462 BRADLEY 



equally important, to be closely related to survival at high tempera- 

 tures (Bradley, 1976). In this context, it is interesting to note that 

 Hamby (1975), working with the marine snail Littorina littorea, 

 found that the temperature at which an animal entered a coma was 

 significantly shifted by acclimation, but the lethal temperature was 

 affected very little. He concluded that the nervous system of 

 Littorina was most vulnerable to thermal extremes, as is the case 

 with other poikilotherms (Prosser, 1973), 



The great genetic variance in temperature tolerance implies 

 seasonal selection caused by changing water temperatures. There 

 have been some laboratory demonstrations of genetic variance 

 associated with environmental variability (McDonald and Ayala, 

 1974; Beardmore, 1961; Gibson and Bradley, 1974). Battaglia (1967; 

 1970) reported visible differences in nature in copepods controlled 

 by single loci and influenced by temperature variation. There are 

 many other instances of genetic variance in other species associated 

 with temperature variability (see Parsons, 1973, for a review). A 

 complete discussion of the relevant theory on genetic and environ- 

 mental variability is inappropriate here, but the reader is referred to 

 Levins (1968) and to the work of Gillespie (e.g., Gillespie, 1974). 



There are relatively few examples of demonstrated genetic 

 variance in temperature tolerance as such. Banta and Wood (1939) 

 reported that work they did in 1928 showed much higher tolerance 

 in one clone of Daphnia magna, obviously caused by a genetic 

 difference. Fatt and Dougherty (1963) found a single locus effect on 

 temperature tolerance in the nematode Caenorhabditis elegans. In a 

 sense, the whole class of temperature-sensitive mutants also demon- 

 strates genetic variance in temperature tolerance. 



One of the earliest demonstrations of genetic variance in 

 temperature tolerance was described by Dallinger (1887) in his 

 presidential address to the Royal Microscopical Society. He collected 

 three species of flagellated bacteria at 15.6°C, gradually raised the 

 temperature, and observed (under the microscope) whether or not 

 they seemed uncomfortable. When they became adjusted, he again 

 raised the temperature, repeating the process until the temperature 

 reached 70°C. Wild specimens did not survive at this temperature, 

 and bacteria living at 70°C did not survive the original 15.6°C 

 optimum. His experiment was ended "with such pain as I presume is 

 natural" when "the accident" happened; this was presumably a 

 malfunction of his thermostat after 7 years of painstaking observa- 

 tion and a remarkable demonstration of adaptation to thermal stress. 



There are far more precedents for the finding of physiological 

 than of genetic flexibility, especially as demonstrated by acclimation. 



