Origin, Classification, and Distribution of Mountain Sheep in North America

An ancestral form of the Siberian snow sheep ( Ovis nivicola ) accessed northwestern North America via the Bering Land Bridge and became the progenitor of the two species (Dall’s [ Ovis dalli ] and bighorn sheep [ Ovis canadensis ]) of North American mountain sheep. The distribution of mountain sheep extends from northern Alaska to the southwestern United States and northern Mexico. Unstable landscapes and climates of the Pleistocene and glacial-induced vicariance, hybridization, and introgression are major influences of North American mountain sheep genetic and phenotypic evolution. The diverse habitats enabled localized populations to evolve genetically distinct genomes, and in some instances formed hybrid zones and subsequent introgressed populations further complicated determining genomic histories and infraspecific genetic relationships. The infraspecific taxonomy of bighorn sheep has been complicated, especially because of named taxa originally based on phenotypic criteria that are not diagnostically meaningful. The use of genetic data since the 1980s has been instrumental in determining infraspecific variation. The application of genetic methods (e.g., karyotyping, mitochondrial DNA sequencing, single-nucleotide polymorphisms) has enabled the identification of unique and distinct populations that represent the overall genetic variation within a species and also are important conservation units. The current classification of American sheep includes two subspecies of thinhorn sheep and five subspecies of bighorn sheep.