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

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  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  speciation Heterogeneous Env’ts: Selection maintains Diversity Multiple Niche Polymorphisms: Coarse vs. Fine – Grained Spatial vs. Temporal AAAaaa Resource distribution Fitness

eg. 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)

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 2a 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 a selection b

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