The Yellow Warbler (Dendroica petechia) is a highly polytypic bird species with a vast breeding range in the Americas. To assess the level of population structuring within the northern part of its range, I surveyed the nucleotide variation present in a 344bp segment of the mitochondrial DNA (mtDNA) control region I (CR-I) from 155 breeding individuals. These birds were caught at seven locations in Canada and Alaska. Fifty-nine haplotypes were observed in this sample, with pairwise distances between haplotypes ranging from 0.29 to 4.35%. The number of nucleotide sites with multiple hits indicates a high rate of evolution in this region. A homologue to the CR-I was also identified and likely originated from a paralogous duplication event, as suggested by the comparison of sequences from the two regions. Significant population structuring in Yellow Warblers across North America was revealed by analyses of nucleotide diversity and molecular variance, which demonstrated the existence of a major subdivision between eastern (Manitoba to Newfoundland) and western (Alaska and British Columbia) warbler populations. This finding provides evidence for very low levels of gene flow between these two groups. Fifteen out of 21 pairs of populations differ significantly in their genetic composition, indicating further structuring at a smaller geographic scale. Within the eastern group of populations, both the high mutation rate of the CR-1 and an isolation-by-distance process seem to be responsible for differences between locations. Phylogenetic analyses indicate that western birds form a monophyletic group whereas eastern birds are paraphyletic with respect to the western ones. However this conclusion remains hypothetical because of a lack of statistical support for the monophyly of western haplotypes. Nevertheless, this situation is consistent with a historical splitting of warbler populations by a vicariant event, possibly of Pleistocene origin, and provides intraspecific support for vicariance as a mechanism leading to isolation and speciation of western warbler taxa, as hypothesized by Mengel (1964). However, other scenarios, such as a founder event in the west from an eastern stock, cannot be excluded, although they are less likely based on mtDNA data. The absence of phylogeographic structure in the East suggests a recent expansion of Yellow Warbler populations from a restricted geographic range. These findings demonstrate that populations of continentally distributed North American passerine species can show high level of population structuring when assayed with an hypervariable molecular marker such as the mtDNA control region I. / Thesis / Master of Science (MSc)