458 BRADLEY 



TABLE 2 



ESTIMATES OF PHYSIOLOGICAL VARIANCE 

 IN MALES AND FEMALES* 



Method 



*Estimates were made using methods 1 and 2 in Appen- 

 dix A. 



Eurytemora collected at different seasons (Bradley, 1975). Since 

 they were treated identically, the consistent difference implied a 

 genetic basis. (2) Genetic variance in temperature tolerance was 

 found when the progeny of animals collected from the intake canal 

 were compared with the progeny of animals collected from the 

 discharge canal of a steam electric-power plant on the Patuxent River 

 in Maryland. Progeny rather than parents were used to avoid effects 

 of exposure to water of different temperatures. The average 

 tolerances of the male progeny collected at the intake and discharge 

 canals were 20.3 and 32.5°C, respectively, and of the female 

 progeny, 17.5 and 27.8°C, respectively. Both these differences were 

 significant (P < 0.01); this indicates that more-tolerant parent 

 animals had been selected by the power plant. The differences among 

 the progeny indicated genetic differences among the parents since all 

 the progeny were treated exactly alike. 



Table 2 shows the estimates of physiological variance in 

 temperature tolerance in males and females, measured by the first 

 two methods in Appendix A. The pair of estimates from test— retest 

 correlations and all estimates obtained from the four experiments on 

 genetic variance indicate that physiological variance within the 

 female group is much higher than that within the male. These 

 variances were observed in the absence of systematic changes in 

 external temperature. The physiological variances of females are 

 comparable to the genetic variances of males (Table 1) and vice versa. 

 Taken together, the estimates in Tables 1 and 2 indicate that 

 Eurytemora has considerable potential for adaptation to thermal 



