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Abstract
Hybridization, the interbreeding of distinctly different taxa, is widespread in nature in plants and animals. Its consequences include complete speciation, the fusion of species through hybridization, and most interesting, a stable but limited zone of hybridization between species that remain distinct. I studied California Quail and Gambel's Quail (C. californica and C. gambelii), which have hybridized in a narrow area for at least 100 years, but retain their morphological and physiological differences.
I first asked how ecological conditions affect the hybrid zone. A transect through the hybrid zone revealed that changes in morphological and genetic character states corresponded to a sharp transition in vegetation, rainfall, and temperature profiles between mesic and xeric biomes. Within the hybrid zone, the exchange of genes between species occurred freely, and interannual climatic fluctuations caused rapid and episodic mixing over the five-year study. Outside of the ecological gradient and area of transition between pure species, gene exchange did not occur.
Second, I addressed how sympatric conditions may lead to behaviors that maintain the hybrid zone. I found that as in allopatry, pairing occurred in coveys, which are flocks of coalesced family groups. Fitness benefits gained from pairing early within a sympatric covey of mixed species favored interspecific pairing over assortative mating.
Third, I asked if mating behavior within the hybrid zone is affected by vocal differences between the species. Sympatric California and Gambel's Quail responded to each other's vocal types as well as to intermediate-calling hybrids.
Finally, I examined individual mating behavior by asking how mixed species coveys form. Evidence showed that mixed species coveys could arise from exposure to heterospecifics that results in acceptance of heterospecifics as potential mates.
I conclude that covey formation is pivotal to understanding the stable but limited zone of hybridization in this system. In sympatry, exposure to heterospecifics may influence perception of trait differences and species discrimination, and result in formation of mixed species coveys. In general, when genetic penalties to interbreeding are weak or absent, mating behaviors specific to local conditions maintain hybrid zones.
