Morphology, genetics, and ecology of pocket gophers (genus Geomys) in a narrow hybrid zone

A population of hybrid pocket gophers ( Geomys bursarius × G. lutescens ) exists 1–2 km west of Oakdale, Antelope County, Nebraska, U.S.A. The hybrids occur in soil that has characteristics intermediate between that occupied by G. lutescens (sand) and by G. bursarius (silt loam); the vegetation associations on the different soils are Sandhills Prairie and Tall-grass Prairie, respectively, with mixed prairie on the intermediate soils. Hybrids are identifiable on the basis of both qualitative and quantitative morphological characteristics, allozymes and karyology. Concordance between morphological, allozymic, karyotypic and ecological data sets is very high. Hybrids appear to reproduce normally and survive well; i.e. they suffer no obvious loss of fitness. Backcrossing to either parental type is apparently rare. The parental species each support obligate parasitic lice ( Geomydoecus : Trichodectidae) of different species; these species are not sister species. We suggest that hybrid zones resulting from primary and secondary contact may be distinguished by (1) concordance of clines in different character sets, (2) fossil and biogeographic data, and (3) parasite data. We conclude that this zone resulted from secondary contact, and that the zone is maintained either by selection against hybrids (less likely) or by hybrid superiority (more likely).