Hybrid zones are, roughly speaking, narrow regions in which genetically distinctpopulationsmeet, mate, andproducehybrids.They areoften only a few hundred meterswide andyet may be severalhundred kilometerslong. They are found in a wide varietyof organisms:Tables 1 and 2 list about 150 reasonably clear examples, in which there is a spatialtransitionbetween two hybridizing forms. Such a widespreadandstrikingphenomenonrequiresexplanation; more importantly, offers us severalways of understanding natureand origin of it the species. First, hybridzones pose interestingquestions for the taxonomist, for they contrasttwo views of the species: as a set of populations delimited by genetic barriersto gene exchange; and as a set of populationsmaintainedin a particular stableequilibrium selection. Second, the wide rangeof genotypes by found in a hybrid zone can be used to analyze the genetic differences and selective forces thatseparate taxainvolved. This may allow some inferences the aboutthe way these differencesevolved and, by extrapolation,aboutthe way fully isolated species diverge from each other. Finally, models of parapatric speciation, and of Wright's "shiftingbalance," involve the formation, move- ment, and modification of hybrid zones. Hybrid zones must be understood before the plausibility of these models can be judged.


Analysis of Hybrid Zones

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