Geographic variation refers to the widespread phenomenon of variation within the range of species. Understanding the origin and maintenance of such variation is a primary goal of attempts to understand the evolutionary process itself. This project is designed to clarify evolutionary processes that shape the geography of intraspecific genetic variation in three codistributed species of birds, namely, the Fox, Song, and Chipping sparrows. In morphology, these species exhibit both marked geographic differentiation (Fox and Song sparrows) and phenotypic uniformity (Chipping sparrow) in their continent-wide distributions. In samples of birds taken throughout the range of each species, genetic differentiation in mitochondrial DNA (mtDNA) and a series of genes that encode soluble proteins, called allozymes will be measured. Such descriptive data on genetic variation within species provide a foundation for understanding the origin and evolutionary significance of geographic variation, and its relationship to the process of adaptation and speciation. Examination of genetic data is crucial because evolution ultimately occurs at this level in populations. By examining genetic variation in samples of these three species, it will be possible to determine if geographic distance alone or "vicariant" events in earth history (such as the formation of mountain ranges) play a primary role in geographic differentiation, the initial step in evolutionary divergence. No studies of avian genetic variation within and among codistributed species have been conducted on the geographic, continent-wide scale of the proposed study. This study will provide the most detailed information on avian population genetic structure yet available, for both proteins and at the level of the genetic code itself, (mt)DNA. The use of three species that occur over the same range increases the statistical power of interpreting patterns of variation. The results will offer significant insight into the evolutionary process in general as well as the birds under study.
