EVOLUTION IN CONSTANT AND FLUCTUATING ENVIRONMENTS: THERMAL TOLERANCES OF DESERT PUPFISH ( CYPRINODON )

What are the evolutionary and ecological consequences of inhabiting constant, as opposed to fluctuating, enviromnents? Students of evolution since Lamarck and Darwin have recognized that characteristics of organisms which are no longer functional tend to be eliminated (e.g, many cave dwelling species have lost their eyes and pigment) . This suggests that organisms which once inhabited fluctuating environments should lose their tolerance to those conditions that they no longer encounter during their subsequent evolution in less variable environments. In ecological terms this means that the tolerance of a population for a particular parameter should correspond to the breadth of the niche in that parameter. In an ecological context tolerance usually has two components, a genetically determined capacity to withstand a certain range of a parameter and a phenotypic capacity, frequently termed acclimation, to shift tolerance within this range as a result of previously experienced conditions. Levins (1969) has recently found a fairly good correspondence between these two components of thermal tolerance and the breadth of the thermal parameter of the niche in Puerto Rican Drosophila. The present paper compares the thermal tolerances of populations of desert pupfish that have been isolated for varying periods of time in thermally constant environments with those of populations inhabiting waters that fluctuate tremendously in temperature. Pupfish (Cyprinodlm ne'lladensis and related species) of the Death Valley region