Death Valley seems an unlikely spot to go fishing. Nonetheless, seven species of pupfish survive in North America's lowest, hottest spot as remnants from the cooler, damper Pleistocene Epoch. For the most part, these species exist in isolation and have been left to adapt to minute details of their local environment—sort of the fish version of Darwin's finches. Surprisingly, however, even within a species, when environmental variables such as water temperature or food supply vary, morphological changes are evident within a few generations. This phenotypic plasticity calls into question not only environmental management practices for species preservation but also just what it is to be a species.
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