Lecture 12 Speciation 1 Geographic variation genetic behavioural

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 Speciation speciation requires isolating mechanism: eventual genetic barrier to interbreeding

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. )

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 aa selected against (low fitness) 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 This genotype is uncommon AA Fixation 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 • • 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.