Fig. 10. Hypothesized effects of evolution at moderately high temperatures on the Hsp70 versus temperature norm of reaction. The solid line represents an idealized norm of reaction for wild-type Drosophila melanogaster reared at typical culture temperatures, 25°C or below, and approximates diverse data for Hsp70 expression and heat-shock factor (HSF) activation in this species (Bettencourt et al., 1999; Dahlgaard et al., 1998; Feder et al., 1997; Lerman and Feder, 2001; Lindquist, 1980). Given this norm of reaction, D. melanogaster cultured at a constant 28°C (Cavicchi 28°C lines; Bettencourt et al., 1999) or 31–32°C (T strain; T32) would constitutively undergo low levels of Hsp70 expression, incurring the deleterious consequences of Hsp70 (see Discussion) but none of the benefits realized only at higher temperatures. It is suggested that evolution at these moderately high temperatures acts to modify the norm of reaction (broken line) so that heat shocks at temperatures above 35°C result in lower concentrations of Hsp70 than in the wild type.