• Cline;
  • ecotone;
  • El Niño;
  • hybridization;
  • introgression;
  • local adaptation;
  • speciation

Abstract Allopatric species commonly interbreed in a restricted margin between their ranges. The particular factors that permit interbreeding between species determine the extent of hybridization and its significance for evolution and conservation. Using California quail and Gambel's quail (Callipepla californica and C. gambelii) that naturally hybridize in a narrow region between relatively mesic and xeric environments, I assessed the exchange of genetic and phenotypic traits in relation to vegetative and climatic features (temperature and precipitation) that characterize the area of range overlap, and I examined genetic and phenotypic traits within the hybrid zone over a five-year period in relation to variation in precipitation. Using microsatellite markers, this study reveals that genetic, plumage, and morphometric traits are tightly associated with vegetation, rainfall, and temperature profiles through the abrupt transition from one parental species to the other across the hybrid zone. Results show that the hybrid zone has remained clinal, stationary, and bounded over the five-year study period. There was no evidence of introgression outside the narrow hybrid zone. Interannual climatic fluctuations are associated with internal hybrid zone dynamics but did not alter the shape and position of the zone. A transect through the hybrid zone revealed rapid and episodic genetic mixing within the zone. Possible long-term consequences of this restricted hybridization for the evolution of the two parental species are discussed in the light of changing environments.