Cynthia L. Bronson
Department of Evolution, Ecology, and Organismal Biology, The Ohio State University, Columbus, OH 43210, UNITED STATES OF AMERICA.
This dissertation uses an avian hybrid zone as a study system in which to explore potential causes for the configuration and movement of such zones. Stable hybrid zones are commonly considered to reflect a balance between selection and dispersal. Both exogenous and endogenous selection factors have been advanced to explain barriers to gene flow across narrow hybrid zones. Exogenous selection involves adaptation to local environments. In contrast, endogenous selection involves adaptation or coadaptation on the genomic level regardless of the external environment.
The focal avian species of this dissertation, black-capped (Poecile atricapilla) and Carolina (Poecile carolinensis) chickadees, are permanent residents in Ohio. Currently, their distributions abut in an east-west hybrid zone in the northern aspect of the state. The black-capped chickadee resides north of the hybrid zone and the Carolina chickadee south. Based on historical data, the hybrid zone has moved northward about 100 km in the past 70 years.
The first phase of this dissertation characterized, in a limited manner, a transect of the hybrid zone and identified patterns in reproductive success across the zone. A distinctive trough in reproductive success in the hybrid zone was observed and correlated with both location and parental genetics.
The second phase consists of a controlled experiment. The genetics of the chickadees within the hybrid zone (endogenous selection) and the location of the hybrid zone (exogenous selection) were tested against each other to determine which was more influential on the observed pattern of decreased reproductive success within the hybrid zone. In support of endogenous selection, transplanted hybrid pairs produced fewer nestlings and fledglings than did transplanted pairs of either parental species.
The final phase consists of a controlled, aviary experiment to determine if female mate preference might be a causal mechanism for the northward movement of the hybrid zone. The relative importance of morphological appearance (possibly denoting species identity) and social dominance of males were assessed. Without apparent knowledge of dominance, females preferred black-capped chickadee males. With knowledge of dominance, females preferred the dominant male which, when the dyad was of comparable size, was the Carolina male.