GEE, JM; Princeton University: The causes and consequences of hybridization in California and Gambel’s Quail

The maintenance of two distinct species is dependent on barriers that limit gene flow between them, yet isolating mechanisms rarely work in an all-or-none fashion. California Quail and Gambel�s Quail (C. californica and C. gambelii) have hybridized in a narrow area for at least 100 years, but they retain their morphological and physiological differences. I examined what factors cause the persistence of the hybrid zone and whether exchange of genes is confined to the hybrid zone. Using microsatellite markers and morphological measures, I showed that the hybrid zone is bounded, clinal and stable. A gradient in vegetation, precipitation and temperature profiles between mesic (California Quail) and xeric (Gambel�s Quail) habitats coincided with the abrupt transition in genetic and morphological character states. I suggest that natural selection acting on morpho-physiological adaptations may be a barrier to the interchange of genes. Social conditions in sympatry influence mating behavior and 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. Acoustic signals that are genetically determined did not form a behavioral barrier to reproduction in mixed species coveys. Signal recognition between vocal types may result from sexual imprinting. This study demonstrated that in the hybrid zone, California Quail and Gambel�s Quail chicks were frequently raised by heterospecific foster parents. I conclude that covey formation is pivotal to understanding the stable but limited zone of hybridization in this system. In general, when genetic penalties to interbreeding are weak or absent, mating behaviors specific to local conditions maintain hybrid zones.