SPECIATION IN FISHES OF THE GENERA CYPRINODON AND EMPETRICHTHYS , INHABITING THE DEATH VALLEY REGION

Abstract

During the Pleistocene history of the desert basins of western North America, particularly in the Great Basin, block faulting produced dozens of independent valleys separated by precipitous mountain ranges. In Pluvial 2 periods many of these valleys held lakes and large streams, but the desiccation of postglacial tinjes disrupted the drainages between, as well as within, the once continuous basins. Isolated populations of animals and plants resulted, and the Great Basin has become a land of relicts. The Death Valley region, lying in the southwestern extension of the Great Basin, provides an excellent example of such disconnected drainages and faunas. During late Pleistocene time, an integrated river system spread over this desert region to unite the four now isolated basins of what may be called the Death Valley system (Map 1). Although only a limited fish fauna inhabits this area, it is rich in material for the student of evolution and is highly endemic. The living fishes comprise 3 families, 5 genera, 10 species and about 24 kinds including subspecies. One genus, Empetrichthys, is endemic, and nine species are confined to the system. High endemism on the species level suggests a Wisconsin or pre-Wisconsin origin of the fauna.