ABSTRACT
To understand the basic reasons for the endangered and threatened status of some of the fishes native to the Colorado River Basin, it is helpful to have some concept of the geologic history of the basin and the long evolutionary history of the species specializing and adapting to an environment which essentially no longer exists. That is, the evolutionary programming resulting in such unusual species as the squawfish, the razorback sucker, and the bonytail and humpback chubs has dictated life histories and ecologies discordant with present conditions.
