Chromosome Studies of Pocket Gophers, Genus Thomomys. I. The Specific Status of Thomomys umbrinus (Richardson) in Arizona

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Abstract
The complexities of morphology and, hence, taxonomy of pocket gophers (genus Thomomys) in southern Arizona are reflected by extreme interpopulation chromosomal variation in both T. bottae (2n = 76) and T. umbrinus (2n = 78). The variation consists of differing numbers of morphological types of chromosomes for nearly each population karyotype. The known range of variation in either species is less than the amount of difference between the two. A somewhat strict ecological separation exists between T. bottae and T. umbrinus in areas of sympatry or near sympatry, with the former preferring the more friable soils of the valley floors and mountain tops and the latter confined to the indurate soils of the oak zones at intermediate elevations. Chromosomal and ecological concordance support the interpretation that T. bottae and T. umbrinus are distinct species. Limited hybridization between the two species at one locality of sympatric contact, however, is known.