Hybrid zones: zones of interbreeding between differentiated populations

• 1. Hybrid zone narrow– A. hybrid zone recently formed.

– B. fitness of hybrids may be lower than parental individuals.

– If so, then reinforcement can take place.

– Differences between parental populations are enhanced.

– Final stages of speciation.

• 2. Hybrid zone broad– A. Fitness of hybrids equal to parental species.

– Parental populations converge phenotypically.

– Differences decrease.

– B. Fitness of hybrids greater than parental species.

– Stable hybridization zone or speciation.

• A hybrid zone reflecting secondary contact• Populations diverge in allopatry

• Range shifts bring populations into sympatry

• Pattern of variation

• Low variation in F1s

• High variation in F2s, F3s, etc.

• Measure of variation (continuous characters)

– CV = coefficient of variation

– CV = (s/mean) x 100

Aspidoscelis tigris

Secondary contact

center

Aspidoscelis tigris

Clinal variation in southern Arizona

Southwest AZ South-centralAZ

Primary contactPopulations divergewhile in contact

Aspidoscelis tigris

Release of F2 variationMimulus lewisii Mimulus cardinalis

Greya (moth genus): mtDNA data

Interpretations different with different species concepts