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Show Gerald R. Smith Museum of Zoology University of Michigan Ann Arbor, Michigan 48109 HISTORICAL BIOGEOGRAPHY OF GREAT BASIN FISHES Hubbs' and Miller's works on Great Basin fishes are model studies of hydrographic history and rates of evolutionary divergence. These studies relied on evaluation of consistency between amounts of fish divergence and timing of pluvial and post- pluvial events. Hubbs and Miller hypothesized evolutionary rates much more rapid than previously thought possible. Their method relied on the assumption that the interval of divergence began with the historically most recent hydrographic connection, at which time homogeneous ancestral populations could be assumed. This approach and its key assumption have become so standard that they are rarely questioned or tested. Alternative hypotheses about rates of evolution require a choice of multiple separation times, estimated from data independent of the hypotheses. Additional sources of data on times and rates- fossils, molecules, and geological data are helpful. In this paper fossil fishes in stratigraphic context introduce independent time estimates, thereby enabling continuation of the tradition of Great Basin fish studies. Hubbs and Miller recognized that the few fossil fishes then known suggested that Great Basin fish lineages have been essentially modern since the Miocene. The Neogene fossil fish record in the Great Basin now includes about 40 lineages, from over 45 localities. The pattern of change within lineages suggests that supposedly rapid post- pluvial changes are not representative of the slow rates of change over the past several million years. Observed external and osteological changes, inferred to have occurred in post- pluvial time, have not changed the lineages substantially. Generalizations about rates of evolution in the Great Basin benefit from a limited number of fish lineages, especially minnows, suckers, trout, and pupfish, almost completely isolated in numerous replicated basins. Hubbs' and Miller's research emphasized changes at the level of races and subspecies; these changes were believed to have occurred in 102 to 104 years. Miocene to Pleistocene fossils of western North American fishes, in contrast, suggest unexpectedly slow changes at the species level, i. e., over 105 to 107 years, with infrequent evolution of new species. The latter observations contrast with expectations for a system with extreme isolation and high endemism. Resolution of these seeming conflicts requires clarification of species concepts and processes for origin of fish species. The evolutionary species concept, which emphasizes long term independence, proves useful for Great Basin fishes, if modified to allow for introgressive origin of species changes. The biological species concept emphasizes reproductive isolation; it is applicable only if modified to permit introgression between evolutionary species. The paleontological chronospecies concept is not helpful because it leads to arbitrary species and inflated estimates of diversity. Patterns of Great Basin fishes through time as well as independent evidence from molecular studies suggest that lineage evolution involved as much recombination of introgressive input as new mutations. |