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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