GENETIC AND PHYSIOLOGICAL FLEXIBILITY 465 



flexibility (acclimation), then the effective tolerance of the popula- 

 tion might not increase much since acclimation increases the range of 

 tolerance. If genetic variance in temperature tolerance cannot, in 

 fact, be exhausted, there may well be a negative correlation between 

 reproduction and tolerance so that the most tolerant genotypes 

 reproduce less than average. I should point out that the two most 

 obvious models explaining the large residual genetic variance are both 

 unlikely. Heterozygous superiority, which results in large variance 

 caused by genie segregation, is unlikely in view of the apparently 

 small average deviation of heterozygotes from the means of 

 homozygotes (dominance variance) (Table 1). The other model, 

 seasonal selection, also seems unlikely because, since heat and cold 

 tolerance do not seem to be opposite characteristics, selection would 

 not be reversed seasonally and genetic variance would not be 

 maintained. 



Finally, if genetic variance for heat tolerance can be exhausted, 

 the question of reverse adaptation arises. Is adaptation to more 

 "normal" conditions compromised by adaptation to extreme stress 

 conditions? Adaptation of species or communities to novel stresses 

 might be tolerable if there was a guarantee that further stresses were 

 not on the way. 



ACKNOWLEDGMENTS 



This work could not have been done without the assistance of 

 Richard Imbach, Kenneth Keeling, Denise Markoff, Richard Muths, 

 and Gregory Taylor and the financial support of the Office of Water 

 Research and Technology, U. S. Department of the Interior (Grant 

 B-021-MD) and the National Science Foundation (Grant 

 BMS-75-20282). 



Finally, I thank Carol Ferrigno for her patience in typing the 

 manuscript and Tim Ford for preparing the figures. 



REFERENCES 



Baldwin, J., and P. W. Hochachka, 1970, Functional Significance of Isoenzymes 

 in Thermal Acclimation: Acetylcholinesterase from Trout Brain, Biochem. 

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Banta, A. M., and T. R. Wood, 1939, Genetical Studies in Sexual Reproduction, 

 in Studies on the Physiology, Genetics, and Evolution of Some Cladocera, 

 A. M. Banta (Ed.), Paper No. 39, Department of Genetics, Carnegie Institute 

 of Washington, Washington, D. C. 



Battaglia, B., 1967, Genetic Aspects of Benthic Ecology in Brackish Waters, in 

 Estuaries, H. W. Lauff (Ed.), pp. 574-577, Publication No. 83, American 

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