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McBe and Vlijenhcek ton Sea sink due to human intervention may also have prevented the pattern of local differentiation observed in Death Valley. It is instructive that the most isolated population of C. macularius, in Quitobaquito Spring in the upper Sonoyta basin of Arizona, does exhibit a level of genetic differentiation that is roughly compazable to that ob- served among Death Valley pupfish populations (Turner 1983). 'This isolated population was recently described as a distinct subspecies, G macularius eremus (Miller & Fuiman 1987). The Stream Hierarchy Model The second pattern of population structure occurs in desert fish that inhabit one or more dendritic river sys- tems, with varying degrees of connectedness and gene flow (Fig. 2a). In this case, local demes are only partially isolated from other gene pools, with some probability of gene flow among them. Periodic or permanent connec- tions allow some migration and gene flow among habi- tats, and local population divergence is less extreme than in the Death Valley model. A hierazchical genetic structure results, wherein higher frequencies of gene flow correspond to greater similarity among demes. This hierarchy of genetic divergence is a function of geographic connectedness of habitats: localities that are frequently connected and exchange population mem- bers will have greater genetic (and presumably pheno- typic) similarities than localities that infrequently or never experience gene exchange. In such a hierarchical system, total genetic diversity of a species (Ht) consists ofwithin-colony diversity (H~) plus differences among colonies within a river (D~) plus differences among rivers within major drainage sys- tems (D,~) plus divergence among drainages occupied by the species as a whole (Dst). More complex hieraz- chies may develop depending upon levels of connectiv- ity and extent of the species' range. During wet years, patches connect and allow migration by fishes; dry years isolate habitats and stop gene exchange. Natural cli- matic variation thus connects and separates local demes in a hierarchical structure that is a function of geo- graphic distance and river discharge. This model is typ- ified by numerous species in the Gila River drainage of southern Arizona and probably represents fishes in many lotic systems in the southwest. Interruption of the stream hierazchy by habitat alter- ation can change patterns of gene flow among localities and disrupt natural genetic structure in two ways. Move- ment of fishes among naturally isolated populations in- troduces gene flow that historically might not have oc- curred, thus decreasing Dn, Dom, or Dst. Conversely, impoundment of rivers, pumping of springheads, or other alteration of drainage patterns can fragment and Conservation Genetics of Desert Fishes 161 Figure 3. Distribution of Poeciliopsis occidentalis in Arizana The historical distribution, as of about 190, is shaded; circles are distribution as of late 1987. Solid circles are the Gila subspecies (P. o. occi- dentalis, genetic group I) and open circles are the Yaqui subspecies (P. o. sonoriensis, genetic group II). isolate populations that would normally experience gene exchange (Fig. 2b). Such actions should cause a general decline in local genetic diversity (H~) and a corresponding increase in divergence among colonies within a drainage system (Dn and Dom). Habitat destruction resulting in population segrega- tion is presently the greatest threat to the preservation of genetic diversity in river-dwelling fishes of the Amer- ican Southwest. Because of limited water availability, many southwestern streams have been dammed or di- verted, completely changing flow characteristics and connectivity patterns (Minckley & Meffe 1987), and ex- tensive groundwater pumping has depleted ancient aquifers and caused many isolated springs to cease flow- ing (Williams et al. 1985; Pfister & Unkel, in press). An example of hierarchical population structure in an endangered fish species occurs in the Sonoran topmin- now, Poeciliopsts occidentalis This livebeazing fish is native to the lower Colorado River drainage of Arizona and New Mexico and to several major drainages in So- nora, Mexico. It was extremely abundant through the Gila River basin of southern Arizona, including numer- ous springs along watercourses, as recently as'the 1940s (Hubbs & Miller 1941; Minckley & Deacon 1968; Fig. Conservation Biology Volume 2, No. 2, June 1988