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 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

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 AAAaaa selected against (low fitness)  AA  A Aa aa  a Assortative mating Fixation Speciation

Problem… If a is rare, selection against Aa removes it from the pool AA Aa  AA aa Fixation No Speciation This 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.