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<br />244
<br />
<br />MICHAEL E. DOUGLAS ET AL.
<br />
<br />evolution of biogeographic distributions (shallow history is
<br />defined as the most recent glacial/interglacial period, 85,000
<br />years ago to present). In this sense, we concur with Riddle
<br />(1996), who advocated caution in accepting hypotheses of
<br />Pleistocene diversification without first critically testing al-
<br />ternative, temporally deeper models. The fact that we ac-
<br />commodate both types of models (among others) in our an-
<br />alyses is also noteworthy. Finally, our results statistically
<br />support, rather than simply propose, vicariance as a primary
<br />factor in phenotypic evolution of one group of western Amer-
<br />ican freshwater fishes. In many ways, the last is most im-
<br />portant because it speaks of process, which merits further
<br />examination.
<br />We know that from Miocene to Recent times aquatic hab-
<br />itats in the American West suffered numerous and extensive
<br />disruptions from episodic orogeny and progressive aridity
<br />(Armentrout et al. 1979; Axelrod 1979). Some lakes, springs,
<br />and rivers disappeared. Others remained isolated in inter-
<br />montane basins or dwindled to disjunct segments separated
<br />from others by dry channels. Only a few continued to flow
<br />from the mountains through lowlands to the sea. Over time,
<br />drainage divides eroded, basins filled with alluvium, and old
<br />channels were reintegrated piecemeal to form new and com-
<br />plex watersheds. Today, many western American rivers exist
<br />not as units intact since their inception, but as composites of
<br />more than one original system (Minckley et al. 1986).
<br />Fishes must have persisted within the drainage fragments
<br />almost as an addendum to hydrologic chaos. As intercon-
<br />nections developed, fishes were either passively transferred
<br />from one system to another or provided opportunity to ac-
<br />tively disperse into new systems. As a result, fishes of the
<br />American West frequently exhibit confusing patterns of di-
<br />versity when analyzed at the morphological level. As shown
<br />here, these patterns are difficult or impossible to interpret
<br />without know ledge of the past because they often run counter
<br />to conventional logic. Yet, to infer past process from existing
<br />pattern requires an ability to simultaneously erect and test
<br />multiple hypotheses, each of which offers an interpretation
<br />of the pattern but from a different perspective.
<br />In this study, the vicariance hypothesis is the only model
<br />statistically proficient in explaining diversity of fish pheno-
<br />types in the Gila River Basin (Table 1). Further, of paleo-
<br />hydrographic reconstructions available, those of the mid-
<br />Miocene and Pliocene epochs are associated significantly
<br />with present-day geographic distributions of phenotypes (Ta-
<br />ble 2), and of these the Pliocene is paramount. Simply stated,
<br />Pliocene hydrography statistically explains the existing pat-
<br />tern of phenotypic diversity in three forms of the minnow
<br />genus Gila.
<br />
<br />ACKNOWLEDGMENTS
<br />
<br />This work could not have been done without the extensive
<br />research of J. Dale Nations and his colleagues (Nations et al.
<br />1982 and references therein), who provided a spatial and
<br />temporal geologic backdrop for our endeavors. R. R. Miller,
<br />University of Michigan Museum of Zoology, graciously pro-
<br />vided access to specimens, field notes (including those of C.
<br />L. Hubbs), and personal research notes on the genus Gila.
<br />We thank the individuals involved in the review process for
<br />
<br />helping resolve organizational and textual problems. This
<br />study was supported by several organizations, agencies, and
<br />individuals over many years, and especially benefited from
<br />those who discussed with us substantial differences of opin-
<br />ion with our conclusions. We thank them all and hope this
<br />contribution will stimulate further research and debate on this
<br />remarkable group of minnows.
<br />
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