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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 = σ (SD of dispersal dist)
√ s (selection coefficient against Aa)
- Hybrid Zone = 2° contact or start of parapatric speciation??
Parapatric Speciation
Adjacent Populations
CLINE evolves in hybrid zone
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
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
Inversions
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
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
AA Aa aa
Resource distribution Fitness Docsity.com
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)
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
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
(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
