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Douglas et al., 1994). This is especially germane for westem North America, where the availability and reliability of aquatic systems have been compromised over millenia by tectonic and climatic events (Douglas et al., 2003; Oakey and Douglas, 2003). These limited sources of water evolved in concert with (and provide habitat for) an endemic, isolated, and depauperate native fish communities. Today, these fishes seemingly lack those competitive abilities and predator defenses developed by fishes in more speciose regions (Moyle et al., 1986). Similarly, parasites harbored by exotic fishes can also be extremely detrimental to natives (Meffe, 1985). The Asian tapeworm (Bothriocephalus acheilognath~), for example, is an intestinal parasite apparently introduced into the Colorado River Basin circa 1990. It is now pandemic within western stream ecosystems (Heckman et al., 1993; Browder and Hoffnagle, 1997; Clarkson et al., 1997). Its presence in native fishes is a cause of considerable concern (Amin and Minckley, 1996), for infection will emaciate the host and lead, under conditions of stress, to significant mortality (Granath and Esch, 1983; Riggs and Esch, 1987). Again, these effects do not occur immediately, but are instead mediated slowly and over considerable time. A third mechanism by which exotic fishes gradually eliminate endemics, and one that provides the focus for this paper, is through introgressive hybridization. Here we define "introgressive hybridization" as "...the infiltration of genes of one species by the intermediacy of hybrids into another species, resulting in the genetic modification of the latter" (Hanson, 1962). It is particularly prevalent in westem North America, because anthropogenic alterations of habitat and introductions of non-native species have characterized this region. Both factors will increase the rate at which species hybridize (Rhymer and Simberloff, 1996). The situation is particularly acute in fishes due simply to their external mode of fertilization coupled with weakly developed reproductive isolating mechanisms (Hubbs, 1955; Campton, 1987). Furthermore, the situation becomes exacerbated when the introduced form has evolved allopatrically but is closely related to the indigenous form. The result is often that the more abundant exotic may ultimately swamp out and thus eliminate the endemic (Rhymer and Simberloff, 1996). 5