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 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

Resource distribution Fitness AA Aa aa 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

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

Damselflies Cont’d Past Present Past Present Allopatric w Recontact (no interbreeding) Sympatric w High Isol’n 1b 1a 1a 1b 2a 2b Allopatric w Low Isol’n (interbreeding) Fused 2b 2b 2a Sympatric sp. only ever show spp. with high isolation

1a 1b 1a1b But, doesn’t explain… Hybrid Zone If mate then allopatric w low isolation If won’t mate – sympatric w high isol’n

Genetic Models of Speciation a Freq of x Fitness 1) Divergence model • isolated popn • Select’n for lower x • divergence to equilibria a & b b

2) Peak Shift b selection drift a • 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 P2 P1

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

Lecture 13: Speciation Continued