Transcript Lecture 12: Speciation

Lecture 12: Speciation
1) Geographic variation
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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)
Polytypic species (Rassenkreis)
Heliconius butterflies
Cougars
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
Concordant Clines
e.g. Orioles
• Colour
• Enzyme frequencies
• West to east
Pleiotropy may cause
concordance
Discordant Clines
e.g. Rat Snake
• Blotching
• Colour
• Striping
Due to geographic variation in selection pressure
More Examples
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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
Famous Clines
Bergman’s rule:
• warm-blooded spp.
• larger in colder environments
Allen’s rule:
• warm-blooded species
• shorter protruding body parts
relative to body size in colder
Gloger’s rule:
• More darkly pigmented in
more humid climates
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
Allopatric
Speciation
Model
Allopatric Speciation
Best known & easiest understood mode of speciation
Lots of evidence:
• Ring Species (Ensatina spp.)
Ensatina eschscholtzii complex
(a)
(b)
(c)
(d)
(e)
(f)
(g)
(a)
(b)
(f)
(g)
(e)
(c)
(d)
Model: Island Archipelagoes
1. Invasion
2. Divergence
3. Reinvasion
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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
Reinforcement model
AA
Aa
aa
selected against
(low fitness)
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AA
Aa

A
aa

a
Assortative
mating
Fixation
Speciation
Problem…
• If a is rare, selection against Aa removes it
from the pool
AA
Aa
aa

AA
Fixation
This genotype is
uncommon
No Speciation
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
Peripheral Isolate Model
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Mayr 1954
“founder effect”
“peripatric speciation”
Observed: local, isolated populations
peripheral to the main range are often
divergent
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
Speyeria spp.