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Nadler, C. F., R. S. Hoffmann, and K. R. Greer (Dept. Medicine, Northwestern Univ. Med. School, Chicago, 60611; Mus. Nat. History, Univ. Kansas, Lawrence, 66044; and Montana Dept. Fish and Game, Bozeman, 59715) 1971. Chromosomal divergence during evolution of ground squirrel populations. Syst. Zool., 20:298–305.—Two subspecies of Spermophilus richardsonii, occurring in the Madison River valley of southwestern Montana differ in chromosome number, S. r. aureus being 2n = 34, and S. r. richardsonii, 2n = 36. The zone of contact between them is 20-25 miles long; only one of 21 colonies investigated contained chromosomal hybrids (2n = 35). Cytological and morphological differences suggest that aureus and richardsonii passed through a period of divergence while isolated from one another, and that the present contact is secondary.
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Nadler et al. (1971) studied this question.
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