Divergent mitochondrial lineages arose within a large, panmictic population of the Savannah sparrow (Passerculus sandwichensis).

Unusual patterns of mtDNA diversity can reveal interesting aspects of a species' biology. However, making such inferences requires discerning among the many alternative scenarios that could underlie any given mtDNA pattern. Next-generation sequencing methods provide large, multi-locus datasets with increased power to resolve unusual mtDNA patterns. A mtDNA-based phylogeography of the Savannah sparrow (Passerculus sandwichensis) previously identified two sympatric, but divergent (~2%) clades within the nominate subspecies group and a third clade that consisted of birds sampled from northwest Mexico. We revisited the phylogeography of this species using a population genomic dataset to resolve the processes leading to the evolution of sympatric and divergent mtDNA lineages. We identified two genetic clusters in the genomic dataset corresponding to (1) the nominate subspecies group, and (2) northwestern Mexico birds. Following divergence, the nominate clade maintained a large, stable population, indicating that divergent mitochondrial lineages arose within a panmictic population. Simulations based on parameter estimates from this model further confirmed that this demographic history could produce observed levels of mtDNA diversity. Patterns of divergent, sympatric mtDNA lineages are frequently interpreted as admixture of historically isolated lineages. Our analyses reject this interpretation for Savannah sparrows and underscore the need for genomic datasets to resolve the evolutionary mechanisms behind anomalous, locus-specific patterns. This article is protected by copyright. All rights reserved.