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The chromosomes of Ovis nivicola, described for the first time, exhibit 2n = 52, the lowest diploid number to be reported for wild sheep and goats. The new chromosomal data, together with a review of the fossil history of the genus, lead us to conclude that the bighorned wild sheep (subgenus Pachyceros) evolved their distinctive characteristics while isolated in the ice-free Beringian refugium, and then migrated southward into western North America when the glacial barriers melted, as first suggested by Cowan (1940).

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