Evolutionary history of migratory and nonmigratory populations of prairie warblers (Dendroica discolor)

The cohesion of species is a function of interbreeding among constituent populations. Gene flow smooths clinal variation and must be interrupted for significant population differentiation and speciation to occur. Therefore, within a species, an abrupt, discontinuous geographic distribution of variation suggests an interruption of gene flow and the potential for evolutionary divergence. In this dissertation I describe the geographic pattern of genetic and morphological variation in prairie warblers, a species that exhibits discontinuous geographic variation in migratory behavior. Members of one widely-distributed subspecies are highly migratory (Dendroica discolor discolor), while members of a peripheral subspecies in Florida are sedentary (D. d. paludicola). Their geographic ranges are separated by approximately 200 km.;I used mitochondrial DNA sequences to infer the relationship of migratory and non-migratory prairie warblers. A phylogeny of DNA haplotypes indicates that there has been no long-term genetic isolation of these forms. However, a population genetic analysis suggests there is a significant reduction of gene flow between discolor and paludicola. This result is surprising, given prairie warblers' capacity for long-distance movements and no obvious extrinsic barriers to mating.;Using museum specimens collected throughout the species' range, I described variation in external morphology. My analysis indicates that the morphologies of migratory and non-migratory populations differ substantially. The pattern of geographic variation in morphology is coincident with variation in migratory behavior. The concordant genetic and morphological data suggest that these behaviorally differentiated taxa may be sufficiently isolated to continue to diverge from one another.;Finally, I used computer simulations of gene genealogies to infer parameters of population growth for prairie warblers and two other bird species. These simulations showed that populations of D. d. discolor increased at least ten-fold during the Pleistocene. Calidris alpina and Carduelis chloris showed no evidence of population growth. I also evaluated the efficiency of this method for recovering information about historical demographies.;These studies add to our understanding of spatial and temporal aspects of population differentiation and speciation. Extensions of this research would involve sampling at a finer geographic scale and studies of development of taxon-specific behavior and morphology.