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Lecture 12: Speciation
- Geographic variation
- genetic, behavioural, developmental differences over geographic range
- important to the study of evolution
- describes the course of evolutionary change
- new species formed by same processes that give rise to variation among conspecifics
Patterns in Geographic Variation
Among Species
- Parapatric – different forms meet & interbreed
- Sympatric – different forms “meet” but don’t breed
- Allopatric – geographically separated …never get the chance to mate
- Polytypic species – spp. with several defined subspecies (geographic races)
- Superspecies –monophyletic group of closely, mostly allopatric species (i.e. used to be one spp., usually post-mating isolation)
Superspecies (Artenkreis)
Parus spp. Gasterosteus aculeatus
Forms of Geographic Variation
Cline: gradient of variation in genotype/phenotype
Types of Clines:
- Concordant: > 1 characters vary along the same transect
- Discordant: characters vary independently
- Shallow: gradual changes in character
- Stepped: abrupt changes in character
Discordant Clines
e.g. Rat Snake
- Blotching
- Colour
- Striping
Due to geographic variation in selection pressure
More Examples
Gradual Cline CLOVER
- cyanide production
- ↑ N to S cline
- Balance costs & benefits
- Benefit: protection against herbivory
- Cost: frost sensitivity
Discordant Cline RABBIT TICKS “body size”: ↑ S to N “appendage size”: ↑ W to E Trade-off b/w fat storage & desiccation
Variation + Reproductive Isolation → Speciation
speciation requires isolating mechanism: eventual genetic barrier to interbreeding
Speciation
Speciation can be classified by geographic
characteristics or genetics:
Allopatric speciation Parapatric speciation Peripatric speciation Sympatric speciation
Model: Island Archipelagoes
- Invasion
- Divergence
- Reinvasion 1. 2. 3. Docsity.com
Recontact of Divergent Species
- Speciation complete: no hybridization
- Speciation incomplete: hybridization
- Hybrid Zones: regions of 2° contact b/w previously isolated pop’ns w viable hybrids
- Fitness of hybrids determines incidence of hybrid zones
- If reduced fitness: isolation reinforced by selection
Problem…
- If a is rare, selection against Aa removes it from the pool
AA
Aa → AA
aa
Fixation
This genotype is^ No Speciation uncommon Docsity.com
Gene flow between divergent populations will:
- equalize gene frequencies
- reduce isolation
- make one species (hybrid zone)
Reinforcement must act quickly because of
competing effects of gene flow & isolation
Peripatric Speciation
- “new” environment homogeneous
(few conflicting selective pressures)
- population small
- founded by few individuals
- low genetic diversity
- genetic drift
- must be no gene flow
Mechanism
- Genetic change in large pop’ns is slow (gene flow)
- Faster in small populations (genetic drift)
- Epistasis: synergistic effect of 2+ loci on phenotype
(may help pass fitness “valley”)
- Rapid speciation
- Evidence: flycatcher
lizards
- Model allows for “non-adaptive” speciation
