for host selection.

EXCEPTIONAL CIRCUMSTANCES!!

e.g. Lacewings

- colour & niche & seasonal diffns, currently sympatric, assortative mating b/c poor camouflage of heterozygote

- NOT proof

Host shifts – new food plant

e.g. Apple pest – from Hawthorn

- breed on fruit type born on

- different development times for 2 fruits

- Assortative mating but hybridize in lab

- What maintains Diversity?

- envtl segregn, difft devt times \ maybe don’t need more selection for isolation

Little evidence: SYMPATRIC SPECIATION

Both PARAPATRIC & SYMPATRIC models require REINFORCEMENT

CHARACTER DISPLACEMENT suggests REINFORCEMENT

Isolating characters:

SYMPATRIC > ALLOPATRIC b/c threat of hybridization lowers fitness

e.g. Damselflies:

- Wing Colour (Courtship – increased diffn in colour with sympatry)

- Interpopulation comparisons convincing

- Interspecific comparisons ….not

convincing

However, doesn’t explain

GENETIC MODELS OF SPECIATION

1) DIVERGENCE MODEL

- isolated popn

- selectn for lower x

- divergence to equilibria a & b.

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