Lecture 12: Speciation

1) 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)

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

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 Speciationspeciation 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)

(b)

(c)

(c)

(a)

(d)

(d)

(e)

(e)

(f)

(f)

(g)

(g)

Model: Island Archipelagoes

1. Invasion

2. Divergence

3. Reinvasion

1. 2. 3.

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 aaselected against

(low fitness)

AA A

Aa

aa aAssortative

mating

Fixation

Speciation

Problem…

• If a is rare, selection against Aa removes it from the pool

AA

Aa AA

aa Fixation

No SpeciationThis genotype is uncommon

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

• 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.