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<br />2750 <br /> <br />Evolution: DeMarais et al. <br /> <br />would produce coincidental sorting of unique features in <br />morphology, allozymes, and mtDNA to form G. r. robusta <br />and G. elegans, as well as the morphological and allozymic <br />intermediacy of MRN and G. seminuda. <br />Morphology, allozymes, and mtDNA are all consistent <br />with past hybridization between G, elegans and G. r. ro- <br />busta. Various features of body shape place G. seminuda <br />almost exactly intermediate between G. elegans and G. r. <br />robusta (Fig, 1) and similar to artificially produced hybrids of <br />that same parentage, while MRN, although clearly G. r. <br />robusta-like, tends toward G. elegans. Allozymes provide <br />the same pattern of intermediacy. MRN and G. seminuda <br />exhibit a mixture of alleles diagnostic for G. elegans and G. <br />r. robusta in the Colorado River basin below Grand Canyon <br />(B.D.D., unpublished data). Results for G. elegans are, <br />however, based on only a single population of this rare <br />species. <br />Unlike morphology or allozymes, mtDNA is nonrecombi- <br />nant, and strict maternal inheritance prevents its expression <br />as a mosaic of characters within individuals. The mtDNA of <br />only one of the hybridizing species may be present within <br />hybrid zones, depending on the direction and extent of <br />introgression (e.g., see refs. 39-41), or both mtDNAs may <br />persist long after the disappearance of one parental taxon <br />(e.g., see refs. 42 and 43). By itself, the presence of only G. <br />elegans-Iike mtDNA in G. seminuda and MRN has little <br />meaning; however, the strong contrast between morpholog- <br />ical and mtDNA characteristics in these forms (such that G. <br />r. robusta-like or intermediate morphologies are coupled with <br />G. elegans-like mtDNAs) is compelling evidence for their <br />hybrid origin. <br />Hybridization and Evolution. Acquisition of ecological di- <br />vergence and attainment of reproductive isolation are the two <br />major problems that must be overcome before hybrid speci- <br />ation can successfully occur (3). If either fails, recombinant <br />phenotypes are swamped out of existence via gene flow with <br />parental forms. Hybrid recombinants generally show ecolog- <br />ical divergence from both parental forms and seem most <br />suited for hybridized habitats (1). In fact, the presence of <br />such a suitable habitat has been argued as the primary limiting <br />factor for hybrid speciation (ref. 44 and references therein). <br />Modern examples of widespread hybridization between <br />fish species can usually be traced to stocking of alien species <br />and/or anthropogenic habitat alterations. Interspecific (and <br />even intergeneric) hybridization is common (45, 46), with <br />most such incidents resulting only in first-generation hybrids <br />or local and transitory hybrid swarms. <br />The early discovery and description of G. seminuda (19) <br />preclude the possibility of introductions and/or man-induced <br />habitat change as promoting their hybrid origin. Further- <br />more, neither G. r. robusta nor G. elegans has ever been <br />collected from the Virgin-River system (47), LaRivers (16) <br />noted G. elegans in the lower Virgin River, but this may have <br />been speculation since it was common in the adjacent main- <br />stem Colorado River before impoundment of Lake Mead and <br />persisted in the reservoir until the 1960s (ref. 48; W.L.M., <br />unpublished data). <br />Natural phenomena surely had consequences similar to <br />those of human perturbations (45) and perhaps with far <br />broader implications, such as bursts of speciation (44). Drain- <br />age transfers through stream capture or tectonism forced <br />allopatric faunas into contact, and progressive desertification <br />shrank aquatic habitats more than enough to hinder ecolog- <br />ical segregation (49). Dramatic environmental change has <br />been the rule in western North America throughout the <br />Tertiary period (50), and repeated cycles of isolation and <br />sympatry must have been common during the evolutionary <br />history of modem western fishes. Differentiating taxa, re- <br />peatedly forced into contact, almost certainly exchanged <br />genetic material, and some stocks of hybrid origin must have <br /> <br />~ <br /> <br />Proc. Natl. Acad. Sci. USA 89 (1992) <br /> <br />persisted. These may have had a selective advantage over <br />parental forms since interspecific gene flow increases genetic <br />variability faster than mutation alone, thereby allowing rapid <br />responses to changing environments (1, 51, 52), <br />Before human perturbations, both G. elegans and G, r, <br />robusta were widely distributed in the Colorado basin. The <br />former lived in mainstem rivers and their largest tributaries, <br />and the latter occurred syntopically, while occupying smaller <br />streams as well (53). Opportunities for hybridization were <br />thus abundant, and the origin of G. seminuda/MRN through <br />such an event would be predicted in intermediate-sized <br />tributaries during regional drought or other natural catastro- <br />phes, <br />No physical barrier is known that would have impeded <br />continual gene flow or actual migration of either G, elegans <br />or G. r. robusla into the Virgin River. Yet, G. seminuda has <br />established and continues to maintain independence from <br />these taxa, It has stabilized morphologically, maintained <br />Hardy-Weinberg equilibrium, and sustained a distinct and <br />unique distribution parapatric to both parents for the > 100 <br />years since it came to the attention of ichthyologists, <br />The taxonomic status of stabilized hybrid derivatives such <br />as G. seminuda will likely remain a point of contention. <br />However, current relegation of G. seminuda to a subspecies <br />of G. r. robusta is inappropriate, iff or no other reason than <br />arbitrariness; it could just as easily be considered a subspe- <br />cies of G. elegans. More importantly, subspecific designation <br />obscures both the evolutionary origin and independence of G. <br />seminuda as a distinct lineage. Because G. seminuda exhibits <br />phenotypic, genetic, and geographic integrity, its reelevation <br />to specific rank is warranted. <br />We thus conclude that G. seminuda and MRN comprise a <br />distinct species, originating, through Pleistocene or post- <br />Pleistocene hybridization between G. elegans and a local <br />form of G. robusta and promoted by habitat changes brought <br />on by regional aridification. <br />Conservation Implications. Demonstration of hybrid origin <br />for G. seminuda and MRN has specific ramifications for their <br />conservation status. When G. seminuda was listed as endan- <br />gered (54), MRN was considered an "undescribed" subspe- <br />cies of G. robusta and specifically excluded from protection. <br />However, information presented here indicates a close rela- <br />tionship between these forms. Therefore, the listing of G, <br />seminuda should be amended to include the Moapa River <br />population. <br />Evidence for hybrid origin also has general implications for <br />conservation of the entire grouj). Most Gila native to the <br />Colorado River basin are threatened with extinction, with <br />several members listed or candidates for listing as endan- <br />gered (G. cypha, G. elegans. G. r. robusta, Gila robusta <br />jordani. G. seminuda). The tremendous morphological di- <br />versity of this group is paralleled by its genetic complexity, <br />and we anticipate that hybridization played an important role <br />in generating and maintaining this complexity, While the <br />general validity of this hypothesis remains to be tested, it still <br />requires serious consideration, particularly when recovery <br />programs are implemented and further studies are designed, <br />If hybridization is indeed an important mode of evolution in <br />western fishes, then protection of distinct forms coupled with <br />an active bias against suspected hybrids could prove detri- <br />mental to the entire complex. Not only could valid species of <br />hybrid origin (such as G, seminuda) be eliminated, but a <br />valuable mode of evolution could be truncated. Given the <br />potential impact of such a program, research emphasis should <br />instead be placed on determining the extent of hybridization <br />and its effect on the evolution of these fishes. Meanwhile, it <br />would be prudent to protect the entire complex so that <br />valuable genetic variation distributed throughout the basin <br />will not be lost forever. <br />